CODEWITHSUNDEEP
linuxperf
Ben J
Ben J

Reputation: 2382

Read performance counters periodically in linux

Is there a way to read performance counters periodically in linux?

Something like perf stat with the ability to sample every X cycles is what I'm looking for.

Basically I would like to be able to read the instruction counter (number of instructions executed) every X amount of cpu cycles for some program.

Upvotes: 7

Views: 4597

Answers (3)

Moncef M.
Moncef M.

Reputation: 1293

Good news: In the next kernel (Linux 3.9), perf stat will have an option -I msecs to print event deltas at regular time intervals.

https://patchwork.kernel.org/patch/2004891/

$ perf stat -I 1000 -e cycles noploop 10
noploop for 10 seconds
1.000086918         2385155642 cycles                    #    0.000 GHz
2.000267937         2392279774 cycles                    #    0.000 GHz
3.000385400         2390971450 cycles                    #    0.000 GHz
4.000504408         2390996752 cycles                    #    0.000 GHz
5.000626878         2390853097 cycles                    #    0.000 GHz

http://man7.org/linux/man-pages/man1/perf-stat.1.html

-I msecs, --interval-print msecs

Print count deltas every N milliseconds (minimum: 10ms)

Upvotes: 8

Guru Prasad
Guru Prasad

Reputation: 4223

You can easily modify perf stat to do this.

In fact, I have a crude modification already implemented and would be glad to share this change with you..

The changes I made are mostly in the run_perf_stat function within the while(!done) loop

Just move the lines below the while(!done) {sleep(1);} to inside the loop and change the sleep to a nanosleep with the time period that you wish to sample at

That should make perf print the output on STDOUT(or STDERR)

If you wish to store these values, I suggest you create a 2-Dimensional array of type struct stats, update this with every sample and write periodically to a file

Upvotes: 0

jpa
jpa

Reputation: 12156

It seems that the perf tool in Linux works by recording an event when the counters reach a specific value, rather than sampling at regular intervals.

Command perf record -e cycles,instructions -c 10000 stores an event every 10000 cycles and every 10000 instructions. It can be run against a new command or an existing pid. It records to perf.data in current directory.

Analyzing the data is another matter. Using perf script gets you quite close:

ls 16040 2152149.005813: cycles:          c113a068  ([kernel.kallsyms])
ls 16040 2152149.005820: cycles:          c1576af0  ([kernel.kallsyms])
ls 16040 2152149.005827: cycles:          c10ed6aa  ([kernel.kallsyms])
ls 16040 2152149.005831: instructions:          c1104b30  ([kernel.kallsyms])
ls 16040 2152149.005835: cycles:          c11777c1  ([kernel.kallsyms])
ls 16040 2152149.005842: cycles:          c10702a8  ([kernel.kallsyms])
...

You need to write a script that takes a bunch of lines from that output and counts the number of 'cycles' and 'instructions' events in that set. You can adjust the resolution by changing the parameter -c 10000 in the recording command.

I verified the analysis by running perf stat and perf record against ls /. Stat reported 2 634 205 cycles, 1 725 255 instructions, while script output had 410 cycles events and 189 instructions events. The smaller the -c value, the more overhead there seems to be in the cycles reading.

There is also a -F option to perf record, which samples at regular intervals. However, I could not find a way to retrieve the counter values when using this option.

Edit: perf stat apparently works on pids also, and captures data until ctrl-c is pressed. It should be quite easy to modify the source so that it always captures for N seconds and then run it in a loop.

Upvotes: 5

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