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This post is about one of the fundamentally important properties of a database: how IO is done. The test case I studied is doing a simple full table scan of a single large table. In both Oracle and postgres the table doesn’t have any indexes or constraints, which is not a realistic example, but this doesn’t change the principal topic of the study: doing a table scan.

I used a publicly available dataset from the US bureau of transportation statistics called FAF4.5.1_database.zip
The zipped file is 347MB, unzipped size 1.7GB.

In both cases Oracle Linux 7.7 (64 bit) is used, running in VirtualBox, with the storage being a USB3 SSD. Number of CPUs is 4, memory size is 6G. Filesystem type: xfs.
The Oracle version used is Oracle 19.5, the Postgresql version used is 12.1.
For Postgresql, the postgresql.conf file is not changed, except for max_parallel_workers_per_gather which is set to 0 to make postgres use a single process.
For Oracle, the parameters that I think are important: filesystemio_options=’setall’. Oracle is used filesystem based (so no ASM).

This is the table definition for Oracle:

create table faf451 (
  fr_origin varchar2(3),
  dms_orig varchar2(3),
  dms_dest varchar2(3),
  fr_dest varchar2(3),
  fr_inmode varchar2(1),
  dms_mode varchar2(1),
  fr_outmode varchar2(1),
  sctg2 varchar2(2),
  trade_type varchar2(1),
  tons number,
  value number,
  tmiles number,
  curval number,
  wgt_dist number,
  year varchar2(4)
);

This is the table definition for Postgresql:

create table faf451 (
  fr_origin varchar(3),
  dms_orig varchar(3),
  dms_dest varchar(3),
  fr_dest varchar(3),
  fr_inmode varchar(1),
  dms_mode varchar(1),
  fr_outmode varchar(1),
  sctg2 varchar(2),
  trade_type varchar(1),
  tons double precision,
  value double precision,
  tmiles double precision,
  curval double precision,
  wgt_dist double precision,
  year varchar(4)
);

In order for the data to be easy loadable into postgres using copy from, I had to remove ‘””‘ (double double quotes) for the empty numeric fields. In oracle I could say “optionally enclosed by ‘”‘”. For Oracle I used an external table definition to load the data.

Now, before doing any benchmarks, I have an idea where this is going. Oracle is using direct IO (DIO) so linux page cache management and “double buffering” are avoided. Also, oracle will be doing asynchronous IO (AIO), which means submitting is separated from waiting for the notification that the submitted IOs are ready, and on top of that oracle will submit multiple IO requests at the same time. And again on top of that, oracle does multi-block IO, which means that instead of requesting each 8K database block individually, it will group adjacent blocks and request for these in one go, up to a size of combined blocks of 1MB, which means it can requests up to 128 8K blocks in one IO. Postgres will request every block synchronous, so 1 8K block at a time, and waiting for each request to finish. That makes me have a strong idea where this is going.

It should be noted that postgres explicitly is depending on the operating system page cache for buffering as a design principle. Because of DIO, blocks that are read by oracle are not cached in the operating system page cache.

I executed my benchmark in the following way:
– A run for every size is executed 5 times.
– At the start of every run for a certain size (so before every “batch” of 5 runs), the page cache is flushed: (echo 3 > /proc/sys/vm/drop_caches).
– Before each individual run, the database cache is flushed (systemctl restart postgresql-12 for postgres, alter system flush buffer_cache for oracle).

I started off with 2G from the dataset, and then simply performed a ‘copy from’ again to load the same dataset into the table in postgres. Oracle required a bit more of work. Oracle was able to save the same data in way less blocks; the size became 1.18G. In order to have both postgres and oracle scan the same amount of data, I calculated roughly how much rows I needed to add to the table to make it 2G, and copied that table to save it as a 2G table, so I could insert that table to increase the size of the test table by 2G. This way in both oracle and postgres I could test with a 2G table and add 2G at a time until I reached 20G.

These are the results. As you can see in the legenda: oracle is orange, postgres is blue.
postgres oracle scan results(click graph to load full picture)

What we see, is that postgres is a bit slower with the first run of 5 for the smaller dataset sizes, which becomes less visible with larger datasets.
Also, postgres is way faster if the dataset fits into the page cache and it has been read into it. This is logical because postgres explicitly uses the page cache as a secondary cache, and the test is the only activity on this server, so it hasn’t been flushed by other activity.

What was totally shocking to me, is postgres is performing alike oracle and both roughly are able to perform at the maximum IO speed of my disk: 300MB/s, especially when the dataset is bigger, alias beyond the page cache size.

It wasn’t shocking that oracle could reach the total bandwidth of the disk: oracle uses all the techniques to optimise IO for bandwidth. But how can postgres do the same, NOT deploying these techniques, reading 8K at a time??

The first thing to check is whether postgres is doing something else than I suspected. This can simply be checked with strace:

poll_wait(3, [{EPOLLIN, {u32=18818136, u64=18818136}}], 1, -1) = 1
recvfrom(11, "Q\0\0\0!select count(*) from faf451"..., 8192, 0, NULL, NULL) = 34
lseek(20, 0, SEEK_END)                  = 335740928
lseek(20, 0, SEEK_END)                  = 335740928
kill(1518, SIGUSR1)                     = 0
pread64(5, "\f\0\0\0\310ILc\0\0\0\0D\1\210\1\0 \4 \0\0\0\0\230\237\312\0000\237\312\0"..., 8192, 846061568) = 8192
pread64(5, "\f\0\0\0HcLc\0\0\0\0D\1\210\1\0 \4 \0\0\0\0\230\237\312\0000\237\312\0"..., 8192, 846069760) = 8192
pread64(5, "\f\0\0\0\260|Lc\0\0\0\0D\1\210\1\0 \4 \0\0\0\0\230\237\312\0000\237\312\0"..., 8192, 846077952) = 8192
pread64(5, "\f\0\0\0000\226Lc\0\0\0\0D\1\210\1\0 \4 \0\0\0\0\230\237\312\0000\237\312\0"..., 8192, 846086144) = 8192
…etc…

The above strace output shows only 4 rows of pread64() calls, but this goes on. So no “secret” optimisation there.

Luckily, my VM has a new enough version of Linux for it to be able to use eBPF, so I can use biosnoop. Biosnoop is a tool to look at IO on one of the lower layers of the linux kernel, the block device interface (hence ‘bio’). This is the biosnoop output:

# /usr/share/bcc/tools/biosnoop
TIME(s)        COMM           PID    DISK    T  SECTOR    BYTES   LAT(ms)
0.000000000    postmaster     4143   sdb     R  66727776  708608     5.51
0.006419000    postmaster     4143   sdb     R  66731720  77824     11.06
0.006497000    postmaster     4143   sdb     R  66734432  786432    11.03
0.011550000    postmaster     4143   sdb     R  66731872  1310720   16.17
0.013470000    postmaster     4143   sdb     R  66729160  1310720   18.86
0.016439000    postmaster     4143   sdb     R  66735968  1310720   14.61
0.019220000    postmaster     4143   sdb     R  66738528  786432    15.20

Wow…so here it’s doing IOs of up to 1MB! So somewhere between postgres itself and the block device, the IOs magically grew to sizes up to 1MB…that’s weird. The only thing that sits between postgres and the block device is the linux kernel, which includes page cache management.

To get an insight into that, I ran ‘perf record -g -p PID’ during the scan, and then perf report to look at the recorded perf data. This is what is I found:

Samples: 21K of event 'cpu-clock', Event count (approx.): 5277000000
  Children      Self  Command     Shared Object       Symbol                                                                  ◆
-   41.84%     3.63%  postmaster  libpthread-2.17.so  [.] __pread_nocancel                                                    ▒
   - 38.20% __pread_nocancel                                                                                                  ▒
      - 38.08% entry_SYSCALL_64_after_hwframe                                                                                 ▒
         - 37.95% do_syscall_64                                                                                               ▒
            - 35.87% sys_pread64                                                                                              ▒
               - 35.51% vfs_read                                                                                              ▒
                  - 35.07% __vfs_read                                                                                         ▒
                     - 34.97% xfs_file_read_iter                                                                              ▒
                        - 34.69% __dta_xfs_file_buffered_aio_read_3293                                                        ▒
                           - 34.32% generic_file_read_iter                                                                    ▒
                              - 21.10% page_cache_async_readahead                                                             ▒
                                 - 21.04% ondemand_readahead                                                                  ▒
                                    - 20.99% __do_page_cache_readahead                                                        ▒
                                       + 14.14% __dta_xfs_vm_readpages_3179                                                   ▒
                                       + 5.07% __page_cache_alloc                                                             ▒
                                       + 0.97% radix_tree_lookup                                                              ▒
                                       + 0.54% blk_finish_plug                                                                ▒

If you look at rows 13-15 you see that the kernel is performing readahead. This is an automatic function in the linux kernel which looks if the requests are sequential of nature, and when that’s true performs readahead, so that the scan is made faster.

The Oracle database log writer is the process that fundamentally influences database change performance. Under normal circumstances the log writer must persist the changes made to the blocks before the actual change is committed. Therefore, it’s vitally important to understand what the log writer is exactly doing. This is widely known by the Oracle database community.

The traditional method for looking at log writer performance is looking at the wait event ‘log file parallel write’ and the CPU time, and comparing that to the ‘log file sync’ alias “commit wait time”. If ‘log file parallel write’ and ‘log file sync’ roughly match, a commit is waiting on the log writer IO latency, if it isn’t then it’s unclear, and things get vague.

Now we get to the gist of this blogpost: since Oracle 12, there are additional statistics in V$SESSTAT and v$SYSSTAT that describe where the log writer spending it’s time on. However, and sadly, these are not documented by Oracle. The statistics are:

– redo write gather time
– redo write schedule time
– redo write finish time
– redo write time (usec)
– redo write total time

Probably ‘redo write time (usec)’ doesn’t belong to these statistics because the timing is as good as the same as ‘redo write finish time’. Also the statistic ‘redo write time (usec)’ is updated by ‘kcrfw_postprocess_write’ right after the main writing function (kcrfw_do_write), whilst the other redo write timing statistics are updated/made visible by ksu_dispatch_tac outside of the actual writing functions, at the approximate rate of every 3 seconds.

All these statistics contain cumulative timing figure in microseconds (usec).

All these timings start at the start of the function kcrfw_redo_write_driver. This is the main function that handles all the redo writing, including investigating the fixed area structure (kcrfs_) and the shared pool structures (made visible using X$KCRFSTRAND) to see if there are changes.

1. redo write gather time.
This times reading the structures that control allocation of the redo structures, validating foreground processes are done writing the change vectors and applying these to the blocks, updating the LWN SCN. Part of this also happens when there is nothing to write, but then the time is not added to this statistic, only if it found something to write.

2. redo write schedule time.
This includes the time of 1, but now it found something to write, entered the writing function (kcrfw_do_write), and updated the ksfd IO related structures for writing.

3. redo write finish time.
This includes the time of 1 and 2, but this additionally times all the IO related functions for writing the changes it found in the public redo strands, as indicated by the shared pool and fixed area structures.

4. redo write time (usec).
This probably is not part of this group of statistics, but it works in exactly the same way: it’s a cumulative figure, it counts time since the start of the kcrfw_redo_write_driver and includes a tiny bit more code, it times until it has left the kcrfw_do_write function and enters the kcrfw_postprocess_write function.

5. redo write total time.
This includes the time of 1, 2 and 3, but additionally times the kcrfw_post function, which performs the signalling of the foreground processes that have committed and wait for a semop() signal.

This means that the Oracle database since Oracle 12 provides the timing information to see where the log writer is spending it’s time on. This can prevent a lot of guesswork.

This post is about a change in how the time is measured for the event ‘log file parallel write’. This is important for the performance tuning of any change activity in an Oracle database, because with the default commit settings, a foreground session that commits changes waits in the wait event ‘log file sync’, which is a wait on logwriter activity, for which the wait event ‘log file parallel write’ always has been the indicator of the time spend on IO.

Log file sync
First things first: a foreground session normally waits on the wait event ‘log file sync’ when it commits waiting for its change vectors to be written to the online redologfile(s) by the logwriter. It is wrong to always assume a ‘log file sync’ will be present. If, somehow, the logwriter manages to increase the ON DISK SCN to or beyond the foreground session’s commit SCN, there will be no ‘log file sync’ wait event.

I found the absence of the ‘log file sync’ wait event by artificially slowing the committing session down. However, with the improvements in parallelism in the Oracle code, and even more with the improvements on the hardware layer for lowering write latency (especially with persistent memory), this might be a situation that is actually visible in the near future. This is true for at least version 11.2.0.4 and higher (probably earlier too, but these are versions that I verified).

Log file parallel write
However, this post is about the event that in a lot of cases is the culprit for the ‘log file sync’ time: it’s about the ‘log file parallel write’ wait event, which takes place in the logwriter (when the logwriter itself writes, it could delegate that to the logwriter worker processes, LGnn), which normally would be measured by Oracle database tuners to validate logwriter IO latency in case of foreground sessions showing high ‘log file sync’ waits indicating waiting on the logwriter.

I was looking at the logwriter code path using the ‘debug trace’ tool from intel pin tools and used Tanel Poder’s snapper using begin and end snapshots on the logwriter process on Linux, non ASM, Oracle 19.5. I put the logwriter into single logwriter mode (“_use_single_log_writer”=true) and performed redo, which is bound to be picked up by the logwriter.

There I noticed the statistics indicated a write to have been taking place, whilst there was no ‘log file parallel write’ wait event. Weird… So I dug deeper and looked at the functions taking place. These are the functions that I found: https://gitlab.com/snippets/1926207

The important bits are:
– In the function ksfdaio() the IO request is submitted:
ksfdaio>ksfdgo>ksfd_skgfqio>skgfqio>skgfr_lio_listio64>io_submit
– And in the function ksfdblock() the IO request result is read (“reaped”):
ksfdblock>ksfdpoll>ksfdwtio>skgfrwat>skgfrliopo>io_getevents

But no wait event is called *at all!*

This is a clear change from earlier versions, where the logwriter write would always yield a ‘log file parallel write’ wait event. But when did this change happen? To answer that question, I installed version 12.1.0.2 (no patches) to look at the code path and found: https://gitlab.com/snippets/1926211

The important bits are:
– The wait interface is invoked: kslwtbctx()
– In the function ksfdgo() the IO request is submitted:
ksfd_sbio>ksfdgo>ksfd_skgfqio>skgfqio>skgfr_lio_listio64>io_submit
– And in the function ksfdwtio() to IO request result is read (“reaped”):
ksfd_sbio>ksfdwtio>skgfrwat>skgfospo>skgfrliopo>io_getevents
– And the wait interface is ended: kslwtectx()

Ah! So a clear change! So when did this change happen? I installed 12.2.0.1 and found the same ‘waiting’ codepath, then installed 18.3, and found the waiting codepath. So the change probably happened in Oracle 19! So I installed 19.3, and found the waiting codepath once again!! So was it a change in PSU 19.4 or 19.5? I installed 19.5, and found the waiting codepath too.

O.M.G. Why am I seeing different behaviour between two databases on the exact same version? Well, I set some undocumented parameters, so I replicated these in my newly installed database…and it didn’t change.

What else is different? Well…one is a multi-tenant database, and the other isn’t…but surely that doesn’t change the IO codepath?? Of course this is easy to check, so I dropped the non-CDB database and created a CDB one, and…it now showed the same “non waiting” codepath.

So, apparently, using the oracle database in multi-tenant mode changes the IO codepath behaviour. I checked the codepath going down in versions, and it turns out this change appeared in 12.2.0.1.0, so essentially in Oracle 12.2 (Oracle 18 and 19 are actually Oracle 12.2.0.2 and Oracle 12.2.0.3). To be clear on this: I didn’t find this change in 12.1.0.2.

Does this mean the wait events ‘log file parallel write’ does not show up at all? No, if the call io_getevents after io_submit does not return all submitted IOs, it must wait for it, and at that point invoke the wait interface and then go into a blocking io_getevents call, so the actual waiting is timed. This is shown in this snippet: https://gitlab.com/snippets/1926212

This shows ksfdblock>ksfdpoll>ksfdwtio>skgfrwat>skgfrliopo>io_getevents, which returns 0 (what means that it didn’t found any IOs in the completion queue), which then returns up to the ksfdpoll function, invokes the wait interface, kslwtbctx, and then dives to io_getevents again, but now in blocking mode to wait for the IO.

I also replayed the tests on ASM, which shows the exact same (non-)waiting behaviour.

Conclusion
For Oracle versions 12.1.0.2 up to 19.5, the wait event ‘log file parallel write’ includes both the submission of the IO request (io_submit call) and the waiting for the submitted IOs to finish (io_getevents call). This means the waiting time of the event in this case is the total latency of the IO request, including operating system processing.

Starting from version 12.2, only if the multi-tenant option is used, this is changed, and the wait event ‘log file parallel write’ only is shown if the submitted IOs are not available after they are submitted, and thus the logwriter process has to wait for them. If the wait is shown, the time in the event is the actual waiting time for the IOs to finish from the perspective of the logwriter process, not the IO latency time.

Of course if you have multiple log groups on disks with different latencies, this wait event waits for all IOs to finish, which means it times the slowest IO of all submitted IOs.

I was asked some time ago what the Oracle database event ‘TCP socket (KGAS)’ means. This blogpost is a deep dive into what this event times in Oracle database 12.1.0.2.180717.

This event is not normally seen, only when TCP connections are initiated from the database using packages like UTL_TCP, UTL_SMTP and the one used in this article, UTL_HTTP.

A very basic explanation is this event times the time that a database foreground session spends on TCP connection management and communicating over TCP, excluding client and database link (sqlnet) networking. If you trace the system calls, you see that mostly that is working with a (network) socket. Part of the code in the oracle database that is managing that, sits in the kernel code layer kgas, kernel generic (of which I am quite sure, and then my guess:) asynchronous services, which explains the naming of the event.

This is what the Oracle online manual (https://docs.oracle.com/database/121/REFRN/GUID-203ACA60-9912-4493-9B79-EA4CDE89A78D.htm#REFRN00642 – Oracle is notorious for breaking links) says about ‘TCP socket (KGAS)’:

C.3.157 TCP Socket (KGAS)
A session is waiting for an external host to provide requested data over a network socket. The time that this wait event tracks does not indicate a problem, and even a long wait time is not a reason to contact Oracle Support. It naturally takes time for data to flow between hosts over a network, and for the remote aspect of an application to process any request made to it. An application that communicates with a remote host must wait until the data it will read has arrived. In addition, on Microsoft Windows, a separate thread monitors the arrival of traffic. This thread spends most of its life in waits tracked by the TCP Socket (KGAS) wait event.

Wait Time: The total elapsed time for the network connection to be established or for data to arrive from over the network

Parameter Description
P0 For Oracle internal use only. Values 8, 9, and 10 occur within the special thread present on Microsoft Windows; other P0 values occur in normal user sessions.

P1 For Oracle internal use only

Overall, the basic explanation that Oracle provides is mostly correct. I think the text saying to not contact Oracle support is not relevant, but maybe there is a need to relieve Oracle support. In my tests, I found that the full TCP connection lifecycle (creation, usage and removal) is put under this event, for which the text seems to emphasise on waiting for a remote host, which would be the most logical culprit for wait times, but other issues could lead to wait times additionally. This means the wait event itself is not explaining what it is showing, outside of TCP connection management.

The wait time explanation is nearly complete. If it would say something like ‘all TCP connection management and usage’ it would have fully covered it, it now excludes disconnecting and sending, because it explicitly mentions creating the connecting and receiving (waiting for data).

I do not understand what is meant with P0 and P1. I think it is p1 and p2 of the wait event, but naming it P0 and P1 is weird. When looking at the explanation it reads to me ‘we do not wish to explain anything to you’.

So, that means I am going to find this out myself….

If you are interested in this, or do want to write articles like this too, I urge you to replay this on your own system.

First of all, create a small setup which you can use to actually execute UTL_HTTP. The example essentially is taken from father of code examples, Tim Hall/Oracle base. Thank you Tim!
First setup the database to allow a user (in my case ‘ts’) to create the objects and use the network:

grant create sequence to ts;
grant create procedure to ts;
grant create table to ts;
grant alter session to ts;
begin
  dbms_network_acl_admin.create_acl (
    acl => 'anything.xml',
    description => 'allow anything',
    principal => 'TS',
    is_grant => true,
    privilege => 'connect'
  );
end;
begin
  dbms_network_acl_admin.assign_acl (
    acl => 'anything.xml',
    host => '*'
  );
end;

Then connect as the actual user (ts), and create the objects and the procedure that uses UTL_HTTP:

drop table http_clob_test;
create table http_clob_test (
        id number(10),
        url varchar2(255),
        data clob,
        constraint http_clob_test_pk primary key (id)
);
drop sequence http_clob_test_seq;
create sequence http_clob_test_seq;
CREATE OR REPLACE PROCEDURE load_html_from_url (p_url  IN  VARCHAR2) AS
  -- examples by tim hall
  -- https://oracle-base.com/articles/misc/retrieving-html-and-binaries-into-tables-over-http
  l_http_request   UTL_HTTP.req;
  l_http_response  UTL_HTTP.resp;
  l_clob           CLOB;
  l_text           VARCHAR2(32767);
BEGIN
  DBMS_LOB.createtemporary(l_clob, FALSE);

  -- Make a HTTP request and get the response.
  l_http_request  := UTL_HTTP.begin_request(p_url);
  l_http_response := UTL_HTTP.get_response(l_http_request);

  -- Copy the response into the CLOB.
  BEGIN
    LOOP
      UTL_HTTP.read_text(l_http_response, l_text, 32766);
      DBMS_LOB.writeappend (l_clob, LENGTH(l_text), l_text);
    END LOOP;
  EXCEPTION
    WHEN UTL_HTTP.end_of_body THEN
      UTL_HTTP.end_response(l_http_response);
  END;

  -- Insert the data into the table.
  INSERT INTO http_clob_test (id, url, data)
  VALUES (http_clob_test_seq.NEXTVAL, p_url, l_clob);

  -- Relase the resources associated with the temporary LOB.
  DBMS_LOB.freetemporary(l_clob);
EXCEPTION
  WHEN OTHERS THEN
    UTL_HTTP.end_response(l_http_response);
    DBMS_LOB.freetemporary(l_clob);
    RAISE;
END load_html_from_url;
/

The first thing to do is turn on sql_trace at level 8 to include waits:

set serverout on
alter session set events 'sql_trace level 8';
exec load_html_from_url('http://orafun.info/');
alter session set events 'sql_trace off';

If you look at the relevant piece, which means where it shows the wait events, it looks like this:

WAIT #139864521752120: nam='TCP Socket (KGAS)' ela= 128265  =2  =0  =0 obj#=662 tim=86395107497
WAIT #139864521752120: nam='TCP Socket (KGAS)' ela= 395  =5  =0  =0 obj#=662 tim=86395110191
WAIT #139864521752120: nam='TCP Socket (KGAS)' ela= 150  =6  =0  =0 obj#=662 tim=86395111115
WAIT #139864521752120: nam='TCP Socket (KGAS)' ela= 131998  =6  =0  =0 obj#=662 tim=86395243764
WAIT #139864521752120: nam='TCP Socket (KGAS)' ela= 269  =4  =0  =0 obj#=662 tim=86395245182
WAIT #139864521752120: nam='direct path write temp' ela= 4137 file number=201 first dba=257795 block cnt=1 obj#=662 tim=86395250494
WAIT #139864521752120: nam='TCP Socket (KGAS)' ela= 352  =3  =2  =0 obj#=662 tim=86395251294

What is shown here is some quite spectacular differences in elapsed time. Also, the only way to understand what is actually done flagged as ‘TCP Socket (KGAS)’ is the value following ‘ela’, which is the event p1 value.
The pattern is:

- 2
- 5
- 6
- 6
- 4
- 3

It’s relatively simple to guess what a few of these are:

- 2 - connect
- 5 - send
- 6 - \
- 6 - |   receiving ?
- 4 - /
- 3 - close

But if you include the timing, there must be more into play:

- 2 - ela= 128265   connect
- 5 - ela= 395      send
- 6 - ela= 150      \
- 6 - ela= 131998   |   receiving ?
- 4 - ela= 14       /
- 3 - ela= 177     close

2/connect: In order to build up a connection, a tcp connection needs to be created and established. That takes some time.
5/send: Sending from the perspective of a userland process is writing into a socket, which will get send by the operating system independently. This means sending from a userland process normally takes relative little time, because it’s not waiting for actually sending it.
6,4/receive: At this time, this doesn’t make sense to me.
3/close: Closing for a userland process is a simple, swift task. The operating system will keep the port open for some time, etc. but this is not visible for the user land application.

Let’s pull an old trick out of the hat: use strace (system call tracing) with an emphasis on writing on an oracle session that has SQL trace with waits enabled set. This will show the system calls executed, and show exactly when the oracle engine ends a wait, so we can reasonably well establish a relation between wait events and system calls. I say “reasonably well”, because we can’t see when Oracle truly started timing the wait event (kslwtbctx), only the output to trace file as part of ending the wait event (kslwtectx).

The way I done it, is using the command ‘strace -e write=all -p 18513 -o utl_http_test.txt’. Obviously 18513 is the process ID of the database foreground process. The results of the strace are in utl_http_test.txt.

Now open utl_http_test.txt and search for KGAS. The full output is way too much text, let me show some of the output which I think is noteworthy. Again: this is selective, partial output.
I do maintain the order in which the calls are visible.

1. TCP Socket (KGAS) p1=2, earlier annotated as ‘connect’

-- try to find a socket that has been created by nscd (name server caching deamon)
-- two times?
--
socket(AF_LOCAL, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 9
connect(9, {sa_family=AF_LOCAL, sun_path="/var/run/nscd/socket"}, 110) = -1 ENOENT (No such file or directory)
close(9)                                = 0
socket(AF_LOCAL, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 9
connect(9, {sa_family=AF_LOCAL, sun_path="/var/run/nscd/socket"}, 110) = -1 ENOENT (No such file or directory)
close(9)                                = 0
--
-- obtain file status of resolv.conf (hostname resolving configuration file)
--
stat("/etc/resolv.conf", {st_mode=S_IFREG|0644, st_size=93, ...}) = 0
--
-- open and read host.conf (another hostname resolving configuration file)
--
open("/etc/host.conf", O_RDONLY|O_CLOEXEC) = 9
fstat(9, {st_mode=S_IFREG|0644, st_size=9, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f34bf377000
read(9, "multi on\n", 4096)             = 9
read(9, "", 4096)                       = 0
close(9)                                = 0
--
-- open and read resolv.conf (hostname resolving configuration)
--
open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 9
fstat(9, {st_mode=S_IFREG|0644, st_size=93, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f34bf377000
read(9, "# Generated by NetworkManager\nse"..., 4096) = 93
read(9, "", 4096)                       = 0
close(9)                                = 0
--
-- open /etc/hosts (ip address to hostname mapping locally)
--
open("/etc/hosts", O_RDONLY|O_CLOEXEC)  = 9
fstat(9, {st_mode=S_IFREG|0644, st_size=200, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f34bf377000
read(9, "127.0.0.1   localhost localhost."..., 4096) = 200
read(9, "", 4096)                       = 0
close(9)
--
-- at this point two dynamic loadable libraries are read: libnss_dns.so.2 and libresolv.so.2
--
-- this is the DNS lookup of orafun.info
-- again, this is done twice, just like the use of /var/run/nscd/socket above?
--
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 9
connect(9, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("10.0.2.3")}, 16) = 0
poll([{fd=9, events=POLLOUT}], 1, 0)    = 1 ([{fd=9, revents=POLLOUT}])
sendto(9, "=#\1\0\0\1\0\0\0\0\0\0\6orafun\4info\0\0\1\0\1", 29, MSG_NOSIGNAL, NULL, 0) = 29
 | 00000  3d 23 01 00 00 01 00 00  00 00 00 00 06 6f 72 61  =#...........ora |
 | 00010  66 75 6e 04 69 6e 66 6f  00 00 01 00 01           fun.info.....    |
poll([{fd=9, events=POLLIN}], 1, 5000)  = 1 ([{fd=9, revents=POLLIN}])
ioctl(9, FIONREAD, [45])                = 0
recvfrom(9, "=#\201\200\0\1\0\1\0\0\0\0\6orafun\4info\0\0\1\0\1\300\f\0"..., 2048, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("10.0.2.3")}, [16]) = 45
close(9)                                = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 9
connect(9, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("10.0.2.3")}, 16) = 0
poll([{fd=9, events=POLLOUT}], 1, 4971) = 1 ([{fd=9, revents=POLLOUT}])
sendto(9, "o=\1\0\0\1\0\0\0\0\0\0\6orafun\4info\0\0\34\0\1", 29, MSG_NOSIGNAL, NULL, 0) = 29
 | 00000  6f 3d 01 00 00 01 00 00  00 00 00 00 06 6f 72 61  o=...........ora |
 | 00010  66 75 6e 04 69 6e 66 6f  00 00 1c 00 01           fun.info.....    |
poll([{fd=9, events=POLLIN}], 1, 4970)  = 1 ([{fd=9, revents=POLLIN}])
ioctl(9, FIONREAD, [109])               = 0
recvfrom(9, "o=\201\200\0\1\0\0\0\1\0\0\6orafun\4info\0\0\34\0\1\300\f\0"..., 65536, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("10.0.2.3")}, [16]) = 109
close(9)                                = 0
--
-- an epoll is created at file descriptor 9 (epoll: I/O event notification facility)
--
epoll_create(82)                        = 9
fcntl(9, F_SETFD, FD_CLOEXEC)           = 0
--
-- an IPV6 socket is created at file descriptor 11, 
-- bound to the IPV6 equivalent of localhost (::1),
-- destination port 0, source port 63257,
-- and is NOT connected.
--
socket(AF_INET6, SOCK_DGRAM, IPPROTO_IP) = 11
bind(11, {sa_family=AF_INET6, sin6_port=htons(0), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = 0
getsockname(11, {sa_family=AF_INET6, sin6_port=htons(63257), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, [28]) = 0
getpeername(11, 0x7ffdea6ba0f8, 0x7ffdea6ba1c8) = -1 ENOTCONN (Transport endpoint is not connected)
getsockopt(11, SOL_SOCKET, SO_SNDBUF, [262144], [4]) = 0
getsockopt(11, SOL_SOCKET, SO_RCVBUF, [262144], [4]) = 0
fcntl(11, F_SETFD, FD_CLOEXEC)          = 0
fcntl(11, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
--
-- File descriptor 11 is added to the epoll at file descriptor 9.
--
epoll_ctl(9, EPOLL_CTL_ADD, 11, {EPOLLIN, {u32=3110993336, u64=139864426020280}}) = 0
--
-- A connection is created to the true destination (orafun.info/18.218.92.122).
-- This connection gets file descriptor 12.
-- Destination port 80 (http), source port 11751.
--
socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 12
fcntl(12, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
connect(12, {sa_family=AF_INET, sin_port=htons(80), sin_addr=inet_addr("18.218.92.122")}, 16) = -1 EINPROGRESS (Operation now in progress)
times(NULL)                             = 438106227
mmap(NULL, 786432, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f34b959b000
poll([{fd=12, events=POLLOUT}], 1, 60000) = 1 ([{fd=12, revents=POLLOUT}])
getsockopt(12, SOL_SOCKET, SO_ERROR, [0], [4]) = 0
fcntl(12, F_GETFL)                      = 0x802 (flags O_RDWR|O_NONBLOCK)
fcntl(12, F_SETFL, O_RDWR)              = 0
getsockname(12, {sa_family=AF_INET, sin_port=htons(11751), sin_addr=inet_addr("10.0.2.15")}, [16]) = 0
getsockopt(12, SOL_SOCKET, SO_SNDBUF, [87040], [4]) = 0
getsockopt(12, SOL_SOCKET, SO_RCVBUF, [374400], [4]) = 0
setsockopt(12, SOL_TCP, TCP_NODELAY, [1], 4) = 0
fcntl(12, F_SETFD, FD_CLOEXEC)          = 0
--
-- And this is the wait event written by the process: TCP Socket (KGAS), p1=2
--
write(7, "WAIT #139864521752120: nam='TCP "..., 95) = 95
 | 00000  57 41 49 54 20 23 31 33  39 38 36 34 35 32 31 37  WAIT #1398645217 |
 | 00010  35 32 31 32 30 3a 20 6e  61 6d 3d 27 54 43 50 20  52120: nam='TCP  |
 | 00020  53 6f 63 6b 65 74 20 28  4b 47 41 53 29 27 20 65  Socket (KGAS)' e |
 | 00030  6c 61 3d 20 31 32 38 32  36 35 20 20 3d 32 20 20  la= 128265  =2   |
 | 00040  3d 30 20 20 3d 30 20 6f  62 6a 23 3d 36 36 32 20  =0  =0 obj#=662  |
 | 00050  74 69 6d 3d 38 36 33 39  35 31 30 37 34 39 37     tim=86395107497  |

So yes, I am not sure if all of this is in the wait event, but there is a lot of stuff happening to build a connection to the remote server.

In order to find out why the lookup which is tried via the NSCD socket at the beginning, and later via DNS, is done twice, I ran the same procedure again and used tcpdump to look at the actual network traffic. This explained a lot:

# tcpdump -n host 10.0.2.3 and port 53
09:14:02.923389 IP 10.0.2.15.16819 > 10.0.2.3.domain: 15651+ A? orafun.info. (29)
09:14:02.948791 IP 10.0.2.3.domain > 10.0.2.15.16819: 15651 1/0/0 A 18.218.92.122 (45)
09:14:02.952304 IP 10.0.2.15.54590 > 10.0.2.3.domain: 28477+ AAAA? orafun.info. (29)
09:14:02.979534 IP 10.0.2.3.domain > 10.0.2.15.54590: 28477 0/1/0 (109)

In other words: first a DNS A record is requested (TCPv4 DNS name lookup), which results in the IPv4 ip address, then a DNS AAAA record is requested (TCPv6 DNS name lookup), which resulted in no ip address. In other words: orafun.info only has an IPv4 ip address. So the two lookups actually do have a function.

2. TCP Socket (KGAS) p1=5, earlier annotated as ‘send’
These are the systemcalls that are visible and quite probably related to the send wait event:

--
-- file descriptor 12 holding the connection to the destination server is added to the epoll at file descriptor 9
--
epoll_ctl(9, EPOLL_CTL_ADD, 12, {EPOLLIN, {u32=3110998864, u64=139864426025808}}) = 0
--
-- Then the http get request is sent to the destination server at its normal file descriptor, 12.
--
write(12, "GET / HTTP/1.1\r\nHost: orafun.inf"..., 56) = 56
 | 00000  47 45 54 20 2f 20 48 54  54 50 2f 31 2e 31 0d 0a  GET / HTTP/1.1.. |
 | 00010  48 6f 73 74 3a 20 6f 72  61 66 75 6e 2e 69 6e 66  Host: orafun.inf |
 | 00020  6f 0d 0a 43 6f 6e 6e 65  63 74 69 6f 6e 3a 20 63  o..Connection: c |
 | 00030  6c 6f 73 65 0d 0a 0d 0a                           lose....         |
--
-- And this is the wait event written by the process: TCP Socket (KGAS), p1=5
--
write(7, "WAIT #139864521752120: nam='TCP "..., 92) = 92
 | 00000  57 41 49 54 20 23 31 33  39 38 36 34 35 32 31 37  WAIT #1398645217 |
 | 00010  35 32 31 32 30 3a 20 6e  61 6d 3d 27 54 43 50 20  52120: nam='TCP  |
 | 00020  53 6f 63 6b 65 74 20 28  4b 47 41 53 29 27 20 65  Socket (KGAS)' e |
 | 00030  6c 61 3d 20 33 39 35 20  20 3d 35 20 20 3d 30 20  la= 395  =5  =0  |
 | 00040  20 3d 30 20 6f 62 6a 23  3d 36 36 32 20 74 69 6d   =0 obj#=662 tim |
 | 00050  3d 38 36 33 39 35 31 31  30 31 39 31              =86395110191     |

3. TCP Socket (KGAS) p1=6, earlier annotated as ‘receive’

--
-- Calling epoll_wait with timeout set to 0, so it doesn't block.
--
epoll_wait(9, [], 82, 0)                = 0
--
-- And this is the wait event written by the process: TCP Socket (KGAS), p1=6
-- 
write(7, "WAIT #139864521752120: nam='TCP "..., 92) = 92
 | 00000  57 41 49 54 20 23 31 33  39 38 36 34 35 32 31 37  WAIT #1398645217 |
 | 00010  35 32 31 32 30 3a 20 6e  61 6d 3d 27 54 43 50 20  52120: nam='TCP  |
 | 00020  53 6f 63 6b 65 74 20 28  4b 47 41 53 29 27 20 65  Socket (KGAS)' e |
 | 00030  6c 61 3d 20 31 35 30 20  20 3d 36 20 20 3d 30 20  la= 150  =6  =0  |
 | 00040  20 3d 30 20 6f 62 6a 23  3d 36 36 32 20 74 69 6d   =0 obj#=662 tim |
 | 00050  3d 38 36 33 39 35 31 31  31 31 31 35              =86395111115     |

I have been thinking a lot about this seemingly weird call. It calls epoll_wait, but indicates it doesn’t want to wait (timeout=0), and even if epol_wait would have returned anything, indicated by a return code > 0, the epoll_event pointer is not set (indicated by []). The epoll file descriptor is used, but the only working file descriptor in the epoll is file descriptor 12, which has just been sent a http GET command, so the functionality of epoll is used.

This doesn’t make sense, unless you think about the asynchronous IO implementation of Oracle (see a lot of my earlier investigations), for which (in the case of asynchronous IO) io_getevents was called in a similar matter, timeout set to 0, to be able to do more requests while earlier IO requests are executed by the kernel. So my current theory here is that if multiple requests are happening, this mechanism provides a way to handle them.

If you have a simple single request, like in my case, this systemcall seems redundant. And because it queries the epoll file descriptor right after the request, it returns zero events, because there hardly has been any time after sending the http GET request.

4. Second TCP Socket (KGAS) p1=6, earlier annotated as ‘receive’

--
-- Calling epoll_wait with timeout set to 30000 (milliseconds).
--
epoll_wait(9, [{EPOLLIN, {u32=3110998864, u64=139864426025808}}], 82, 30000) = 1
--
-- And this is the second wait event written by the process: TCP Socket (KGAS), p1=6
-- 
write(7, "WAIT #139864521752120: nam='TCP "..., 95) = 95
 | 00000  57 41 49 54 20 23 31 33  39 38 36 34 35 32 31 37  WAIT #1398645217 |
 | 00010  35 32 31 32 30 3a 20 6e  61 6d 3d 27 54 43 50 20  52120: nam='TCP  |
 | 00020  53 6f 63 6b 65 74 20 28  4b 47 41 53 29 27 20 65  Socket (KGAS)' e |
 | 00030  6c 61 3d 20 31 33 31 39  39 38 20 20 3d 36 20 20  la= 131998  =6   |
 | 00040  3d 30 20 20 3d 30 20 6f  62 6a 23 3d 36 36 32 20  =0  =0 obj#=662  |
 | 00050  74 69 6d 3d 38 36 33 39  35 32 34 33 37 36 34     tim=86395243764  |

This is the second time epoll_wait is called, and this one is blocking, because timeout has been set to 30000 milliseconds. If you look at the ela time, this took some time, and this now makes perfect sense: this system calls waits for an event to become available in the epoll, so it waits for the response of the remote http server. Please mind this call just notifies the userland process that the response is ready, the received data yet has to be read.

5. TCP Socket (KGAS) p1=4, earlier annotated as ‘receive’

--
-- At this point we know there is a response. First the original file descriptor is removed from the epoll:
--
epoll_ctl(9, EPOLL_CTL_DEL, 12, 0x7ffdea6b9710) = 0
--
-- The the response is read from file descriptor 12:
--
read(12, "HTTP/1.1 200 OK\r\nServer: nginx/1"..., 4096) = 2687
--
-- Then file descriptor 12 is added to the epoll again.
--
epoll_ctl(9, EPOLL_CTL_ADD, 12, {EPOLLIN, {u32=3110998864, u64=139864426025808}}) = 0
--
-- And a wait event written by the process: TCP Socket (KGAS), p1=4
--
write(7, "WAIT #139864521752120: nam='TCP "..., 92) = 92
 | 00000  57 41 49 54 20 23 31 33  39 38 36 34 35 32 31 37  WAIT #1398645217 |
 | 00010  35 32 31 32 30 3a 20 6e  61 6d 3d 27 54 43 50 20  52120: nam='TCP  |
 | 00020  53 6f 63 6b 65 74 20 28  4b 47 41 53 29 27 20 65  Socket (KGAS)' e |
 | 00030  6c 61 3d 20 32 36 39 20  20 3d 34 20 20 3d 30 20  la= 269  =4  =0  |
 | 00040  20 3d 30 20 6f 62 6a 23  3d 36 36 32 20 74 69 6d   =0 obj#=662 tim |
 | 00050  3d 38 36 33 39 35 32 34  35 31 38 32              =86395245182     |

So, what p1 set to 4 actually means, is that once the connection did return data, which is checked using epoll, and visible with p1 set to 6, it is read into the process. This is also the reason this takes very little time, this is the time to read data from kernelspace to user space, and to manage the connection’s file descriptor. It is taken off the epoll in order not to disturb it, and it is added again because there could be another request.

6. TCP Socket (KGAS) p1=3, earlier annotated as ‘close’

--
-- file descriptor 12 removed from the epoll
--
epoll_ctl(9, EPOLL_CTL_DEL, 12, 0x7ffdea6bac20) = 0
--
-- file descriptor 12 is closed, closing the network connection
--
close(12)                               = 0
--
-- And a wait event written by the process: TCP Socket (KGAS), p1=3
--
write(7, "WAIT #139864521752120: nam='TCP "..., 92) = 92
 | 00000  57 41 49 54 20 23 31 33  39 38 36 34 35 32 31 37  WAIT #1398645217 |
 | 00010  35 32 31 32 30 3a 20 6e  61 6d 3d 27 54 43 50 20  52120: nam='TCP  |
 | 00020  53 6f 63 6b 65 74 20 28  4b 47 41 53 29 27 20 65  Socket (KGAS)' e |
 | 00030  6c 61 3d 20 33 35 32 20  20 3d 33 20 20 3d 32 20  la= 352  =3  =2  |
 | 00040  20 3d 30 20 6f 62 6a 23  3d 36 36 32 20 74 69 6d   =0 obj#=662 tim |
 | 00050  3d 38 36 33 39 35 32 35  31 32 39 34              =86395251294     |

I don’t think this part holds any surprises. The network file descriptor is first removed from the epoll, and then it is closed, ending the TCP connection that was setup to perform a http request (in my case, I didn’t test, but I believe you will see the same with for example a SMTP connection, or any other type of TCP connection).

Summary

The basic message of this article is not surprising, nor does it conflict with current knowledge. Whenever you see a wait event ‘TCP Socket (KGAS)’, it means a foreground process is performing TCP networking via PLSQL. This wait event is a single event for creating, sending, receiving and closing a connection.

The true information of this article is how you can use the p1 value of the event to learn what actually the foreground is doing, and thus should give you more information to troubleshoot in the case of long waiting times.

TCP Socket (KGAS) p1 values:
1 - ?
2 - perform DNS lookup and create connection
3 - close connection
4 - copy TCP response into process space
5 - send request
6 - wait for TCP response to become available
7 - ?
8 - ? \
9 - ? | According to documentation, windows only in a 'special thread'.
10- ? /

This post was triggered upon rereading a blogpost by Mike Dietrich called databases need patched minimum april 2019. Mike’s blogpost makes it clear this is about databases that are connected using database links, and that:
– Newer databases do not need additional patching for this issue (11.2.0.4, 12.1.0.2, 12.2 and newer).
– Recent PSU patches contain a fix for certain older versions (11.1.0.7, 11.2.0.3 and 12.1.0.1).
– This means versions 11.2.0.2 and earlier 11.2 versions, 11.1.0.6 and earlier and anything at version 10 or earlier can not be fixed and thus are affected.

But what is the actual issue?

The first link in the article: Recommended patches and actions for Oracle databases versions 12.1.0.1, 11.2.0.3 and earlier – before June 2019 (Doc ID 2361478.1) provides essentially the same information as Mike’s blogpost, however it additionaly mentions that the interoperability of database clients with database servers is not affected.

Mike’s article mentions the following:
The alert refers to an SCN issue which came up a while ago. The system change number (SCN) is a logical, internal timestamp used by the Oracle Database. SCNs order events that occur within the database, which is necessary to satisfy the ACID properties of a transaction. The database uses SCNs to query and track changes.

So I guess it has something to do with SCNs. Most of the links are about SCNs. The MOS article that is most descriptive is: System Change Number (SCN), Headroom, Security and Patch Information (Doc ID 1376995.1).

This article describes a lot of details:
– SCNs are necessary for the database to keep changes organised. I got an article that explains that SCNs are not unique to a transaction, but are “just” granular enough to keep changes organised.
– SCNs are an ever increasing number. SCNs are never decreased!
– SCNs have a hard limit, which version specific, and is based on the number of bits for the number:
– – The general limit is 2^48.
– – From version 12.2 on, with compatibility set to 12.2 or higher, the limit is 2^63.
– SCNs have a per second increasing soft limit, called ‘RSL’ or ‘reasonable SCN limit’, which is version specific:
– – The general soft limit is 16384 (16k) SCNs per second calculated by the number of second from Januari 1st, 1988 times 16384.
– – From version 12.2 on, with compatibility set to 12.2 or higher, the limit is 98304 (96k) SCNs per second calculated by the number of seconds from Januari 1st, 2008 times 98304.
– The RSL can not be exceeded, if a database session tries to go beyond the soft limit, an ORA 600 [2252] is raised and the transaction is rolled back. This will not corrupt data (but obviously the data in the transaction is not applied).
– The difference between the current SCN and the RSL SCN is known as ‘SCN headroom’.
– There have been bugs that can increase SCNs at a higher rate, decreasing the SCN headroom or even reaching the soft limit, but these have all been solved in the Januari 2012 CPU/PSU/patch bundles.
– When databases communicate which each other via a database link, the SCNs of both databases are synchronised by picking the highest of the two.

But it doesn’t really explain why patches must be applied before June 2019. However, another article is more concrete on that: Recommended patching and actions for Oracle database versions 12.1.0.1, 11.2.0.3 and earlier – before June 2019 (Doc ID 2335265.1). The first interesting thing mentioned is:

3. What is the change introduced by the patches listed above?
These patches increase the database’s current maximum SCN (system change number) limit.
At any point in time, the Oracle Database calculates a “not to exceed” limit for the number of SCNs a database can have used, based on the number of seconds elapsed since 1988.

So, this means the patched database have a change (increase) in the RSL.

These recommended patches enable the databases to allow for a higher current maximum SCN limit. The rate at which this limit is calculated can be referred to as the “SCN rate” and these patches help allow higher SCN rates to enable databases to support many times higher transaction rates than earlier releases.

And this means the RSL is increased from the 16k per second since Januari 1988 to the 96k per second since Januari 2008.

Please note that the patches only increase the max limit but the current SCN is not impacted. So, if all your databases don’t have any major change in transaction rate, the current SCN would still remain below the current maximum SCN limit and database links between newer (or patched) and unpatched databases would continue to work. The patches provide the safety measure to ensure that you don’t have any issue with dblinks independent of any possible future change in your transaction rate.

With the patches applied, this change in current maximum SCN limit will happen automatically starting 23rd June 2019.

That is important information! So with the patch applied to some databases and not applied to some other databases and patched and non-patched databases have database links between them, everything should remain working. This is true for any database at this moment, because the change of the limit will happen on the 23rd of June 2019, which at the time of writing is in the future.

Once the change of limit has happened at the 23rd of June 2019, database links between older, non-patched versions of the database and newer or patched versions of the database could be affected if after the 23rd of June 2019 the SCN rate is increased in a newer or patched database and it runs beyond the headroom available in databases with the 16k rate, because the earlier mentioned SCN synchronisation then will fail because it’s beyond the 16k rate database headroom meaning it can not synchronise with the newer dataebase.

So the problem we are talking about here is two databases talking with each other over a database link, which have a different RSL, which could lead to a situation that one database is at an SCN number which is too high for the other older or non-patched database, meaning the communication will fail, which will persist until the older or non-patched databases is able to reach the SCN number of the newer database over time, if that is possible at all.

A thing that is not clear to me at this point: the documentation to me seems to suggest that Oracle version 12.2 with compatibility set to 12.2 or higher versions already allows the higher number of SCNs per second. (the above point: – – From version 12.2 on, with compatibility set to 12.2 or higher, the limit is 98304 (96k) SCNs per second calculated by the number of seconds from Januari 1st, 2008) If that is true, the issue that is warned for could potentially already happen!

Luckily, there is are ways to investigate this:

The reported “newer” versions and the versions that are patched for the rate change have an SGA variable “kcmscnc_” that lists the current SCN rate of the database. There are 3 values that I have seen for “kcmscnc_”:
1: This is the rate of 16k per second since Januari 1st 1988.
2: ?
3: This is the rate of 96k per second since Januari 1st 2008.

SQL> select ksmfsnam, ksmfsadr, ksmfssiz from x$ksmfsv where ksmfsnam like 'kcmscnc_';
KSMFSNAM							 KSMFSADR	    KSMFSSIZ
---------------------------------------------------------------- ---------------- ----------
kcmscnc_							 000000006001579C	   4
SQL> oradebug peek 0x6001579C 4
[06001579C, 0600157A0) = 00000001

So, this databases is capable of switching (because kcmscnc_ exists), and is currently running with the 16k per second threshold.

In fact, I tested this on 11.2.0.4, 12.1.0.2 and 18.3.0.0, all these version report currently (currently is before June 2019) “1” or “scheme 1”. So the above mentioned rate of 96k for 12.2 and above does currently not apply for the soft limit or SCN headroom calculation for any database, including 12.2 and 18.3.

There is a less “hardcore” way to obtain this information, via the DBMS_SCN. This package equally to the “kcmscnc_” variable only exists if the database is of a version or patch version high enough to switch:

declare
  v_rsl number;
  v_headroom_in_scn number;
  v_headroom_in_sec number;
  v_cur_scn_compat number;
  v_max_scn_compat number;
begin
  dbms_scn.getcurrentscnparams(v_rsl, v_headroom_in_scn, v_headroom_in_sec, v_cur_scn_compat, v_max_scn_compat);
  dbms_output.put_line('reasonable scn limit (soft limit): '||to_char(v_rsl,'999,999,999,999,999,999'));
  dbms_output.put_line('headroom in scn                  : '||to_char(v_headroom_in_scn,'999,999,999,999,999,999'));
  dbms_output.put_line('headroom in sec                  : '||v_headroom_in_sec);
  dbms_output.put_line('current scn compatibility scheme : '||v_cur_scn_compat);
  dbms_output.put_line('max scn compatibility scheme     : '||v_max_scn_compat);
end;
/

SQL> /
reasonable scn limit (soft limit):	 16,104,861,483,008
headroom in scn 		 :	 16,104,860,802,471
headroom in sec 		 : 982962695
current scn compatibility scheme : 1
max scn compatibility scheme	 : 3

PL/SQL procedure successfully completed.

This is executed in a version 18.3.0.0 database. So the conclusion here is that currently all versions up to 18.3.0.0 are still compatible, because they all use the same SCN limit per second, which is referred to as ‘scheme 1’. However, on June 23, 2019 newer versions of the database will switch to a new scheme, which is referred to as ‘scheme 3’, which allows an more headroom.

First of all, I hope a lot of databases out there have enough headroom in the first place, and a modest rate of SCNs it is using per second, which means it doesn’t advance into the RSL. In such a case, when you got older versions that can not switch to scheme 3 and newer versions that will, I can see no reason to worry at all.

The second case is when your database is close to running out of headroom currently in scheme 1, and the SCN rate in the database is also close the limit, so you truly should worry when your database switches to scheme 3, it might actually run over the 16k per second limit, and when it does that long enough to run over RSL of scheme 1, communication over a database link between the current scheme 3 database with a scheme 1 database will be disrupted until the scheme 3 database SCN is low enough again to fit the scheme 1 RSL. If the SCN rate persists in the scheme 3 database, communication will be impossible between the scheme 1 and a scheme 3 database.

So, at this point it should be clear that the most important statistic for determining issues between scheme 1 and after June 2019 scheme 3 databases is the current headroom in your databases. For all of the databases involved that will have a database link between a scheme 1 and a scheme 3 database, you should investigate their SCN number and SCN rate. If some of these databases have run into the soft limit ora-600, ora 600 [2252], these are potential candidates for running over the soft limit when they switch to scheme 3.

However, the most important statistic is to see how close the current SCN is to the scheme 1 soft limit. This can be done using the following SQL (this SQL does not need a newer version of the database, and is tested with version 11.2.0.2 and higher):

col "RSL scheme 1" format 9,999,999,999,999,999
col "current value" format 9,999,999,999,999,999
select dbms_flashback.get_system_change_number "current value",
       ((((to_number(to_char(sysdate,'YYYY'))-1988)*12*31*24*60*60) +
       ((to_number(to_char(sysdate,'MM'))-1)*31*24*60*60) +
       (((to_number(to_char(sysdate,'DD'))-1))*24*60*60) +
       (to_number(to_char(sysdate,'HH24'))*60*60) +
       (to_number(to_char(sysdate,'MI'))*60) +
       (to_number(to_char(sysdate,'SS')))) * (16*1024)) "RSL scheme 1",
       round(dbms_flashback.get_system_change_number/((((to_number(to_char(sysdate,'YYYY'))-1988)*12*31*24*60*60) +
       ((to_number(to_char(sysdate,'MM'))-1)*31*24*60*60) +
       (((to_number(to_char(sysdate,'DD'))-1))*24*60*60) +
       (to_number(to_char(sysdate,'HH24'))*60*60) +
       (to_number(to_char(sysdate,'MI'))*60) +
       (to_number(to_char(sysdate,'SS')))) * (16*1024))*100,5) "% to RSL scheme 1"
from dual;

If a database is close the RSL (roughly speaking beyond 90%-95%), the next thing to do is measure if the database keeps on using SCNs and keeps on being close to the RSL. If that is true, an additional increase in SCN usage could in the current situation using scheme 1 lead to an ora-600 [2252], but if that database has switched to scheme 3 after June 2019, there will not be anything keeping that database from going beyond an SCN number that will exceed the RSL of scheme 1, which will then cause issues if that database has a database link with a scheme 1 database.

Is there anything you can do if you suspect or know a database will go over the scheme 1 RSL? Purely for this issue, the obvious solution would be to make sure you are on a version that will switch to scheme 3 on June 2019, so at least after June 2019 it will not run into ora-600 [2252].

However, if such a scheme 3 database needs to connect to an older scheme 1 database, you have two choices:
1. Potentially run over the the scheme 1 limit and disrupt the database link communication.
2. Stop a newer database from switching to scheme 3, potentially disrupt changes in the current database, but it guarantees the database link will always work.

To look into the switch to scheme 3, which oracle calls ‘auto rollover’, the following SQL can be used:

declare
  v_autorollover_date date;
  v_target_compat number;
  v_is_enabled boolean;
begin
  dbms_scn.getscnautorolloverparams(v_autorollover_date, v_target_compat, v_is_enabled);
  dbms_output.put_line('auto rollover date      : '||to_char(v_autorollover_date,'YYYY-MM-DD'));
  dbms_output.put_line('target scheme	        : '||v_target_compat);
  dbms_output.put_line('rollover enabled (1=yes): '||sys.diutil.bool_to_int(v_is_enabled));
end;
/
SQL> /
auto rollover date	: 2019-06-23
target scheme		: 3
rollover enabled (1=yes): 1

PL/SQL procedure successfully completed.

If you want to prevent a database from rolling over to scheme 3, the procedure dbms_scn.disableautorollover can be used:

exec dbms_scn.disableautorollover;

Obviously, the procedure enableautorollover does the opposite. Please mind to contact Oracle support before doing this with your production database, this is an undocumented procedure at this time.

Also mind that if you create a new database after June 23, 2019, with a new or patched version that can switch to scheme 3, it will probably be running scheme 3 by default. If you want to be absolutely sure it will not exceed the scheme 1 limit, you can revert it to scheme 1 manually using the alter database set scn compatibility N command in mount mode:

SQL> startup mount;
ORACLE instance started.

Total System Global Area 1048574496 bytes
Fixed Size		    8665632 bytes
Variable Size		  281018368 bytes
Database Buffers	  616562688 bytes
Redo Buffers		  142327808 bytes
Database mounted.
SQL> alter database set scn compatibility 1;

Database altered.

SQL> alter database open;

Database altered.

For this too I would strongly advise to contact Oracle support first. The purpose of this blogpost is to define the problem, show all the technical details that have to do with it, and show all the tools that are part of it. There is in no way anything in this article to tell you what you should do, it just shows everything that surrounds the switch to scheme 3 in June 2019.

Another view that might be beneficial is x$kcmscn. This view seems to be created to help looking if a scheme 3 database can connect to a scheme 1 database:

col cur_max_scn format 999,999,999,999,999
col pre_11_2_0_2_cur_max_scn format 999,999,999,999,999
select * from x$ksmscn;
ADDR		       INDX    INST_ID	   CON_ID CUR_MAX_RATE	  CUR_SCN
---------------- ---------- ---------- ---------- ------------ ----------
	 CUR_MAX_SCN DIFF_IN_DAYS DIFF_IN_HOURS PRE_11_2_0_2_CUR_MAX_SCN
-------------------- ------------ ------------- ------------------------
PRE_11_2_0_2_DIFF_IN_DAYS
-------------------------
00007F773DEDAE10	  0	     1		0	     0	   800918
  16,108,830,064,640	    11379	 273112       16,108,830,064,640
		186444783

Because currently (before June 2019) every database by default will be in scheme 1, the cur_max_scn and pre_11_2_0_2_cur_max_scn are identical. I even believe the column naming is wrong, the first version that can switch if it is patched to a high enough PSU version is 11.2.0.3, I do believe the column name is suggesting scheme 1 databases are databases of a version lower than 11.2.0.2, not including 11.2.0.2.

Conclusion.
I think there’s been a lot of fuzz for something that in most cases is not an issue. This article is supposed to give you all the knowledge and the tools to determine how it looks like in your situation.

This might be an issue if you happen to have one or more databases that are high on SCN numbering, and continues to take a lot of SCN numbers, and will be converted to a scheme 3 database on June 29, 2019 and is suspected to increase on taking SCN numbers for whatever reason AND it has a database link to a scheme 1 database that remains scheme 1. That’s a lot of ifs.

On the other hand you only need one database to be high in SCN numbering which continues to take a lot of SCNs keeping it close to the soft limit, which will propagate its SCN to other databases if it is linked, or the required properties of the problem spread out over multiple linked databases.

Again, I do not advise anything in this article, the purpose here is to provide all the details that surround it so you can make the best decision for yourself.

Starting from Oracle 12, in a default configured database, there are more log writer processes than the well known ‘LGWR’ process itself, which are the ‘LGnn’ processes:

$ ps -ef | grep test | grep lg
oracle   18048     1  0 12:50 ?        00:00:13 ora_lgwr_test
oracle   18052     1  0 12:50 ?        00:00:06 ora_lg00_test
oracle   18056     1  0 12:50 ?        00:00:00 ora_lg01_test

These are the log writer worker processes, for which the minimal amount is equal to the amount public redo strands. Worker processes are assigned to a group, and the group is assigned to a public redo strand. The amount of worker processes in the group is dependent on the undocumented parameter “_max_log_write_parallelism”, which is one by default.

The actual usage of the worker processes is dependent in the first place on the value of the undocumented parameter “_use_single_log_writer”, for which the default value is ‘ADAPTIVE’, which means it’s switching automatically between ‘single log writer mode’, which is the traditional way of the LGWR process handling everything that the log writer functionality needs to do, and the ‘scalable log writer mode’, which means the log writer functionality is presumably using the log writer worker processes.

Other values for “_use_single_log_writer” are ‘TRUE’ to set ‘single log writer mode’, or ‘FALSE’ to set ‘scalable log writer mode’ fixed.

I assume most readers of this blog will know that the master log writer idle work cycle is sleeping on a semaphore (semtimedop()) under the wait event ‘rdbms ipc message’ for 3 seconds, then performs some “housekeeping”, after which it’ll sleep again repeating the small cycle of sleeping and housekeeping. For the log writer worker processes, this looks different if you look at the wait event information of the log writer worker processes:

135,59779,@1    14346                    DEDICATED oracle@memory-presentation.local (LGWR)	    time:1909.44ms,event:rdbms ipc message,seq#:292
48,34282,@1     14350                    DEDICATED oracle@memory-presentation.local (LG00)	    time:57561.85ms,event:LGWR worker group idle,seq#:150
136,24935,@1    14354                    DEDICATED oracle@memory-presentation.local (LG01)	    time:112785.66ms,event:LGWR worker group idle,seq#:74

The master log writer process (LGWR) has been sleeping for 1.9s when I queried the database, and it will sleep for 3 seconds, and then do some work and sleep again. However, the log writer worker processes have been sleeping for much longer: LG00 for 57.6s and LG01 for 112.8s, and the event is different: ‘LGWR worker group idle’. How is this implemented? Let’s look!

$ strace -p $(pgrep lg01)
strace: Process 14354 attached
semtimedop(360448, [{27, -1, 0}], 1, {3, 0}) = -1 EAGAIN (Resource temporarily unavailable)
semtimedop(360448, [{27, -1, 0}], 1, {3, 0}) = -1 EAGAIN (Resource temporarily unavailable)

I used strace on the LG01 process, and it’s still doing the same as most idle background processes are doing: sleeping on a semaphore for 3 seconds. But, it does not end its wait like LGWR does, the event the log writer worker processes are waiting in keeps on being timed.

Using a pin tools debugtrace shows the following:

 | | < semtimedop+0x000000000023 returns: 0xffffffffffffffff
 | | > __errno_location(0x38000, 0x7ffce278c328, ...)
 | | | > fthread_self(0x38000, 0x7ffce278c328, ...)
 | | | < fthread_self+0x000000000024 returns: 0
 | | < __errno_location+0x000000000010 returns: 0x7f7e930a26a0
 | < sskgpwwait+0x00000000014e returns: 0
 < skgpwwait+0x0000000000e0 returns: 0
 > ksuSdiInProgress(0x19e80, 0x19e80, ...)
 < ksuSdiInProgress+0x000000000035 returns: 0
 > sltrgftime64(0x19e80, 0x19e80, ...)
 | > clock_gettime@plt(0x1, 0x7ffce278c3a0, ...)
 | | > clock_gettime(0x1, 0x7ffce278c3a0, ...)
 | | < clock_gettime+0x000000000069 returns: 0
 | < clock_gettime+0x00000000003a returns: 0
 < sltrgftime64+0x00000000004c returns: 0x19c253f3ff
 > kslwo_getcbk(0xa2, 0xd80fa62, ...)
 < kslwo_getcbk+0x000000000017 returns: 0
 > kgslwait_last_waitctx_time_waited_usecs(0x7f7e930a29a0, 0x6dfd01c0, ...)
 < kgslwait_last_waitctx_time_waited_usecs+0x000000000045 returns: 0x25e5e80
 > kskiorm(0x6d1854a8, 0, ...)
 < kskiorm+0x00000000001e returns: 0
 > kfias_iswtgon_ksfd(0x6d1854a8, 0, ...)
 < kfias_iswtgon_ksfd+0x00000000002b returns: 0
 > kxdbio_has_work(0x7ffce278c3c4, 0x6003d010, ...)
 < kxdbio_has_work+0x000000000027 returns: 0
 > skgpwwait(0x7ffce278c630, 0x7f7e930a7ca0, ...)
 | > kslwait_conv_wait_time(0x2dc6c0, 0x7f7e930a7ca0, ...)
 | < kslwait_conv_wait_time+0x000000000027 returns: 0x2dc6c0
 | > sskgpwwait(0x7ffce278c630, 0x7f7e930a7ca0, ...)
 | | > semtimedop(0x38000, 0x7ffce278c328, ...)
 | | < semtimedop+0x000000000023 returns: 0xffffffffffffffff

And a full stack trace of a log writer worker look like this:

$ pstack $(pgrep lg01)
#0  0x00007feda8eaebda in semtimedop () at ../sysdeps/unix/syscall-template.S:81
#1  0x0000000010f9cca6 in sskgpwwait ()
#2  0x0000000010f9a2e8 in skgpwwait ()
#3  0x0000000010a66995 in ksliwat ()
#4  0x0000000010a65d25 in kslwaitctx ()
#5  0x00000000031fb4d0 in kcrfw_slave_queue_remove ()
#6  0x00000000031fad2a in kcrfw_slave_group_main ()
#7  0x00000000012160fa in ksvrdp_int ()
#8  0x000000000370d99a in opirip ()
#9  0x0000000001eb034a in opidrv ()
#10 0x0000000002afedf1 in sou2o ()
#11 0x0000000000d0547a in opimai_real ()
#12 0x0000000002b09b31 in ssthrdmain ()
#13 0x0000000000d05386 in main ()

If you combine the pstack backtrace and the debugtrace information, you see that the idle cycle does not leave the ‘ksliwat’ function, so the wait event is not finished. Quickly looking at the other functions, it’s easy to spot it reads the system clock (sltrgftime64), updates some information (kgslwait_last_waitctx_time_waited_usecs) and then performs some proactive IO checks (kskiorm, kfias_iswtgon_ksfd, kxdbio_has_work) after which it calls the post/wait based functions to setup the semaphore again.

Conclusion so far is the log writer workers do perform a 3 second sleep just like the master log writer, however the wait event ‘LGWR worker group idle’ is not interrupted like ‘rdbms ipc message’ is for the master log writer. This means the wait time for the event for each worker process indicates the last time the worker process actually performed something. A next logical question then is: but what do the log writer worker processes perform? Do they entirely take over the master log writer functionality, or do they work together with the master log writer?

In order to fully understand the next part, it is very beneficial to read up on how the log writer works in ‘single log writer’ mode, where the master log writer handling the idle and work cycle itself:
https://fritshoogland.wordpress.com/2018/02/20/a-look-into-into-oracle-redo-part-4-the-log-writer-null-write/
https://fritshoogland.wordpress.com/2018/02/27/a-look-into-oracle-redo-part-5-the-log-writer-writing/

If you want to perform this investigation yourself, make sure the database is in ‘scalable log writer’ mode, by setting “_use_single_log_writer” to FALSE. This is exactly what I did in order to make sure a log write is done in ‘scalable log writer’ mode.

Now let’s first apply some logic. Above the idle cycle of a log writer worker process is shown. Based on the ‘log writer null write’ blog post, we know that the log writer does advance the LWN and On-disk SCN every 3 seconds. Clearly, the log writer worker process does not do that. So that must mean the master log writer is still performing that function. It would also make very much sense, because it doesn’t matter for scalability if the master log writer performs the function of advancing the LWN and On-disk SCN or a worker process, nothing is waiting on it. Plus, if the master log writer performs most of its functions just like in ‘single log writer’ mode, the change to scalable mode would mean no change for client processes, any committing process must semop() the log writer to start writing.

Let’s look at the relevant debugtrace output of the master log writer in scalable log writer mode:

 | > kcrfw_redo_write_driver(0, 0, ...)
 | | > kcrfw_handle_member_write_errors(0, 0, ...)
 | | < kcrfw_handle_member_write_errors+0x000000000020 returns: 0x600161a0
 | | > kcmgtsf(0, 0, ...)
 | | | > sltrgatime64(0, 0, ...)
 | | | | > sltrgftime64(0, 0, ...)
 | | | | | > clock_gettime@plt(0x1, 0x7fff1fe13010, ...)
 | | | | | | > clock_gettime(0x1, 0x7fff1fe13010, ...)
 | | | | | | < clock_gettime+0x000000000069 returns: 0
 | | | | | < clock_gettime+0x00000000003a returns: 0
 | | | | < sltrgftime64+0x00000000004c returns: 0x53747fe42
 | | | < sltrgatime64+0x00000000003e returns: 0x155d4fd
 | | < kcmgtsf+0x00000000032f returns: 0x3a182314
 | | > kcrfw_slave_adaptive_updatemode(0, 0x600161a0, ...)
 | | < kcrfw_slave_adaptive_updatemode+0x000000000080 returns: 0x7efe34d1f760
 | | > kcrfw_defer_write(0, 0x600161a0, ...)
 | | < kcrfw_defer_write+0x000000000038 returns: 0x7efe34d1f760
 | | > kcrfw_slave_queue_find(0, 0x600161a0, ...)
 | | < kcrfw_slave_queue_find+0x0000000000f1 returns: 0
 | | > kcrfw_slave_queue_setpreparing(0, 0x1, ...)
 | | < kcrfw_slave_queue_setpreparing+0x000000000021 returns: 0
 | | > kcrfw_slave_group_switchpic(0, 0x1, ...)
 | | < kcrfw_slave_group_switchpic+0x000000000050 returns: 0x699b4508
 | | > skgstmGetEpochTs(0, 0x1, ...)
 | | | > gettimeofday@plt(0x7fff1fe13070, 0, ...)
 | | | < __vdso_gettimeofday+0x0000000000fe returns: 0
 | | < skgstmGetEpochTs+0x000000000049 returns: 0x20debfd6192e5
 | | > kcsnew3(0x600113b8, 0x7fff1fe13228, ...)
 | | | > kcsnew8(0x600113b8, 0x7fff1fe13070, ...)
 | | | | > kslgetl(0x60049800, 0x1, ...)
 | | | | < kslgetl+0x00000000012f returns: 0x1
 | | | | > kslfre(0x60049800, 0x1, ...)
 | | | | < kslfre+0x0000000001e2 returns: 0
 | | | < kcsnew8+0x000000000117 returns: 0
 | | | > ub8_to_kscn_impl(0x66c3c7, 0x7fff1fe13228, ...)
 | | | < ub8_to_kscn_impl+0x000000000031 returns: 0
 | | < kcsnew3+0x00000000006f returns: 0x8000
 | | > ktfwtsm(0x3a182314, 0x7fff1fe13228, ...)
 | | | > kcmgtsf(0x2, 0x7fff1fe13228, ...)
 | | | | > sltrgatime64(0x2, 0x7fff1fe13228, ...)
 | | | | | > sltrgftime64(0x2, 0x7fff1fe13228, ...)
 | | | | | | > clock_gettime@plt(0x1, 0x7fff1fe12fe0, ...)
 | | | | | | | > clock_gettime(0x1, 0x7fff1fe12fe0, ...)
 | | | | | | | < clock_gettime+0x000000000069 returns: 0
 | | | | | | < clock_gettime+0x00000000003a returns: 0
 | | | | | < sltrgftime64+0x00000000004c returns: 0x537484a6d
 | | | | < sltrgatime64+0x00000000003e returns: 0x155d511
 | | | < kcmgtsf+0x0000000001b2 returns: 0x3a182314
 | | | > kcmtdif(0x3a182314, 0x3a182314, ...)
 | | | < kcmtdif+0x00000000001b returns: 0
 | | | > ksl_get_shared_latch_int(0x60050340, 0x6ddb1408, ...)
 | | | < ksl_get_shared_latch_int+0x00000000016b returns: 0x1
 | | <> kslfre(0x60050340, 0x66c3c7, ...)
 | | < kslfre+0x0000000001e2 returns: 0
 | | > kcn_stm_write(0x7fff1fe13228, 0x66c3c7, ...)
 | | | > kstmgetsectick(0x7fff1fe13228, 0x66c3c7, ...)
 | | | < kstmgetsectick+0x00000000003a returns: 0x5ae4c494
 | | | > ksl_get_shared_latch_int(0x6004ee40, 0x6ddb1408, ...)
 | | | < ksl_get_shared_latch_int+0x00000000016b returns: 0x1
 | | <> kslfre(0x6004ee40, 0x2244, ...)
 | | < kslfre+0x0000000001e2 returns: 0
 | | > kcrfw_redo_write_initpic(0x699b4508, 0x7fff1fe13228, ...)
 | | | > kscn_to_ub8_impl(0x7fff1fe13228, 0x7fff1fe13228, ...)
 | | | < kscn_to_ub8_impl+0x00000000003e returns: 0x66c3c7
 | | < kcrfw_redo_write_initpic+0x0000000000dc returns: 0x3a182314
 | | > kscn_to_ub8_impl(0x7fff1fe13228, 0, ...)
 | | < kscn_to_ub8_impl+0x00000000003e returns: 0x66c3c7
 | | > kcrfw_gather_lwn(0x7fff1fe13268, 0x699b4508, ...)
 | | | > kslgetl(0x6abe4538, 0x1, ...)
 | | | < kslgetl+0x00000000012f returns: 0x1
 | | | > kcrfw_gather_strand(0x7fff1fe13268, 0, ...)
 | | | < kcrfw_gather_strand+0x0000000000c2 returns: 0
 | | | > kslfre(0x6abe4538, 0x17d5f, ...)
 | | | < kslfre+0x0000000001e2 returns: 0
 | | | > kslgetl(0x6abe45d8, 0x1, ...)
 | | | < kslgetl+0x00000000012f returns: 0x1
 | | | > kcrfw_gather_strand(0x7fff1fe13268, 0x1, ...)
 | | | < kcrfw_gather_strand+0x0000000000c2 returns: 0
 | | | > kslfre(0x6abe45d8, 0x137, ...)
 | | | < kslfre+0x0000000001e2 returns: 0
 | | < kcrfw_gather_lwn+0x00000000065c returns: 0xffffffff
 | | > krsh_trace(0x1000, 0x200, ...)
 | | < krsh_trace+0x00000000005d returns: 0
 | | > kspgip(0x71e, 0x1, ...)
 | | < kspgip+0x00000000023f returns: 0
 | | > kcrfw_slave_queue_setpreparing(0, 0, ...)
 | | < kcrfw_slave_queue_setpreparing+0x000000000021 returns: 0
 | | > kcrfw_slave_queue_flush_internal(0x1, 0, ...)
 | | < kcrfw_slave_queue_flush_internal+0x0000000000d7 returns: 0x1
 | | > kcrfw_do_null_write(0, 0, ...)
 | | | > kcrfw_slave_phase_batchdo(0, 0, ...)
 | | | | > kcrfw_slave_phase_enter(0, 0x9b, ...)
 | | | | < kcrfw_slave_phase_enter+0x000000000449 returns: 0
 | | | <> kcrfw_slave_phase_exit(0, 0x9b, ...)
 | | | < kcrfw_slave_phase_exit+0x00000000035a returns: 0
 | | | > kcrfw_post(0, 0, ...)
 | | | | > kcrfw_slave_single_getactivegroup(0, 0, ...)
 | | | | < kcrfw_slave_single_getactivegroup+0x000000000047 returns: 0x6a9a0718
 | | | | > kspGetInstType(0x1, 0x1, ...)
 | | | | | > vsnffe_internal(0x19, 0x1, ...)
 | | | | | | > vsnfprd(0x19, 0x1, ...)
 | | | | | | < vsnfprd+0x00000000000f returns: 0x8
 | | | | | | > kfIsASMOn(0x19, 0x1, ...)
 | | | | | | <> kfOsmInstanceSafe(0x19, 0x1, ...)
 | | | | | | < kfOsmInstanceSafe+0x000000000031 returns: 0
 | | | | | < vsnffe_internal+0x0000000000a7 returns: 0
 | | | | | > kspges(0x115, 0x1, ...)
 | | | | | < kspges+0x00000000010f returns: 0
 | | | | < kspGetInstType+0x0000000000b1 returns: 0x1
 | | | | > kcrfw_slave_phase_enter(0x1, 0x9b, ...)
 | | | | < kcrfw_slave_phase_enter+0x00000000006f returns: 0x9b
 | | | | > kcscu8(0x60016290, 0x7fff1fe12f98, ...)
 | | | | < kcscu8+0x000000000047 returns: 0x1
 | | | | > kcsaj8(0x60016290, 0x7fff1fe12f38, ...)
 | | | | < kcsaj8+0x0000000000dc returns: 0x1
 | | | | > kcrfw_slave_phase_exit(0x1, 0x9b, ...)
 | | | | < kcrfw_slave_phase_exit+0x00000000008e returns: 0
 | | | | > kslpsemf(0x97, 0, ...)
 | | | | | > ksl_postm_init(0x7fff1fe0ac30, 0x7fff1fe12c50, ...)
 | | | | | < ksl_postm_init+0x00000000002b returns: 0
 | | | | < kslpsemf+0x0000000006b5 returns: 0x1f
 | | | | > kcrfw_slave_barrier_nonmasterwait(0x6a9a0720, 0x4, ...)
 | | | | < kcrfw_slave_barrier_nonmasterwait+0x000000000035 returns: 0x600161a0
 | | | < kcrfw_post+0x000000000c1c returns: 0xd3
 | | < kcrfw_do_null_write+0x0000000000b2 returns: 0xd3
 | < kcrfw_redo_write_driver+0x000000000535 returns: 0xd3

The highlighted functions are extra functions executed when the instance is set to scalable log writer mode, or when adaptive mode has set the instance to scalable log writer mode. This means that the changes between the modes is minimal when there’s no writes, and outside of a few extra functions, the log writer does exactly the same.

The absence of any spectacular changes in the behaviour of the log writer when in scalable log writer mode when there are no writes does hint what the actual changes will be of the scalable mode, which is how writing is handled. In single log writer mode, the most time the log writer is process is likely to spend on is writing the change vectors into the online redologfiles, and maybe, if you have a bad application (!) semop()-ing foreground sessions will be second, if there are a large number of processes committing, because every process needs to be semop()-ed individually. These two functions, along with some other functionality are exactly what the log writer worker processes are doing.

This means that foreground processes do nothing different in scalable log writer mode, they signal (semop) the master log writer, which will investigate the public redo strands, and if the master log writer finds change vectors to write, it will assign log writer worker processes to perform the write, and the log writer worker process will semop() the foreground sessions to indicate the redo has been written when the instance is in post/wait mode, or do not semop() when the instance is in polling mode.

This is the entire function flow of a write when the instance is in scalable log writer mode:

 | > kcrfw_slave_queue_insert(0, 0xd3, ...)
 | | > kcrfw_slave_group_setcurrsize(0, 0, ...)
 | | < kcrfw_slave_group_setcurrsize+0x0000000001d1 returns: 0x1
 | | > _intel_fast_memcpy(0x6a9a05f8, 0x7ffdae335fa0, ...)
 | | <> _intel_fast_memcpy.P(0x6a9a05f8, 0x7ffdae335fa0, ...)
 | | <> __intel_ssse3_rep_memcpy(0x6a9a05f8, 0x7ffdae335fa0, ...)
 | | < __intel_ssse3_rep_memcpy+0x000000002798 returns: 0x6a9a05f8
 | | > kcrfw_slave_group_postall(0, 0xf0, ...)
 | | | > ksvgcls(0, 0xf0, ...)
 | | | < ksvgcls+0x000000000021 returns: 0
 | | | > ksl_post_proc(0x6ddb32f0, 0, ...)
 | | | <> kskpthr(0x6ddb32f0, 0, ...)
 | | | <> kslpsprns(0x6ddb32f0, 0, ...)
 | | | | > ksl_update_post_stats(0x6ddb32f0, 0, ...)
 | | | | | > dbgtTrcData_int(0x7f464c0676c0, 0x2050031, ...)
 | | | | | | > dbgtBucketRedirect(0x7f464c0676c0, 0x7ffdae335338, ...)
 | | | | | | < dbgtBucketRedirect+0x000000000050 returns: 0x1
 | | | | | | > dbgtIncInMemTrcRedirect(0x7f464c0676c0, 0x6fa, ...)
 | | | | | | < dbgtIncInMemTrcRedirect+0x000000000035 returns: 0x1
 | | | | | | > skgstmGetEpochTs(0x7f464c0676c0, 0x6fa, ...)
 | | | | | | | > gettimeofday@plt(0x7ffdae334e40, 0, ...)
 | | | | | | | < __vdso_gettimeofday+0x0000000000fe returns: 0
 | | | | | | < skgstmGetEpochTs+0x000000000049 returns: 0x20e067375b55d
 | | | | | | > dbgtrRecAllocate(0x7f464c0676c0, 0x7ffdae3352e0, ...)
 | | | | | | | > dbgtrPrepareWrite(0x7f464c0676c0, 0x65accba0, ...)
 | | | | | | | < dbgtrPrepareWrite+0x00000000011c returns: 0x4
 | | | | | | < dbgtrRecAllocate+0x000000000144 returns: 0x1
 | | | | | | > _intel_fast_memcpy(0x65acda30, 0x7ffdae3353d8, ...)
 | | | | | | <> _intel_fast_memcpy.P(0x65acda30, 0x7ffdae3353d8, ...)
 | | | | | | <> __intel_ssse3_rep_memcpy(0x65acda30, 0x7ffdae3353d8, ...)
 | | | | | | < __intel_ssse3_rep_memcpy+0x000000002030 returns: 0x65acda30
 | | | | | | > dbgtrRecEndSegment(0x7f464c0676c0, 0x7ffdae3352e0, ...)
 | | | | | | < dbgtrRecEndSegment+0x00000000011c returns: 0x77c000a4
 | | | | | < dbgtTrcData_int+0x000000000323 returns: 0x77c000a4
 | | | | < ksl_update_post_stats+0x00000000024f returns: 0x77c000a4
 | | | | > skgpwpost(0x7ffdae335480, 0x7f464c0acca0, ...)
 | | | | <> sskgpwpost(0x7ffdae335480, 0x7f464c0acca0, ...)
 | | | | | > semop@plt(0xc0000, 0x7ffdae335410, ...)
 | | | | | < semop+0x00000000000f returns: 0
 | | | | < sskgpwpost+0x00000000009a returns: 0x1
 | | | < kslpsprns+0x0000000001c3 returns: 0
 | | < kcrfw_slave_group_postall+0x0000000000a8 returns: 0
 | < kcrfw_slave_queue_insert+0x0000000001b6 returns: 0x667bc540

After the instance has established there are change vectors in kcrfw_gather_lwn, in single log writer mode, the function kcrfw_redo_write is called, which will call kcrfw_do_write which handles the writing, and kslpslf to semop any waiting processes among other things. Now in scalable log writer mode, kcrfw_slave_queue_insert is called which assigns work to worker processes, and then kcrfw_slave_group_postall is called to semop one or more worker processes.

The worker processes are sleeping on a semaphore, and if a process gets signalled, it exits the kcrfw_slave_queue_remove function, ends the wait event, and calls kcrfw_redo_write, just like the master log writer process would call in single log writer mode, which includes doing the write (kcrfw_do_write) and posting the foregrounds (kslpslf), exactly all the functions.

Conclusion.
The adaptive scalable log writer processes function has been silently introduced with Oracle 12, although a lot of the used functionality has been available more or less in earlier versions. It is a fully automatic feature which will turn itself on and off based on heuristics. The purpose of this article is to explain how it works and what it is doing. Essentially, all the functionality that surrounds a log writer write has been moved to a worker process, which means the work can be done in parallel with multiple processes, whilst all the work outside of the work around the write, which is not performance critical, is left with the master log writer.

I gotten some requests to provide an overview of the redo series of blogposts I am currently running. Here it is:

https://fritshoogland.wordpress.com/2018/01/29/a-look-into-oracle-redo-part-1-redo-allocation-latches/
https://fritshoogland.wordpress.com/2018/02/05/a-look-into-oracle-redo-part-2-the-discovery-of-the-kcrfa-structure/
https://fritshoogland.wordpress.com/2018/02/12/a-look-into-oracle-redo-part-3-the-log-writer-work-cycle-overview/
https://fritshoogland.wordpress.com/2018/02/20/a-look-into-into-oracle-redo-part-4-the-log-writer-null-write/
https://fritshoogland.wordpress.com/2018/02/27/a-look-into-oracle-redo-part-5-the-log-writer-writing/
https://fritshoogland.wordpress.com/2018/03/05/a-look-into-oracle-redo-part-6-oracle-post-wait-commit-and-the-on-disk-scn/
https://fritshoogland.wordpress.com/2018/03/19/a-look-into-oracle-redo-part-7-adaptive-log-file-sync/
https://fritshoogland.wordpress.com/2018/03/26/a-look-into-oracle-redo-part-8-generate-redo/
https://fritshoogland.wordpress.com/2018/04/03/a-look-into-oracle-redo-part-9-commit/
https://fritshoogland.wordpress.com/2018/04/09/a-look-into-oracle-redo-part-9a-commit-concurrency-considerations/
https://fritshoogland.wordpress.com/2018/04/16/a-look-into-oracle-redo-part-10-commit_wait-and-commit_logging/

Private redo strands, In memory undo and throw away undo: https://fritshoogland.wordpress.com/2016/11/15/redo-a-blogpost/

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