I am trying to familiarize myself with perf and run it against various programs I wrote.
When I launch it against program that is 100% single threaded, perf shows that it takes two cores on machine (task-clock event). Here's the example output:
perf stat -a --per-core python3 test.py
Performance counter stats for 'system wide':
S0-C0 1 19004.951263 task-clock (msec) # 1.000 CPUs utilized (100.00%)
S0-C0 1 5,582 context-switches (100.00%)
S0-C0 1 19 cpu-migrations (100.00%)
S0-C0 1 3,746 page-faults
S0-C0 1 <not supported> cycles
S0-C0 1 <not supported> stalled-cycles-frontend
S0-C0 1 <not supported> stalled-cycles-backend
S0-C0 1 <not supported> instructions
S0-C0 1 <not supported> branches
S0-C0 1 <not supported> branch-misses
S0-C1 1 19004.950059 task-clock (msec) # 1.000 CPUs utilized (100.00%)
S0-C1 1 6,752 context-switches (100.00%)
S0-C1 1 25 cpu-migrations (100.00%)
S0-C1 1 935 page-faults
S0-C1 1 <not supported> cycles
S0-C1 1 <not supported> stalled-cycles-frontend
S0-C1 1 <not supported> stalled-cycles-backend
S0-C1 1 <not supported> instructions
S0-C1 1 <not supported> branches
S0-C1 1 <not supported> branch-misses
19.004688019 seconds time elapsed
It even shows that simple sleep command takes two cores on my computer and I can't explain this. I understand that OS scheduler can reassign active core for any process, but in this case CPU utilization would reflect that.
Can anyone explain this?
According to man page of perf stat subocmmand, you have -a option to profile full system:
http://man7.org/linux/man-pages/man1/perf-stat.1.html
-a, --all-cpus
system-wide collection from all CPUs (default if no target is
specified)
In this "system-wide" mode perf stat (and perf record too) will count events on (or profile for record) all CPUs in the system. When used without additional argument of command, perf will run until interrupted by Ctrl-C. With argument of command, perf will count/profile until the command works. Typical usage is
perf stat -a sleep 10 # Profile counting every CPU for 10 seconds
perf record -a sleep 10 # Profile with cycles every CPU for 10 seconds to perf.data
For getting stats of single command use single process profiling (without -a option)
perf stat python3 test.py
For profiling (perf record) you may run without -a option; or you may use -a and later do some manual filtering in perf report, focusing only on the pids/tids/dsos of your application (This can be very useful if command to profile uses some interprocess requests to other daemons to do lot of CPU work).
--per-core, -A, -C <cpulist>, --per-socket options are only for system-wide -a mode. Try --per-thread with -p pid attach to process option.