This is a small blogpost on using ‘perf’. I got an error message when I tried to run ‘perf top’ systemwide:
# perf top Too many events are opened. Try again after reducing the number of events
What actually is the case here, is actually described in the perf wiki:
Open file limits
The design of the perf_event kernel interface which is used by the perf tool, is such that it uses one file descriptor per event per-thread or per-cpu.
On a 16-way system, when you do:
perf stat -e cycles sleep 1
You are effectively creating 16 events, and thus consuming 16 file descriptors.
The point for this blogpost is perf (in Oracle Linux 7.1) says ‘too many events’, and hidden away in the perf wiki the true reason for the message is made clear: perf opens up a file descriptor per cpu thread, which means that if you are on a big system you might get this message if the open files (file descriptors actually) limit is set lower than the number of cpu threads.
You can see the current set limits using ‘ulimit -a’:
$ ulimit -a core file size (blocks, -c) unlimited data seg size (kbytes, -d) unlimited scheduling priority (-e) 0 file size (blocks, -f) unlimited pending signals (-i) 189909832 max locked memory (kbytes, -l) 21878354152 max memory size (kbytes, -m) unlimited open files (-n) 1024 pipe size (512 bytes, -p) 8 POSIX message queues (bytes, -q) 819200 real-time priority (-r) 0 stack size (kbytes, -s) 10240 cpu time (seconds, -t) unlimited max user processes (-u) 16384 virtual memory (kbytes, -v) unlimited file locks (-x) unlimited
If you are root, you can simply set the ‘open files’ limit higher than the amount of cpu threads, and perf will work:
# ulimit -n 10240