Messages in this thread | | | From | Robert Bernecky <> | Subject | PAPI vs. perf stat | Date | Fri, 8 Jul 2011 21:13:49 +0000 (UTC) |
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This is actually three questions about perf stat:
1. I have been using PAPI and PAPIEX with excellent results, in the sense that I obtained extremely reproducible instruction counts, varying by only a few hundred instructions over billions of instructions executed. This was on an Opteron 165.
I have been forced to move to a new platform and a newer version of Ubuntu, and decided to try out "perf stat" and friends, rather than going through the tedious task of kernel mods for PAPI.
What I now observe (albeit on a new CPU/MB -- AMD Phenom 1075T) with perf stat is disturbing: Instruction counts vary by several percent. E.g., repeated execution of the same binary, foo, gives me:
perf stat foo
71156657 instructions 71628306 instructions 71613890 instructions 71638216 instructions 71731479 instructions 71564788 instructions
This is on a lightly loaded system with web browser, email, and other tasks running, which is the same environment that I was using with PAPI.
I am curious as to why it is that "perf" does not have the same degree of precision as PAPI.
[From looking at the PAPI kernel mods, it seems that HMI counters are saved at task dispatch, then sampled again at interrupt time, and the differences added to task(process?)-specific fields. Hence, the only variance in instruction counts (aside from page faults, etc.) arise from interrupts happening during task execution. Several kernel instructions are executed between the time of interrupt and counter sampling, and similarly at task dispatch time. ]
Is there a way to improve the precision of "perf" measurements?
2. The Opteron 165 under PAPI shows PAPI_VEC_INS (vector instruction counts) as well as PAPI_TOT_INT (total instruction count). "perf list" (on the Phenom 1075T) does show "instructions" but I do not see an entry for vector instruction counts. Any ideas what may be going on here?
3. I have an on-going process running, and would like to make automated measurements of HMI data at desired (not periodic) intervals, from another shell. is there a way to do this with perf? I see that "perf stat -p PIDNUMBER" almost works, but it requires that I manually hit CTRL-C to terminate the sample.
Thanks. Robert
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