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This is a question that I played with for a long time. There have been statements on logical IO performance (“Logical IO is x times faster than Physical IO”), but nobody could answer the question what the actual logical IO time is. Of course you can see part of it in the system and session statistics (v$sysstat/v$sesstat), statistic name “session logical reads”. However, if you divide the number of logical reads by the total time a query took, the logical IO time is too high, because then it assumed all the time the query took was spend on doing logical IO, which obviously is not the case, because there is time spend on parsing, maybe physical IO, etc. Also, when doing that, you calculate an average. Averages are known to hide actual behaviour.

Luckily, with Redhat Enterprise Linux and Oracle Linux version 7, there is the kernel version 3.10 as the stock RedHat kernel version. This kernel version supports systemtap userspace return probes. A systemtap probe is a way to trigger an action when a certain action (the probed event) is started, a return probe is an action triggered when an action is finished. The Oracle UEK kernel version 3 at the time of writing is version 3.8, which does not support this.

My current knowledge is consistent reads are handled by the Oracle database C function kcbgtcr(). Current reads are quite probably handled by the function kcbgcur(). Having said that, I know of at least one exception to this: scans on hybrid columnar compressed segments do not use the kcbgtcr() function.

Please mind all kernel code translations, like kcbgtcr (kernel cache buffers get consistent read) are pure guesses, albeit somewhat educated guesses, as there are a lot of internet publications naming these, including My Oracle Support itself.

With the 3.10 version kernel, we can create a small systemtap script to measure the time between the start and stop of the kcbgtcr routine. If you want to experiment with this, it is probably best to download the latest version of systemtap and compile it yourself.. This is the reason you see /usr/local/bin/stap in the shebang.

#!/usr/local/bin/stap

global latency

probe begin {
	printf("Begin.\n")
}

probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kcbgtcr") {
	latency[pid()] = gettimeofday_us()
	printf(">kcbgtcr\n")
}

probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kcbgtcr").return {
	printf("<kcbgtcr, latency(us): %d\n", gettimeofday_us() - latency[pid()])
}

Now make the script executable, and run it against a database session:

# ./lio.stap -x 3877

Next, I execute a scan in the database foreground session, and watch the systemtap script output:
(execution of the systemtap script can be cancelled by pressing CTRL-c)

...
>kcbgtcr
<kcbgtcr, latency(us): 2
>kcbgtcr
<kcbgtcr, latency(us): 79542
>kcbgtcr
<kcbgtcr, latency(us): 4
>kcbgtcr
<kcbgtcr, latency(us): 2
>kcbgtcr
<kcbgtcr, latency(us): 2
>kcbgtcr
<kcbgtcr, latency(us): 13
>kcbgtcr
<kcbgtcr, latency(us): 1
...

This shows the function being executed by the database session. However, it seems the time spend in the kcbgtcr() function is not consistent.

In order to get a better overview, we can add a histogram of the kcbgtcr latencies:

#!/usr/local/bin/stap
global latency, latency_histogram

probe begin {
	printf("Begin.\n")
}

probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kcbgtcr") {
	latency[pid()] = gettimeofday_us()
	printf(">kcbgtcr\n")
}

probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kcbgtcr").return {
	latency[pid()] = gettimeofday_us() - latency[pid()]
	latency_histogram <<< latency[pid()]
	printf("<kcbgtcr, latency(us): %d\n", latency[pid()])
}

probe end {
	if ( @count(latency_histogram) > 0 ) {
		printf("\n==kcbgtcr latency==\n")
		println(@hist_log(latency_histogram))
	}
}

Next, attach the systemtap script to the database session again, and issue a scan. Once the scan in the database session is finished, press CTRL-c to finish the systemtap script:

...

==kcbgtcr latency==
  value |-------------------------------------------------- count
      0 |                                                       0
      1 |@@@@@@@@@@@@@@@@@@@@                                5998
      2 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@  14284
      4 |                                                     268
      8 |                                                      58
     16 |                                                     137
     32 |                                                      25
     64 |                                                       1
    128 |                                                      15
    256 |                                                      69
    512 |                                                       1
   1024 |                                                       0
   2048 |                                                       0
   4096 |                                                       1
   8192 |                                                       5
  16384 |                                                      35
  32768 |                                                      39
  65536 |                                                       8
 131072 |                                                       0
 262144 |                                                       1

Okay, the majority is 2 microseconds, but outside the 1 and 2 microseconds buckets, there are a lot of executions that totally fall outside of these, up to 262144 microseconds (262 milliseconds)!

What could cause these huge changes in logical IO time?

At this point I am quite much turning the squelch down and make a lot of information visible (this is a warning!). Here is the systemtap script I am using:

#!/usr/local/bin/stap
#
global latency, latency_histogram

probe begin {
	printf("Begin.\n")
}

probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kc*") {
	printf("%s > %s\n", thread_indent(1), probefunc())
	if ( probefunc() == "kcbgtcr" )
			latency[pid()] = gettimeofday_us()
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kc*").return {
	printf("%s < %s", thread_indent(-1), ppfunc())
	if ( ppfunc() == "kcbgtcr" ) {
		latency[pid()] = gettimeofday_us() - latency[pid()]
		latency_histogram <<< latency[pid()]
		printf(" -- latency(us): %d", latency[pid()])
	}
	printf("\n")
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("qe*") {
	printf("%s > %s\n", thread_indent(1), probefunc())
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("qe*").return {
	printf("%s < %s\n", thread_indent(-1), ppfunc())
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kt*") {
	printf("%s > %s\n", thread_indent(1), probefunc())
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kt*").return {
	printf("%s < %s\n", thread_indent(-1), ppfunc())
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kd*") {
	printf("%s > %s\n", thread_indent(1), probefunc())
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kd*").return {
	printf("%s < %s\n", thread_indent(-1), ppfunc())
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("opiosq0") {
	printf("=PARSE\n")
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("opiexe") {
	printf("=EXECUTE\n")
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("opifch2") {
	printf("=FETCH\n")
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("opiclo") {
	printf("=CLOSE\n")
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kslwtbctx") {
	printf("=KSLWTBCTX\n")
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("kslwtectx") {
	printf("=KSLWTECTX\n")
}
probe process("/lib64/libaio.so.1").function("io_submit") {
	printf("=IO_SUBMIT\n")
}
probe process("/lib64/libaio.so.1").function("io_getevents_0_4") {
	printf("=IO_GETEVENTS\n")
}
probe process("/lib64/libc.so.6").function("pread64") {
	printf("=PREAD\n")
}

Warning! This script will run for a long time before it is compiled as kernel module!! Systemtap works by dynamically creating a kernel module from the system tap script, and insert it into the kernel. Because I injected a huge chunk of code to run in the kernel a lot of things are potentially influenced. I found the Oracle database to drop core’s for example.

After running this against a session and doing a very simple full table scan with a count(*), I searched for the logical IO, in other words where the kcbgrcr() function was performed. Here it is:

166149 oracle_5487_tes(5487):     < kdst_fetch
166155 oracle_5487_tes(5487):     > qeaeCn1SerialRowsets
166159 oracle_5487_tes(5487):     < qeaeCn1SerialRowsets
166162 oracle_5487_tes(5487):     > kdst_fetch
166164 oracle_5487_tes(5487):      > kdst_fetch0
166167 oracle_5487_tes(5487):       > kcbipnns
166170 oracle_5487_tes(5487):       < kcbipnns
166173 oracle_5487_tes(5487):       > kcbrls
166177 oracle_5487_tes(5487):        > kcbrls_direct
166181 oracle_5487_tes(5487):        < kcbrls_direct
166183 oracle_5487_tes(5487):       < kcbrls
166185 oracle_5487_tes(5487):       > kdstsnb
166188 oracle_5487_tes(5487):       < kdstsnb
166191 oracle_5487_tes(5487):       > ktrget2
166194 oracle_5487_tes(5487):        > ktsmg_max_query
166197 oracle_5487_tes(5487):        < ktsmg_max_query
166200 oracle_5487_tes(5487):        > kcbgtcr
166204 oracle_5487_tes(5487):         > kcbldrget
166209 oracle_5487_tes(5487):          > kcblgt
166211 oracle_5487_tes(5487):          < kcblgt
166213 oracle_5487_tes(5487):          > kcbzvb
166216 oracle_5487_tes(5487):           > kcbhvbo
166219 oracle_5487_tes(5487):            > kcbhxoro
166222 oracle_5487_tes(5487):            < kcbhxoro
166224 oracle_5487_tes(5487):           < kcbhvbo
166226 oracle_5487_tes(5487):          < kcbzvb
166228 oracle_5487_tes(5487):          > kcbztek_trace_blk
166230 oracle_5487_tes(5487):          < kcbztek_trace_blk
166233 oracle_5487_tes(5487):          > kcbl_objdchk_with_cache_reread
166236 oracle_5487_tes(5487):           > kcbtgobj
166239 oracle_5487_tes(5487):            > kd4obj
166251 oracle_5487_tes(5487):            < kd4obj
166254 oracle_5487_tes(5487):           < kcbtgobj
166257 oracle_5487_tes(5487):          < kcbl_objdchk_with_cache_reread
166260 oracle_5487_tes(5487):         < kcbldrget
166262 oracle_5487_tes(5487):        < kcbgtcr -- latency(us): 62
166265 oracle_5487_tes(5487):        > ktrgcm
166268 oracle_5487_tes(5487):         > kcbcge
166271 oracle_5487_tes(5487):         < kcbcge
166273 oracle_5487_tes(5487):         > ktcckv
166275 oracle_5487_tes(5487):         < ktcckv
166285 oracle_5487_tes(5487):        < ktrgcm
166286 oracle_5487_tes(5487):       < ktrget2
166289 oracle_5487_tes(5487):       > kdr9ir2blk
166291 oracle_5487_tes(5487):       < kdr9ir2blk
166293 oracle_5487_tes(5487):      < kdst_fetch0
166294 oracle_5487_tes(5487):     < kdst_fetch
166297 oracle_5487_tes(5487):     > qeaeCn1SerialRowsets
166300 oracle_5487_tes(5487):     < qeaeCn1SerialRowsets

How to read: “>” means entering a function, “<" means return from a function.
I selected a piece of the systemtap output/tracing where the counting procedure is visible.

The first row is "< kdst_fetch" in other words: returning from kernel data scan table fetch. So this function has performed a fetch. Not surprisingly, the next function is qeaeCn1SerialRowsets, which I think is the count function.

After the count, the kdst_fetch function is entered again, then kdst_fetch0. The next functions are kcbipnns, kcbrls and kcbrls_direct. Probably these functions are related to pinning and releasing blocks. See Alexander Anokhin’s post on that.

A few functions further we encounter kcbgrcr(). Interestingly, it is followed by the kcbldrget() function, which is kernel cache buffers direct path loader get. In other words, Oracle has chosen to do a direct path read, because this is the function that starts off the direct path read code. The next function, kcblget() requests a block. This means that blocks read in this code path are from PGA memory, not SGA (buffer cache) memory.

The next functions, kcbzvb, kcbhvbo and kcbhxoro are the block XOR checking functionality. The function actually performing this (sxorchk) is not visible because I did not probe for it.

The next functions are not clear to me at this point (kcbztek_trace_blk, kcbl_objdchk_with_cache_reread, kcbtgobj, kd4obj). Then the kcbldrget returns, and the kcbgtcr function too.

Here we can see that probing a huge number of functions does influence the performance of a process. While previously we saw kcbgtcr() took 2us, probably because of all the probes the time the kcbgtcr function took 62us.

The next piece of execution is done by the ktrgcm function. This is handling undo and buffer cleanout. After that function, we cross kdr9ir2blk (function unknown), after which kdst_fetch0 and kdst_fetch return, and the count is done using qeaeCn1SerialRowsets.

Okay, so far so good, but the question was: why are there logical IOs/kcbgtcr() executions that take excessively more time?

After a little searching, I was able to find a very good reason:

169252 oracle_5487_tes(5487):     > kdst_fetch
169254 oracle_5487_tes(5487):      > kdst_fetch0
169256 oracle_5487_tes(5487):       > kcbipnns
169258 oracle_5487_tes(5487):       < kcbipnns
169259 oracle_5487_tes(5487):       > kcbrls
169262 oracle_5487_tes(5487):        > kcbrls_direct
169264 oracle_5487_tes(5487):        < kcbrls_direct
169265 oracle_5487_tes(5487):       < kcbrls
169267 oracle_5487_tes(5487):       > kdstsnb
169269 oracle_5487_tes(5487):       < kdstsnb
169270 oracle_5487_tes(5487):       > ktrget2
169272 oracle_5487_tes(5487):        > ktsmg_max_query
169274 oracle_5487_tes(5487):        < ktsmg_max_query
169275 oracle_5487_tes(5487):        > kcbgtcr
169278 oracle_5487_tes(5487):         > kcbldrget
169280 oracle_5487_tes(5487):          > kcblgt
169283 oracle_5487_tes(5487):           > kcblrs
169286 oracle_5487_tes(5487):            > kdsdrcbk
169288 oracle_5487_tes(5487):            < kdsdrcbk
169291 oracle_5487_tes(5487):            > kcbldio
169296 oracle_5487_tes(5487):             > kcfaioe
169299 oracle_5487_tes(5487):             < kcfaioe
169301 oracle_5487_tes(5487):             > kcflbi
169314 oracle_5487_tes(5487):              > kcf_hard_ftype_check
169317 oracle_5487_tes(5487):              < kcf_hard_ftype_check
=IO_SUBMIT
169416 oracle_5487_tes(5487):             < kcflbi
169420 oracle_5487_tes(5487):            < kcbldio
169425 oracle_5487_tes(5487):            > kcblcffln
169429 oracle_5487_tes(5487):            < kcblcffln
169432 oracle_5487_tes(5487):           < kcblrs
169434 oracle_5487_tes(5487):           > kcblsinc
169438 oracle_5487_tes(5487):           < kcblsinc
169440 oracle_5487_tes(5487):           > kcblcio
169443 oracle_5487_tes(5487):            > kcblci
169447 oracle_5487_tes(5487):             > kcflci
=IO_GETEVENTS
=IO_GETEVENTS
169481 oracle_5487_tes(5487):              > kcflwi
=IO_GETEVENTS
=IO_GETEVENTS
=KSLWTBCTX
=IO_GETEVENTS
=IO_GETEVENTS
=KSLWTECTX
179477 oracle_5487_tes(5487):              < kcflwi
179484 oracle_5487_tes(5487):             < kcflci
179488 oracle_5487_tes(5487):            < kcblci
179491 oracle_5487_tes(5487):           < kcblcio
179494 oracle_5487_tes(5487):          < kcblgt
179497 oracle_5487_tes(5487):          > kcbzvb
179509 oracle_5487_tes(5487):           > kcbhvbo
179513 oracle_5487_tes(5487):            > kcbhxoro
179516 oracle_5487_tes(5487):            < kcbhxoro
179518 oracle_5487_tes(5487):           < kcbhvbo
179520 oracle_5487_tes(5487):          < kcbzvb
179539 oracle_5487_tes(5487):          > kcbztek_trace_blk
179544 oracle_5487_tes(5487):          < kcbztek_trace_blk
179549 oracle_5487_tes(5487):          > kcbl_objdchk_with_cache_reread
179555 oracle_5487_tes(5487):           > kcbtgobj
179559 oracle_5487_tes(5487):            > kd4obj
179562 oracle_5487_tes(5487):            < kd4obj
179563 oracle_5487_tes(5487):           < kcbtgobj
179565 oracle_5487_tes(5487):          < kcbl_objdchk_with_cache_reread
179569 oracle_5487_tes(5487):         < kcbldrget
179571 oracle_5487_tes(5487):        < kcbgtcr -- latency(us): 10295
179576 oracle_5487_tes(5487):        > ktrgcm
179580 oracle_5487_tes(5487):         > kcbcge
179582 oracle_5487_tes(5487):         < kcbcge
179585 oracle_5487_tes(5487):         > ktcckv
179587 oracle_5487_tes(5487):         < ktcckv
179589 oracle_5487_tes(5487):        < ktrgcm
179591 oracle_5487_tes(5487):       < ktrget2
179593 oracle_5487_tes(5487):       > kdr9ir2blk
179606 oracle_5487_tes(5487):       < kdr9ir2blk
179609 oracle_5487_tes(5487):      < kdst_fetch0
179611 oracle_5487_tes(5487):     < kdst_fetch
179616 oracle_5487_tes(5487):     > qeaeCn1SerialRowsets
179620 oracle_5487_tes(5487):     < qeaeCn1SerialRowsets

If you go through the calls, you will see that the start is exactly the same, until line 17. After kcbgtcr>kcbldrgt (consistent read request function choosing direct path reads), the kcblgt function does not return immediately, but rather starts off a lot of extra code path.

This code path fetches new blocks. The most striking thing here is that kcbgtcr requests the blocks, and physical IO is done on behalf of the consistent read request, in other words: on behalf of the logical IO. This is obvious if you think about it, if you want to read blocks you have to look if they are available or not, and if they are not available, you have to fetch them.

As a conclusion: if kcbgtcr() together with kcbgcur() and a couple of other functions is considered the logical IO, then a logical IO has a variable time, instead of a consistent one. Because physical IO is requested inside the logical IO request, technically the physical IO is part of the logical IO. This means that it is technically incorrect to state that a physical IO is slower than logical IO, because a physical IO is part of the logical IO request that needed that physical IO. and as such a physical IO can not be slower than a logical IO

This article is written with examples taken from an (virtualised) Oracle Linux 6u6 X86_64 operating system, and Oracle database version 12.1.0.2.1. However, I think the same behaviour is true for Oracle 11 and 10 and earlier versions.

Probably most readers of this blog are aware that a “map” of mapped memory for a process exists for every process in /proc, in a pseudo file called “maps”. If I want to look at my current process’ mappings, I can simply issue:

$ cat /proc/self/maps
00400000-0040b000 r-xp 00000000 fc:00 786125                             /bin/cat
0060a000-0060b000 rw-p 0000a000 fc:00 786125                             /bin/cat
0060b000-0060c000 rw-p 00000000 00:00 0
0080a000-0080b000 rw-p 0000a000 fc:00 786125                             /bin/cat
01243000-01264000 rw-p 00000000 00:00 0                                  [heap]
345b000000-345b020000 r-xp 00000000 fc:00 276143                         /lib64/ld-2.12.so
345b21f000-345b220000 r--p 0001f000 fc:00 276143                         /lib64/ld-2.12.so
345b220000-345b221000 rw-p 00020000 fc:00 276143                         /lib64/ld-2.12.so
345b221000-345b222000 rw-p 00000000 00:00 0
345b800000-345b98a000 r-xp 00000000 fc:00 276144                         /lib64/libc-2.12.so
345b98a000-345bb8a000 ---p 0018a000 fc:00 276144                         /lib64/libc-2.12.so
345bb8a000-345bb8e000 r--p 0018a000 fc:00 276144                         /lib64/libc-2.12.so
345bb8e000-345bb8f000 rw-p 0018e000 fc:00 276144                         /lib64/libc-2.12.so
345bb8f000-345bb94000 rw-p 00000000 00:00 0
7f8f69686000-7f8f6f517000 r--p 00000000 fc:00 396081                     /usr/lib/locale/locale-archive
7f8f6f517000-7f8f6f51a000 rw-p 00000000 00:00 0
7f8f6f524000-7f8f6f525000 rw-p 00000000 00:00 0
7fff2b5a5000-7fff2b5c6000 rw-p 00000000 00:00 0                          [stack]
7fff2b5fe000-7fff2b600000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]

What we see, is the start and end address, the rights (rwx), absence of rights is shown with a ‘-‘, and an indication of the mapped memory region is (p)rivate or (s)hared. In this example, there are no shared memory regions. Then an offset of the mapped file, then the device (major and minor device number). In our case sometimes this is ‘fc:00′. If you wonder what device this might be:

$ echo "ibase=16; FC" | bc
252
$ ls -l /dev | egrep 252,\ *0
brw-rw---- 1 root disk    252,   0 Mar 23 14:19 dm-0
$ sudo dmsetup info /dev/dm-0
Name:              vg_oggdest-lv_root
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        1
Event number:      0
Major, minor:      252, 0
Number of targets: 2
UUID: LVM-q4nr4HQXgotaaJFaGF1nzd4eZPPTohndgz553dw6O5pTlvM0SQGLFsdp170pgHuw

So, this is a logical volume lv_root (in the volume group vg_oggdest).

Then the inode number (if a file was mapped, if anonymous memory was mapped the number 0 is shown), and then the path if a file was mapped. This is empty for anonymous mapped memory (which is memory which is added to a process using the mmap() call). Please mind there are also special regions like: [heap],[stack],[vdso] and [vsyscall].

Okay, so far I’ve shown there is a pseudo file called ‘maps’ which shows mapped memory and told a bit about the fields in the file. Now let’s move on to the actual topic of this blog: the Oracle database SGA memory, and the indicator this is deleted!

In this example I pick the maps file of the PMON process of an Oracle database. Of course the database must use system V shared memory, not shared memory in /dev/shm (which is typically what you see when Oracle’s automatic memory (AMM) feature is used). This is a snippet from the maps file of the pmon process on my server:

 cat /proc/2895/maps
00400000-1093f000 r-xp 00000000 fc:00 1326518                            /u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle
10b3e000-10dbf000 rw-p 1053e000 fc:00 1326518                            /u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle
10dbf000-10df0000 rw-p 00000000 00:00 0
12844000-1289d000 rw-p 00000000 00:00 0                                  [heap]
60000000-60001000 r--s 00000000 00:04 111902723                          /SYSV00000000 (deleted)
60001000-602cc000 rw-s 00001000 00:04 111902723                          /SYSV00000000 (deleted)
60400000-96400000 rw-s 00000000 00:04 111935492                          /SYSV00000000 (deleted)
96400000-9e934000 rw-s 00000000 00:04 111968261                          /SYSV00000000 (deleted)
9ec00000-9ec05000 rw-s 00000000 00:04 112001030                          /SYSV6ce0e164 (deleted)
345b000000-345b020000 r-xp 00000000 fc:00 276143                         /lib64/ld-2.12.so
345b21f000-345b220000 r--p 0001f000 fc:00 276143                         /lib64/ld-2.12.so
...

If you look closely, you see the oracle executable first, with two entries, one being readonly (r-xp), the other being read-write (rw-p). The first entry is readonly because it is shared with other processes, which means that there is no need for all the processes to load the Oracle database executable in memory, it shares the executable with other process. There’s much to say about that too, which should be done in another blogpost.

After the executable there are two anonymous memory mappings, of which one is the process’ heap memory.

Then we see what this blogpost is about: there are 5 mappings which are shared (r–s and rw-s). These are the shared memory regions of the Oracle database SGA. What is very odd, is that at the end of the lines it says “(deleted)”.

Of course we all know what “deleted” means. But what does it mean in this context? Did somebody delete the memory segments? Which actually can be done with the ‘ipcrm’ command…

If you go look at the maps of other Oracle processes and other databases you will see that every database’s shared memory segment are indicated as ‘(deleted)’.

Word of warning: only execute the steps below on a test environment, do NOT do this in a production situation.

In order to understand this, the best way to see what actually is happening, is starting up the Oracle database with a process which is traced with the ‘strace’ utility with the ‘-f’ option set (follow). Together with the ‘-o’ option this will produce a (long) file with all the system calls and the arguments of the calls which happened during startup:

$ strace -f -o /tmp/oracle_startup.txt sqlplus / as sysdba

Now start up the database. Depending on your system you will notice the instance startup takes longer. This is because for every system call, strace needs to write a line in the file /tmp/oracle_start.txt. Because of this setup, stop the database as soon as it has started, on order to stop the tracing from crippling the database performance.

Now open the resulting trace file (/tmp/oracle_startup.txt) and filter it for the system calls that are relevant (calls with ‘shm’ in their name):

$ grep shm /tmp/oracle_startup.txt | less

Scroll through the output until you see a line alike ‘shmget(IPC_PRIVATE, 4096, 0600) = 130777091′:

...
4545  shmget(IPC_PRIVATE, 4096, 0600)   = 130777091
4545  shmat(130777091, 0, 0)            = ?
4545  shmctl(130777091, IPC_STAT, 0x7fff9eb9da30) = 0
4545  shmdt(0x7f406f2ba000)             = 0
4545  shmctl(130777091, IPC_RMID, 0)    = 0
4545  shmget(IPC_PRIVATE, 4096, 0600)   = 130809859
4545  shmat(130809859, 0, 0)            = ?
4545  shmctl(130809859, IPC_STAT, 0x7fff9eb9da30) = 0
4545  shmdt(0x7f406f2ba000)             = 0
4545  shmctl(130809859, IPC_RMID, 0)    = 0
...

What we see here is a (filtered) sequence of systems calls that could explain the status deleted of the shared memory segments. If you look up what process id is in front of these shm system calls, you will see it’s the foreground process starting up the instance. If you look closely, you’ll that there is a sequence which is repeated often:

1. shmget(IPC_PRIVATE, 4096, 0600) = 130777091
The system call shmget allocates a shared memory segment of 4 kilobyte, rights set to 600. The return value is the shared memory identifier of the requested shared memory segment.

2. shmat(130777091, 0, 0) = ?
The system call shmat attaches the a shared memory segment to the process’ address space. The first argument is the shared memory identifier, the second argument is the address to attach the segment to. If the argument is zero, like in the call above, it means the operating system is tasked with finding a suitable (non used) address. The third argument is for flags, the value zero here means no flags are used. The returncode (here indicated with a question mark) is the address at which the segment is attached. This being a question mark means strace is not able to read the address, which is a shame, because we can’t be 100% certain at which memory address this shared memory segment is mapped.

3. shmctl(130777091, IPC_STAT, 0x7fff9eb9da30) = 0
The system call shmctl with the argument IPC_STAT has the function to read the (kernel) shared memory information of the shared memory identifier indicated by the first argument, and write it at the memory location in the third argument in a struct called shmid_ds.

4. shmdt(0x7f406f2ba000) = 0
With this system call, the shared memory segment is detached from the process’ address space. For the sake of the investigation, I assumed that the address in this call is the address which is returned by the shmat() call earlier.

5. shmctl(130777091, IPC_RMID, 0) = 0
This is another shared memory control system call, concerning our just created shared memory segment (shared memory identifier 130777091), with the command ‘IPC_RMID’. This is what the manpage says about IPC_RMID:

       IPC_RMID  Mark the segment to be destroyed.  The segment will only  actually  be  destroyed
                 after the last process detaches it (i.e., when the shm_nattch member of the asso-
                 ciated structure shmid_ds is zero).  The caller must be the owner or creator,  or
                 be privileged.  If a segment has been marked for destruction, then the (non-stan-
                 dard) SHM_DEST flag of the shm_perm.mode field in the associated  data  structure
                 retrieved by IPC_STAT will be set.

What I thought this means was:
It looked like to me the database instance starts building up its shared memory segments per 4096 page. Because IPC_RMID only marks the segment to be destroyed, and because it will only be truly destroyed when there are no processes attached to the shared memory segment, it looked like to me the background processes were pointed to the shared memory segment which was marked destroyed (in some way I hadn’t discovered yet), which meant the shared memory segment would actually survive and all database processes can use it. If ALL the database processes would be killed for any reason, for example with a shutdown abort, the processes would stop being connected to the shared memory segment, which would mean the shared memory segment would vanish automatically, because it was marked for destruction.
Sounds compelling, right?

Well…I was wrong! The sequence of creating and destroying small shared memory segments is done, but it turns out these are truly destroyed with the shmctl(…,IPC_RMID,…) call. I don’t know why the sequence of creating shared memory segments is happening.

I started looking for the actual calls that create the final, usable shared memory segments in the /tmp/oracle_startup.txt file. This is actually quite easy to do; first look up the shared memory segment identifiers using the sysresv utility (make sure the database’s ORACLE_HOME and ORACLE_SID are set):

$ sysresv
...a lot of other output...
Shared Memory:
ID		KEY
197394436	0x00000000
197427205	0x00000000
197361667	0x00000000
197459974	0x6ce0e164
Semaphores:
ID		KEY
1015811 	0xd5cdbca4
Oracle Instance alive for sid "dest"

Actually the ‘sysresv’ utility (system remove system V memory I think is what the name means) has the task of removing memory segments if there is no instance left to use them. It will not remove the memory segments if it finds the instance alive. It prints out a lot of information as a bonus.

Now that we got the shared memory identifiers, simply search in the trace file generated by strace, and search for the creation of the memory segment with the identifiers: (please mind searching with ‘less’ is done with the forward slash)

$ less /tmp/oracle_startup.txt
9492  shmget(IPC_PRIVATE, 905969664, IPC_CREAT|IPC_EXCL|0640) = 197394436
9492  shmat(197394436, 0x60400000, 0)   = ?
9492  times(NULL)                       = 430497743
9492  write(4, " Shared memory segment allocated"..., 109) = 109
9492  write(4, "\n", 1)                 = 1

Aha! here we see shmget() again, but now with a size (905969664) that looks much more like a real shared memory segment size used by the database! After the shared memory identifier is created, the process attaches it to its addressing space with shmat() to a specific memory address: 0x60400000.

The next thing to do, is to look for any shmctl() call for this identifier. Oracle could still do the trick of marking the segment for destruction…
…But…there are no shmctl() calls for this identifier, nor for any of the other identifiers shown with the sysresv utility. This is rather odd, because Linux shows them as “(deleted)”. There ARE dozens of shmat() calls, of the other (background) processes forked from the starting process when they attach to the shared memory segments.

So, conclusion at this point is Linux shows the shared memory segments as deleted in ‘maps’, but the Oracle database does not mark the segments for destruction after creation. This means that either Linux is lying, or something mysterious is happening in the Oracle executable which I didn’t discover yet.

I could only think of one way to verify what is truly happening here. That is to create a program myself that uses shared memory, so I have 100% full control over what is happening, and can control every distinct step.

This is what I came up with:

#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>

int main ()
{
  int segment_id;
  char* shared_memory;
  struct shmid_ds shmbuffer;
  int segment_size;
  const int shared_segment_size = 0x6400;

  /* Allocate a shared memory segment.  */
  segment_id = shmget (IPC_PRIVATE, shared_segment_size,
                     IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR);
  printf ("1.shmget done\n");
  getchar();
  /* Attach the shared memory segment.  */
  shared_memory = (char*) shmat (segment_id, 0, 0);
  printf ("shared memory attached at address %p\n", shared_memory);
  printf ("2.shmat done\n");
  getchar();
  /* Determine the segment's size. */
  shmctl (segment_id, IPC_STAT, &shmbuffer);
  segment_size  =               shmbuffer.shm_segsz;
  printf ("segment size: %d\n", segment_size);
  printf ("3.shmctl done\n");
  getchar();
  /* Write a string to the shared memory segment.  */
  sprintf (shared_memory, "Hello, world.");
  /* Detach the shared memory segment.  */
  shmdt (shared_memory);
  printf ("4.shmdt done\n");
  getchar();

  /* Deallocate the shared memory segment.  */
  shmctl (segment_id, IPC_RMID, 0);
  printf ("5.shmctl ipc_rmid done\n");
  getchar();

  return 0;
}

(I took the code from this site, and modified it a bit for my purposes)
If you’ve got a linux system which is setup with the preinstall rpm, you should be able to copy this in a file on your (TEST!) linux database server, in let’s say ‘shm.c’, and compile it using ‘cc shm.c -o smh’. This will create an executable ‘shm’ from this c file.

This program does more or less the same sequence we saw earlier:
1. Create a shared memory identifier.
2. Attach to the shared memory identifier.
3. Get information on the shared memory segment in a shmid_ds struct.
4. Detach the shared memory segment.
5. Destroy it using shmctl(IPC_RMID).

What I did was have two terminals open, one to run the shm program, and one to look for the results of the steps.

Step 1. (shmget)

$ ./shm
1. shmget done

When looking with ipcs, you can see the shared memory segment which is created because of the shmget() call:

$ ipcs -m

------ Shared Memory Segments --------
0x00000000 451608583  oracle     600        25600      0

when looking in the address space of the process running the shm program, the shared memory segment is not found. This is exactly what I expect, because it’s only created, not attached yet.

Step 2. (shmat)

shared memory attached at address 0x7f3c4aa6e000
2.shmat done

Of course the shared memory segment is still visible with ipcs:

0x00000000 451608583  oracle     600        25600      1

And we can see from ipcs in the last column (‘1′) that one process attached to the segment. Of course exactly what we suspected.
But now that we attached the shared memory to the addressing space, it should be visible in maps:

...
7f3c4aa6e000-7f3c4aa75000 rw-s 00000000 00:04 451608583                  /SYSV00000000 (deleted)
...

Bingo! The shared memory segment is visible, as it should be, because we just attached it with shmat(). But look: it’s deleted already according to Linux!

However I am pretty sure, as in 100% sure, that I did not do any attempts to mark the shared memory segment destroyed or do anything else to make it appear to be deleted. So, this means maps lies to us.

So, the conclusion is the shared memory Oracle uses is not deleted, it’s something that Linux shows us, and is wrong. When looking at the maps output again, we can see the shared memory identifier is put at the place of the inode number. This is handy, because it allows you to take the identifier, and look with ipcs for shared memory segments and understand which specific shared memory segment a process is using. It probably means that maps tries to look up the identifier number as inode number, which it will not be able to find, and then comes to the conclusion that it’s deleted.

However, this is speculation. Anyone with more or better insight is welcome to react on this article.

Sometimes you need to see the difference between two pieces of console output. When I research, this can be two stacktraces, but also /proc//maps and smaps output; really anything. Of course, there’s diff, but the diff output is not very visual. Also, diff doesn’t do diffing between more than two files.

This can be done reasonably simple in vim. Here’s how to do that:
1. start vi; vi
2. do a vertical split using a new buffer; :vnew
3. open the first (left side) file; :r path/file or goto insert mode (esc i) and paste text.
4. goto the second window: ctrl+w ctrl+w
5. open the second (right side) file; :r path/file or goto insert mode (esc i) and paste text.
6. diff the two windows; :windo diffthis
7. turn diff mode off; :windo diffoff

You can also expand your diffing to three windows:
1. goto the rightside; ctrl+w l
2. change new window placement to the right side; :set splitright
3. do another virtual split; :vnew
4. open another file or paste text
5. diff again; : windo diffthis

I wrote this down for myself, but hopefully this helps other people too.

Every DBA working with the Oracle database must have seen memory dumps in tracefiles. It is present in ORA-600 (internal error) ORA-7445 (operating system error), system state dumps, process state dumps and a lot of other dumps.

This is how it looks likes:

Dump of memory from 0x00007F06BF9A9E00 to 0x00007F06BF9ADE00
7F06BF9A9E00 0000C215 0000001F 00000CC1 0401FFFF  [................]
7F06BF9A9E10 000032F3 00010003 00000002 442B0000  [.2............+D]
7F06BF9A9E20 2F415441 31323156 4F2F3230 4E494C4E  [ATA/V12102/ONLIN]
7F06BF9A9E30 474F4C45 6F72672F 315F7075 3735322E  [ELOG/group_1.257]
7F06BF9A9E40 3336382E 36313435 00003338 00000000  [.863541683......]
7F06BF9A9E50 00000000 00000000 00000000 00000000  [................]

The first column is the memory location in hexadecimal.
The second to fifth columns represent the actual memory values in hexadecimal.
The sixth column shows an ASCII representation of the memory contents. If a position does not represent an ASCII character, a dot (“.”) is printed.

Actually, the values in the second to fifth column are grouped in four columns. This is how the values in a column look like:
{hex val}{hex val}{hex val}{hex val}, for example: 00010203 means: 0, 1, 2, 3.

In the ASCII representation (sixth column) the spaces after every four values are not put in.

However, look at the following line:

7F06BF9A9E10 000032F3 00010003 00000002 442B0000  [.2............+D]

And focus on the last four characters:
“..+D” (two non-printables, plus, D)
Now look at the corresponding memory contents from the dump:
“442B0000″ This is: “44 2B 00 00″, which should correspond to “. . + D”.
There is something the matter here: the plus and the D seem to be represented by “00”. That’s not correct.

Let’s see what “442B0000″ actually represents in ASCI:

$ echo -e "\x44\x2B\x00\x00"
D+

Ah! That looks backwards! Let’s take a full line and see what that gives:
(This is the line with memory address 0x7F06BF9A9E20)

$ echo -e "\x2F\x41\x54\x41 \x31\x32\x31\x56 \x4F\x2F\x32\x30 \x4E\x49\x4C\x4E"
/ATA 121V O/20 NILN

So if you want to look at the actual memory contents, you need to start with the column on the left side, read the values from right to left, then go the next column, etc.

Endianness
Actual, I asked my friend Philippe Fierens for a trace file from a SPARC (big endian) platform, to see if the endianness of the platform was causing this. I test my stuff on Linux, which is little endian.

Here’s a little snippet:

Dump of memory from 0xFFFFFFFF7D977E00 to 0xFFFFFFFF7D97BE00
FFFFFFFF7D977E00 15C20000 00000001 00000000 00000104  [................]
FFFFFFFF7D977E10 F4250000 00000000 0B200400 E2EB8A3D  [.%....... .....=]
FFFFFFFF7D977E20 44475445 53540000 32F6D98B 00000590  [DGTEST..2.......]
FFFFFFFF7D977E30 00004000 00000001 00000000 00000000  [..@.............]
FFFFFFFF7D977E40 00000000 00000000 00000000 00000000  [................]

Let’s test the line from address 0xFFFFFFFF7D977E20:

[oracle@bigmachine [v12102] trace]$ echo -e "\x44\x47\x54\x45 \x53\x54\x00\x00 \x32\xF6\xD9\x8B \x00\x00\x05\x90"
DGTE ST 2� �

So, the endianness determines how the raw memory contents should be read.

This post is about memory management on the operating system level of an Oracle database. The first question that might pop in your head is: isn’t this a solved problem? The answer is: yes, if you use Oracle’s AMM (Automatic Memory Management) feature, which let’s you set a limit for the Oracle datababase’s two main memory area’s: SGA and PGA. But in my opinion any serious, real life, usage of an Oracle database on Linux will be (severely) constrained in performance because of the lack of huge pages with AMM, and I personally witnessed very strange behaviour and process deaths with the AMM feature and high demand for memory.

This means that I strongly advise customers to use Oracle’s ASMM (Automatic Shared Memory Management) feature. In the newer versions of 11.2 I found this to be working very well. Earlier versions like 10.2 could suffer from an ever growing shared pool (which also means an ever shrinking buffer cache), especially when bind variables weren’t used. This still could happen, but it seems the SGA memory management feature in 11.2 handles this well in most cases. The ASMM feature means a fixed memory area is allocated for the SGA. SGA allocation has always been fixed outside of the AMM feature, as far as I know.

When ASMM doesn’t work, meaning the memory areas are getting sized wrong and performance is influenced by that, the last option is to size the memory area’s yourself. However, since version 11.2.0.2 Oracle will resize when the memory manager thinks it’s feasible. See Kurt van Meerbeek’s article about that.

That leaves the PGA (Process Global Area) as a memory area on itself. Most databases are using the automatic PGA memory management, which is enabled once the PGA_AGGREGATE_TARGET parameter is set to a non zero value. A common misunderstanding is this setting is actually limiting the overall PGA usage of an instance. The truth is automatic PGA memory management will make attempts to adhere to the PGA_AGGREGATE_TARGET value. These are the actual words in the official Oracle documentation: ‘attempts to adhere’!

This means sort memory, hash memory and bitmap memory will be actively limited in size per process by automatic PGA memory management, any attempt to allocate more than automatic PGA memory management allows will result in moving some contents of these memory areas to the assigned temporary tablespace of the database user, to make room for new data.

However, there are more memory area’s allocatable per process, which are never swapped to disk, thus always will stay in memory, and these could not be limited in an officially supported way prior to Oracle version 12. Two structures which are allocated in PGA and never swapped to disk are PL/SQL collections and PL/SQL tables. Creating and filling these requires the usage of PL/SQL (hence their names); the reason for mentioning this is that if your database is not used by PL/SQL but only SQL, you almost certainly will not run into the problem I describe below.

You might be thinking: wait a minute! Does this mean a developer can just create such a structure, and allocate whatever he/she likes, with all the consequences that it can have, like the operating system starting to swap, and can do that for every single process? Yes, this is what this means. This is why Oracle introduced a parameter called PGA_AGGREGATE_LIMIT with Oracle 12, to effectively limit the overall PGA heap size.

In case you wonder what this means, or even doubting my words, I have written a little program to demonstrate this behaviour.

This is the source code to create my test table T2:

exec dbms_random.seed('abracadabra');
create table t2
as
with generator as (
    select      rownum      id
    from        dual
    connect by
                rownum <= 1000
)
select
    rownum                                                id,
    trunc((rownum-1)/50)                            clustered,
    mod(rownum,20000)                               scattered,
    trunc(dbms_random.value(0,20000))               randomized,
    trunc(sysdate) + dbms_random.value(-180, 180)   random_date,
    dbms_random.string('l',6)                       random_string,
    lpad(rownum,10,0)                               vc_small,
    rpad('x',100,'x')                               vc_padding
from
    generator   g1,
    generator   g2
where
    rownum <= 1000000
;
exec dbms_stats.gather_table_stats(null,'T2');

This is a very smart way to generate a table. I actually borrowed this from Jonathan Lewis.

Next up, I created a small anonymous PL/SQL block to take the contents from the T2 table, and store them in a collection until I hit the limit in the variable ‘grow_until’.

declare
	type sourcetab is table of t2%ROWTYPE;
	c_tmp		sourcetab;
	c_def		sourcetab	:= sourcetab();
	v_b_p		number		:= 0;
	v_c_p		number		:= 0;
	v_b_u		number		:= 0;
	v_c_u		number		:= 0;
	grow_until	number		:= 700000000;
	p_a_t		number;
begin
	select value into v_b_p from v$mystat m, v$statname n where m.statistic#=n.statistic# and name = 'session pga memory max';
	select value into v_b_u from v$mystat m, v$statname n where m.statistic#=n.statistic# and name = 'session uga memory max';
	select value into p_a_t from v$parameter where name = 'pga_aggregate_target';
	select * bulk collect into c_tmp from t2;
	while v_c_p < grow_until loop
		for c in c_tmp.first .. c_tmp.last loop
			c_def.extend(1);
			c_def(c_def.last) := c_tmp(c);
			select value into v_c_p from v$mystat m, v$statname n where m.statistic#=n.statistic# and name = 'session pga memory max';
			select value into v_c_u from v$mystat m, v$statname n where m.statistic#=n.statistic# and name = 'session uga memory max';
			if v_c_p >= grow_until then
				exit;
			end if;
		end loop;
	end loop;
	dbms_output.put_line('vbp : '||v_b_p);
	dbms_output.put_line('vcp : '||v_c_p);
	dbms_output.put_line('vbu : '||v_b_u);
	dbms_output.put_line('vcu : '||v_c_u);
	dbms_output.put_line('pat : '||p_a_t);
end;
/

Please mind the session needs to have create table, create session granted, enough quota in the default tablespace and select on v_$mystat, v_$parameter and v_$statname granted.

This is run on an Oracle 12.1.0.2 database:

TS@v12102 > @pga_filler
vbp : 3535368
vcp : 700051976
vbu : 1103192
vcu : 4755704
pat : 524288000

PL/SQL procedure successfully completed.

The begin sizes of the UGA (vbu) and PGA (vbp) are 1’103’192 and 3’535’368. The PGA_AGGREGATE_TARGET size is set to 524’288’000 (500MB). I did set the grow_until variable to 700’000’000 (roughly 700MB), which is more than PGA_AGGREGATE_TARGET. After running this, it’s easy to spot the values of vcu (UGA allocation) and vcp (PGA allocation). vcu grew to 4’755’704 during the run, however vcp grew to 700’051’976, a little more than 700MB! This shows that the collection is stored in the PGA, and that the collection grew beyond the value set with PGA_AGGREGATE_TARGET.

This behaviour is consistent in versions 12.1.0.1, 11.2.0.4, 11.2.0.3, 11.2.0.2 and 11.2.0.1.

Let me emphasise once again that the above proof of concept code managed to allocate more memory than was set for the overall PGA usage of the entire instance. This can have an enormous, devastating impact on a consolidated database setup (meaning having multiple instances running on a single machine). Typically, once memory consumption of all the processes exceeds physically available memory, the operating system tries to use the swap device, to which it will swap memory pages in and out depending on memory usage of active (=on CPU) processes. Mild swapping shows as severely slowed-down processing (because a number of memory pages for processing need to be read from the swap device and placed in memory, from which the former contents need to be written to the swap device), heavy swapping shows as the machine coming down to a standstill.

Please mind that a diagnosis on the state of memory usage (alias swapping), just by looking at the amount of used swap (as can be seen in the ‘top’ output, or ‘swapon -s’) could be misleading. It’s also important to look at actual swapping in and out, as can be seen with ‘vmstat 1′ (si/so columns) or swap -W. I’ve found several systems which had been running for some time (approximately longer than a month) that had swap usage, sometimes up to 40%, while no ‘active swapping’, so memory pages being transfered to and from the swap device, was happening.

Luckily, starting with Oracle 12 you can actually limit overall PGA usage using the parameter PGA_AGGREGATE_LIMIT. The default value is the greater of (list from Oracle documentation):
a) 2GB
b) 200% of PGA_AGGREGATE_TARGET parameter (or lower if 200% > (90% of physical memory – total SGA size) but not below 100%)
c) 3MB * PROCESSES parameter
The parameter can not set below it’s default value, except when set in a pfile or spfile.

Let’s set the PGA_AGGREGATE_LIMIT to 600MB and see what happens when we start doing a large allocation again:

SQL> alter system set pga_aggregate_limit=600m scope=spfile;

System altered.

SQL> startup force;

Okay, let’s run the pga_filler.sql script again, and try to allocate 900MB. This means the “grow_until” variable must be set to 900000000.
PLEASE MIND this is done as a regular user, the SYS user and background processes other than job queue processes are not subject to the limiting.

TS@v12102 > @pga_filler
declare
*
ERROR at line 1:
ORA-01423: error encountered while checking for extra rows in exact fetch
ORA-00039: error during periodic action
ORA-04036: PGA memory used by the instance exceeds PGA_AGGREGATE_LIMIT
ORA-06512: at line 21

Great! Exactly like we expect, right?
Well…yes, but let’s look at the alert.log

Sat Dec 13 15:08:57 2014
Errors in file /u01/app/oracle/diag/rdbms/v12102/v12102/trace/v12102_ora_4147.trc  (incident=46599):
ORA-04036: PGA memory used by the instance exceeds PGA_AGGREGATE_LIMIT
Incident details in: /u01/app/oracle/diag/rdbms/v12102/v12102/incident/incdir_46599/v12102_ora_4147_i46599.trc
Sat Dec 13 15:09:07 2014
Dumping diagnostic data in directory=[cdmp_20141213150907], requested by (instance=1, osid=4147), summary=[incident=46599].
Sat Dec 13 15:09:09 2014
Sweep [inc][46599]: completed
Sweep [inc2][46599]: completed

Okay, essentially, this tells us nothing interesting, except for the tracefile. Let’s look in/u01/app/oracle/diag/rdbms/v12102/v12102/trace/v12102_ora_4147.trc, being the tracefile as indicated in the above alert.log snippet:

*** 2014-12-13 15:08:57.351
Process may have gone over pga_aggregate_limit
Just allocated 65536 bytes
Dumping short stack in preparation for potential ORA-4036
----- Abridged Call Stack Trace -----
ksedsts()+244<-ksm_pga_limit_short_stack()+1016<-ksm_check_over_limit()+469<-ksmarfg()+574<-kghgex()+1376<-kghfnd()+361<-kghalo()+4422<-kghgex()+414<-kghfnd()+361<-kghalo()+4422<-kghgex()+414<-kghfnd()+361<-kghalo()+4422<-kghgex()+414<-kghalf()+1003<-klmalf()+103
<-kllcqas()+194<-kcblasm()+108<-kxhfNewBuffer()+607<-qerhjSplitBuild()+632
----- End of Abridged Call Stack Trace -----
=======================================
PRIVATE MEMORY SUMMARY FOR THIS PROCESS
---------------------------------------
******************************************************
PRIVATE HEAP SUMMARY DUMP
781 MB total:
   781 MB commented, 646 KB permanent
   208 KB free (0 KB in empty extents),
     779 MB,   2 heaps:   "koh-kghu call  "            57 KB free held
------------------------------------------------------
Summary of subheaps at depth 1
779 MB total:
   778 MB commented, 110 KB permanent
    63 KB free (0 KB in empty extents),
     667 MB, 42786 chunks:  "pmuccst: adt/record       "
      83 MB, 5333 chunks:  "pl/sql vc2                "

Actually, this is the end of the tracefile. It seems that the pga limit dump (the text in between “Process may have gone over pga_aggregate_limit” to the private memory summary heap dumps) occurs several times before an actual ORA-4036 is triggered. In my private test instance, where I am obviously the only user process doing something, I get a pga limit dump approximately 20 times before the ORA-4036 is actually triggered:

sending 4036 interrupt
Incident 46599 created, dump file: /u01/app/oracle/diag/rdbms/v12102/v12102/incident/incdir_46599/v12102_ora_4147_i46599.trc
ORA-04036: PGA memory used by the instance exceeds PGA_AGGREGATE_LIMIT

Did you actually spot the oddity here?

Remember the PGA_AGGREGATE_LIMIT was set to 600M. Now look at the process’ PGA/Private heap summary dump above: it says 781M. Please mind the 781M is the PGA heap of a SINGLE process! When looking at the total PGA allocated for the entire instance, it’s even more:

SYS@v12102 AS SYSDBA> select value/power(1024,2) "MB" from v$pgastat where name = 'maximum PGA allocated';

	MB
----------
1041.16699

So…despite PGA_AGGREGATE_LIMIT set to 600M, according to the v$pgastat view, there’s 1041MB allocated for PGA. Please mind I haven’t looked into how accurate v$pgastat is, but I tend to believe this.

Summary.
I’ve seen PGA_AGGREGATE_TARGET being used as a calculation value for actual PGA usage of an instance. This is simply wrong. The actual amount of PGA memory allocated by the instance is highly depended on what is done, and can be less than PGA_AGGREGATE_TARGET, or more. Automatic PGA can control three per process memory area’s: the sort, hash and bitmap memory area’s. These are sized based on the setting of PGA_AGGREGATE_TARGET and the actual PGA memory usage instance wide. If more memory is needed for sort, hash or bitmap memory than is made available by the memory manager, excess memory needed is allocated in the temporary tablespace. Any other PGA memory allocation is always done, regardless of the setting of PGA_AGGREGATE_TARGET.

Starting with Oracle 12, it seems the actual PGA allocation now can actually be limited with the new parameter PGA_AGGREGATE_LIMIT. However, during some simple testing it shows that actually more memory is allocated than set with PGA_AGGREGATE_LIMIT as limit. I haven’t tested it in more situations, this post is meant to grow awareness that the actual limit as set by PGA_AGGREGATE_LIMIT might not be that hard as you would expect.

Please mind, PGA_AGGREGATE_LIMIT seems to truly limit PGA usage instance wide, not limit the PGA heap per process, as event 10251 (PGA usage limiting way for Oracle 11.2) does. However, once again: PGA_AGGREGATE_LIMIT seems to try to be smart and actually does not limit at the exact size set, but beyond that.

The next post will introduce a way to limit PGA usage in Oracle 11.2. Stay tuned!

I posted a fair amount of stuff on how Oracle is generating IOs, and especially large IOs, meaning more than one Oracle block, so > 8KB. This is typically what is happening when the Oracle database is executing a row source which does a full segment scan. Let’s start off with a quiz: what you think Oracle is the maximum IO size the Oracle engine is capable of requesting of the Operating System (so the IO size as can be seen at the SCI (system call interface) layer? If you made up your answer, remember it, and read on!

The real intention of this blogpost is to describe what is going on in the Oracle database kernel, but also what is being done in the Linux kernel. Being a performance specialised Oracle DBA means you have to understand what the operating system does. I often see that it’s of the utmost importance to understand how an IO ends up as a request at the NAS or SAN head, so you understand what a storage admin is talking about.

Many people (including myself in the past) would state that the maximum IO size on Linux is 1MB. For the Linux 2.6 kernel and higher this statement is incorrect because there is no such thing as a single maximum IO size on Linux. There used to be one in the Linux 2.4 era, which was set with the maxphys parameter, but that time is long gone. In order to find out what it is now, let’s test and see!

First let’s get a Linux system and a big table!
The system I got is a VMWare Fusion VM, running Linux 3.8.13-44.1.5.el6uek.x86_64 (UEK3) on Oracle Linux 6u6. On top of that I am using the Oracle database and grid infrastructure version 12.1.0.2. This system is using udev for providing disk access (as opposed to asmlib)
The redundancy mode of ASM is external, although for reading (what I will be covering) this doesn’t matter.

Now let’s get a normal database session, and use a combination of sql_trace with waits (10046/8) and strace to see how the Oracle database interfaces with the kernel. Please mind I’ve prepared a heap table with no indexes on it, so a count(*) on it always will result in a full table scan. Also, the buffercache is sized small enough (or the table is created large enough, it depends on how you look at it) to have the session make the decision to do a direct path read, instead of a buffered read. If you don’t know what that means: please search this blog on direct path reads, or even better, download my presentation ‘about multiblock reads’.

The direct path read decision is visible via the ‘direct path read’ wait event. If you get a full table scan operation and see ‘db file scattered read’ waits, you are doing a buffered scan.

In the most cases, you will get a maximum value of 1MB if possible, which seems to support the generally assumed 1MB maximum operating system induced IO size. Why? Well, because you probably set the DB_FILE_MULTIBLOCK_READ_COUNT parameter to 128, which means you have explicitly set the Oracle process not to do IO with a size more than 1MB (8192*128). In this blogpost, I explain that Oracle can request IOs bigger than 1MB.

In the blogpost series on extra huge database IOs, I show that Oracle can do huge (1MB+) IOs, but the physical request size (what Oracle actually requests at the SCI layer, visible with the pread/pwrite/io_submit/io_getevents functions) still is 1MB. This limit is imposed by the physical storage structure which the database uses with ASM, called allocation unit (often called ‘AU’). The default size of an allocation unit is 1MB. The allocation unit can be seen in both the database and the ASM instance with the following query:

SYS@+ASM AS SYSASM> select name, allocation_unit_size from v$asm_diskgroup;

NAME			       ALLOCATION_UNIT_SIZE
------------------------------ --------------------
DATA					    1048576

How about doing an unbuffered read on a filesystem? I’ve created a database on an (XFS, but this doesn’t matter AFAIK) filesystem, and tried to set the maximum value to DB_FILE_MULTIBLOCK_READ_COUNT. I’ve done this by setting DB_FILE_MULTIBLOCK_READ_COUNT to 10000 (ten thousand), and then bounce the database to see what the number has become. In my case, the value became 4096. I think this is the limit for Oracle 12.1.0.2 on Linux x86_64, but love to hear if you have gotten different results:

I set 10000:

SYS@fv12102 AS SYSDBA> select name, value from v$spparameter where name like 'db_file_multiblock%';

NAME						   VALUE
-------------------------------------------------- ----------------------------------------------------------------------
db_file_multiblock_read_count			   10000

But Oracle limits this to 4096:

SYS@fv12102 AS SYSDBA> select name, value from v$parameter where name like 'db_file_multiblock%';

NAME						   VALUE
-------------------------------------------------- ----------------------------------------------------------------------
db_file_multiblock_read_count			   4096

Okay. Let’s start our investigation at that point: a database which is set up with a DB_FILE_MULTIBLOCK_READ_COUNT set to 4096, alias 32MB (with a block size of 8KB), and a table which got extents large enough to accommodate huge (32MB) IOs.

Fire up a session regular database session, and enable sql trace at level 8:

$ sqlplus ts/ts@//localhost/v11204
...
SQL> alter session set events 'sql_trace level 8';

Now start another session as root on the database server, and find the PID of the server process of the sqlplus process we just created above. Issue strace with verbose writing setting:

# strace -e write=all -e all -p PID
Process PID attached - interrupt to quit
read(14,

Okay, we are setup and ready to go, but there is one additional thing: the way direct path reads work, they would probably give little waits with fast IO capabilities. One way to get the waits back, is to limit the IO capabilities of the process. Doing so is documented in this article.

Now issue the full table scan on a large table in sqlplus while strace is attached:

SQL> select count(*) from bigtab;

Now take a peek at the strace output!
The output first shows IOs as we expect:

io_getevents(139717184229376, 1, 128, {{0x7f126dd3d780, 0x7f126dd3d780, 33554432, 0}}, {600, 0}) = 1
times(NULL)                             = 431386800
write(7, "\n*** 2014-11-24 13:09:28.028\n", 29) = 29
 | 00000  0a 2a 2a 2a 20 32 30 31  34 2d 31 31 2d 32 34 20  .*** 201 4-11-24  |
 | 00010  31 33 3a 30 39 3a 32 38  2e 30 32 38 0a           13:09:28 .028.    |
lseek(7, 0, SEEK_CUR)                   = 31181
write(7, "WAIT #139717129509840: nam='dire"..., 130) = 130
 | 00000  57 41 49 54 20 23 31 33  39 37 31 37 31 32 39 35  WAIT #13 97171295 |
 | 00010  30 39 38 34 30 3a 20 6e  61 6d 3d 27 64 69 72 65  09840: n am='dire |
 | 00020  63 74 20 70 61 74 68 20  72 65 61 64 27 20 65 6c  ct path  read' el |
 | 00030  61 3d 20 33 39 30 37 33  30 20 66 69 6c 65 20 6e  a= 39073 0 file n |
 | 00040  75 6d 62 65 72 3d 34 20  66 69 72 73 74 20 64 62  umber=4  first db |
 | 00050  61 3d 37 34 31 33 37 36  20 62 6c 6f 63 6b 20 63  a=741376  block c |
 | 00060  6e 74 3d 34 30 39 36 20  6f 62 6a 23 3d 32 30 34  nt=4096  obj#=204 |
 | 00070  37 34 20 74 69 6d 3d 31  39 32 30 30 37 31 30 31  74 tim=1 92007101 |
 | 00080  39 39                                             99                |

What is visible here, is first the reap of an I/O request (with asynchronous IO on Linux this is typically the io_getevents() call). If you take a close look at the arguments of the io_getevents() call (taken from the manpage of io_getevents):

int io_getevents(aio_context_t ctx_id, long min_nr, long nr, struct io_event *events, struct timespec *timeout);

And then focus on the struct io_event:

struct io_event {
         __u64           data;           /* the data field from the iocb */
         __u64           obj;            /* what iocb this event came from */
         __s64           res;            /* result code for this event */
         __s64           res2;           /* secondary result */
};

The above description is taken from the annotated Linux kernel source, as available here: http://lxr.free-electrons.com/source/include/uapi/linux/aio_abi.h#L58 I use this site for navigating the Linux kernel source. What is important, is that the third field (io_event.res) contains the size of the IO request. Having learned this, now look again in the io_getevents call. The size of the IO reaped above is 33554432, which is 33554432/1024/1024=32 MB. Yes, that’s a single IO of 32MB! Also, this is consistent with the wait line a little lower:

 | 00050  61 3d 37 34 31 33 37 36  20 62 6c 6f 63 6b 20 63  a=741376  block c |
 | 00060  6e 74 3d 34 30 39 36 20  6f 62 6a 23 3d 32 30 34  nt=4096  obj#=204 |

Block count = 4096 * 8192 (block size) = 33554432

So, I wonder what you thought was possible, the correct answer on my operating system (Linux x86_64) with Oracle 12.1.0.2 is 32MB. It turned out the big IOs in the ASM case were limited by the allocation unit size of 1MB.

The next thing I’ve wondered is how this matches with the maximum IO size of the disk devices as visible by the Operating System. You can request 32MB, but a normal SCSI disk doesn’t do 32MB IOs. Of course in my case the SCSI disk really is a VMWare virtual disk device.

Let’s keep the 32MB IO in mind, now dive from the top layer, the SCI (system call interface) where an IO enters the kernel to the bottom of the kernel from an IO perspective, to the block device. The block device settings are found in /sys/block/DEVICE/queue. The maximum IO size the device is capable of is found in max_hw_sectors_kb. This is in kilobytes, and read only (can’t change hardware, right?). In my case this is:

[root@bigmachine queue]# cat max_hw_sectors_kb
4096

My disk supports a maximum of 4M for an IO size! But this is not what is used, the actual setting is in max_sectors_kb:

[root@bigmachine queue]# cat max_sectors_kb
512

That’s half a megabyte!

So…we got (up to) 32MB sized IO requests coming in, and a device that is set to 512KB IOs. This means that somewhere between the SCI and the device, there is a mechanism to scatter the request size to the device’s maximum IO size, and once the IO requests are done, going back to gather the IO results to the original request.

There are a couple of layers in the Linux kernel through which the call travels (including common functions):

-SCI/system call interface: system_call, sys_io_submit…. (io_submit, do_io_submit, io_submit_one; these seem to be in the VFS layer)
-VFS/virtual filesystem: aio_run_iocb, do_aio_read, xfs_file_read_iter, generic_file_read_iter, xfs_vm_direct_IO, bio_*, kiocb_batch_refill
-Block layer: blk_finish_plug, blk_flush_plug_list, queue_unplugged, __blk_run_queue, blk_run_queue
-SCSI layer: scsi_*
-Device driver: mptspi_qcmd, mptscsih_qcmd, mpt_put_msg_frame

(note: there seems to be consensus the above mentioned layers exist, although there is different wording and different numbers by different sources. Also, there doesn’t seem to be a very clear description of what is done by which layer, and what typically defines a kernel layer. For some functions it is clear they belong to a certain layer (for example aio_run_iocb in Linux/fs/aio.c, bulk_finish_plug in Linux/block/blk-core.c, etc.), for some layers, like the SCI layer, it seems there isn’t a clear layer definition by looking at where the function is defined. Also please mind the SCSI layer is implemented as a driver, just like the actual device driver for the hardware. This is very understandable, but makes it a bit harder to see it in a layered way)

System Call Interface (SCI)
The request enters kernel space via the SCI. The function of the SCI is to elevate a process to system priority to perform a kernel mode task, like (but not limited to) doing I/O. The system call implementation on Linux makes use of a wrapper function in glibc, which executes the system call on behalf of the user systemcall request. The reason for mentioning this, is that sometimes the glibc wrapper “hides” the real system call, for example calling the semtimedop() function:

(gdb) break semtimedop
Breakpoint 1 at 0x3bb38eb090: file ../sysdeps/unix/syscall-template.S, line 82.
(gdb) c
Continuing.

Breakpoint 1, semtimedop () at ../sysdeps/unix/syscall-template.S:82
82	T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)

Above is a gdb (GNU debugger) session which attaches to an Oracle background process, which I know is sleeping in the system call semtimedop() when idle, A breakpoint is set on the semtimedop function, and the execution of the attached process is resumed. It then breaks on the function, showing the source code at which the break happened. Instead of showing the actual semtimedop function, it shows the pseudo function in glibc which wraps this system call. This hides the arguments of calling the semtimedop() function. My current workaround is to read the kernel registers which “carry” the arguments (RDI, RSI, RDX, RCX, R8, R9 for the first 6 arguments in most cases).

Virtual File System (VFS)
The next layer is virtual filesystem. Here we see functions specific to asynchronous IO or synchronous IO, and doing direct IO or not, and also actual filesystem specific functions (in my case xfs, when ext4 is used, you will see specific functions for that. I highly recommend XFS!). This layer also uses a structure called ‘request_queue’, which keeps track of the actual IO requests for a block device, of which each individual request is a struct ‘request’, which contains one or more structs called ‘bio’ which contains a description of the request, which points to structure called ‘bio_vec’, which points to pages for storing the disk request contents. This is all setup and created in kernel memory by the user process in system mode. It’s my assumption that the properties of the disk device (=maximum advertised IO size) are taken into account when the VFS filesystem implementation creates requests and all necessary structs and memory area’s. Please mind it’s important that enough memory is available to setup the necessary structures, and enough CPU to make this happen. Also some of the crucial structures for doing IO (request, bio, bio_vec) seem to be setup in this layer. An IO can’t be done without a memory area for the IO request to hold the data for sending it to the device (alias a write), or a memory area for the IO request to hold the data which is fetched from the device (alias a read).

The funny thing is that when you use ASM (the simple version 11.2 ASM with a local ASM instance and local disk devices), you will still see some functions of the VFS layer, because you use a disk device which is opened using the local filesystem. Examples of these functions are: aio_run_iocb, do_aio_read.

Block Layer
The next layer is the block layer. Here the request queue is handled, and I/O scheduling is done. Oracle advises the deadline scheduler in all cases. The scheduler works by plugging a request queue, much like a plug in your bathtub, letting the requests enter the queue. Having multiple requests in a queue means it can be optimised by reordering the requests, and merging adjacent requests up to the device’s advertised maximum IO size. Once a request’s timeout expires, or the requesting process finishes submitting IO, the queue is unplugged.

SCSI layer
The SCSI layer is responsible for communicating with SCSI devices to do IOs.

Device driver
The device driver layer is the layer that truly physically communicates with a device, and implements the device specific communication. In my case the functions start with mpt, which is the driver for LSI PCI adapters.

To see how the flow of IO going through the block layer, there is a tool called blktrace. Actually this is a mini-suite of tools consisting of blktrace (tracing the IO requests through the block layer), blkparse (parsing the output of blktrace to make it human readable), btrace (script to combine blktrace and blkparse, and btt (a blktrace output post processing tool)), among others.

In order to use blktrace, the debug file system of the Linux kernel needs to be mounted. Here is how that is done:

# mount -t debugfs debugfs /sys/kernel/debug

If the kernel debugfs is not mounted, you get the following message:

[root@bigmachine ~]# btrace /dev/oracleasm/disk1
Invalid debug path /sys/kernel/debug: 0/Success

I use blktrace in this article for looking at the IO requests to understand what is going on. The workflow for this use of blktrace is:
– create a trace file of the block flow using blktrace
– make the trace file human readable via blkparse or analyse via btt (block trace times)

Actually, you can parse the output of blktrace directly via blkparse using ‘blktrace -d DEVICE – | blkparse -i -‘. To make that even simpler, the script ‘btrace’ is created, to do exactly that.

Here’s how that looks like (depending on the number of processes using it, the output can be huge, this is only a snippet):

[root@bigmachine ~]# btrace /dev/oracleasm/disk1
...
  8,16   0       57     0.260669503  2421  Q  WS 4088 + 8 [asm_gmon_+asm]
  8,16   0       58     0.260672502  2421  G  WS 4088 + 8 [asm_gmon_+asm]
  8,16   0       59     0.260673231  2421  P   N [asm_gmon_+asm]
  8,16   0       60     0.260674895  2421  I  WS 4088 + 8 [asm_gmon_+asm]
  8,16   0       61     0.260675745  2421  U   N [asm_gmon_+asm] 1
  8,16   0       62     0.260677119  2421  D  WS 4088 + 8 [asm_gmon_+asm]
  8,16   0       63     0.260882884     0  C  WS 4088 + 8 [0]
...

What is shown here, is the typical flow of an IO in the block layer:
Q – Queue. A request starts off sending a notification on the intent to queue at the given location.
G – Get request. A struct request is allocated.
P – Plug. When the block device queue is empty, the queue is plugged in order to receive further IOs and have the ability to optimise (merge and/or reorder) them before the data is sent to the device.
I – Insert. A request is sent to the IO scheduler for addition to the internal queue and later service by the driver. The request is fully allocated at this time.
U – Unplug. The start of sending requests to the driver.
D – Driver. A request has been sent to the driver and removed from the queue.
C – Complete. A previously issued request to the driver has been completed.

The main point is here, that you can truly see how the IO requests flow through the block layer and are issued to the storage device, in other words, you can see how the block layer receives the IOs, and what is exactly submitted to the driver as request for the physical storage layer.

This is a microscopic view of the disk IOs. In most cases, when you want to gain information on block layer IO processing, another view on it is provided by processing blktrace output with btt. This is an example output of btt:

First capture IO events using blktrace:

[root@bigmachine ~]# blktrace -w 60 -d /dev/oracleasm/disk1 -o - | blkparse -d sdb.blkparse -i -

In this example I captured IOs for 60 seconds. You can exclude ‘-w 60′, and press interrupt (ctrl-c) when you deem IO recording is enough. This produces a binary file ‘sdb.blkparse’, which can be used btt:

This is the first part, the flow through the block layer until IO completion:

==================== All Devices ====================

            ALL           MIN           AVG           MAX           N
--------------- ------------- ------------- ------------- -----------

Q2Q               0.000000001   0.239795347   3.002829973         238
Q2G               0.000000001   0.159337842   3.011192142         264
G2I               0.000000679   0.000001724   0.000011618         264
I2D               0.000000764   0.000007633   0.000153436         264
D2C               0.000000001   0.103328167   3.012509148         233
Q2C               0.000000001   0.270961298   3.012516496         233

Note: time is in milli seconds.
Q2Q – Time between IO requests.
Q2G – Time it takes for a request struct to be allocated.
G2I – Time it takes for the request to be inserted in the device’s queue.
I2D – Time spend in the device queue waiting to be issued to the driver.
D2C – Time spend between issuing to the driver and completion of the request. This includes controller, storage. This is the same figure as the ‘svctm’ column with iostat -x.
Q2C – Total time spend in block layer and physical IO. This is the same figure as the ‘await’ column with iostat -x.

The second part is the device overhead section:

==================== Device Overhead ====================

       DEV |       Q2G       G2I       Q2M       I2D       D2C
---------- | --------- --------- --------- --------- ---------
 (  8, 16) |  66.6284%   0.0007%   0.0000%   0.0032%  38.1339%
---------- | --------- --------- --------- --------- ---------
   Overall |  66.6284%   0.0007%   0.0000%   0.0032%  38.1339%

This is partly the same as the IO flow table above. This is expressed as a percentage of where the total time of the IO is spend.
Q2G – Request struct allocation.
G2I – Insertion in the device queue.
Q2M – Total time until merge.
I2D – Time spend in the queue until it was dispatched to the driver.
D2C – Time spend on doing the IO after submitting the request to the driver.

This is a quick post on using git on a server. I use my Synology NAS as a fileserver, but also as a git repository server. The default git package for Synology enables git usage on the command line, which means via ssh, or via web-DAV. Both require a logon to do anything with the repository. That is not very handy if you want to clone and pull from the repository in an automated way. Of course there are ways around that (basically setting up password-less authentication, probably via certificates), but I wanted simple, read-only access without authentication. If you installed git on a linux or unix server you get the binaries, but no daemon, which means you can only use ssh if you want to use that server for central git repositories.

Running git via inetd
What I did is using inetd daemon to launch the git daemon. On any linux or unix server with the inetd daemon, and on Synology too, because it uses linux under the covers, it’s easy to setup git as a server.

First, check /etc/services for the following lines:

git               9418/tcp                   # git pack transfer service
git               9418/udp                   # git pack transfer service

Next, add the following line in the inetd.conf (which is /etc/inetd.conf on my synology):

git stream tcp nowait gituser /usr/bin/git git daemon --inetd --verbose --export-all --base-path=/volume1/homes/gituser

What you should look for in your setup is:
– gituser: this is the user which is used to run the daemon. I created a user ‘gituser’ for this.
– /usr/bin/git: of course your git binary should be at that fully specified path, otherwise inetd can’t find it.
– git daemon:
— –inetd: notify the git executable that it is running under inetd
— –export-all: all git repositories underneath the base directory will be available
— –base-path: this makes the git root directory be set to this directory. In my case, I wanted to have all the repositories in the home directory of the gituser, which is /volume1/homes/gituser in my case.

And make the inetd deamon reload it’s configuration with kill -HUP:

# killall -HUP inetd

Please mind this is a simple and limited setup, if you want to set it up in a way with more granular security, you should look into gitolite for example.

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