Re: [perfmon2] Question about event "BRANCH_INSTRUCTIONS_RETIRED" on x86 machine

[陳韋任 <ch...@ii...>]

Hi, Stephane

> The PMU takes some variable amount of time to interrupt after an overflow.
> During that time, execution continues.

  So the skid has nothing to do with "micro-op"?
 
> This "imprecision" cannot be corrected by software but only thru hardware.
> That's why you have Intel PEBS, for instance.

  You mean PEBS makes the skid disappered? Does `perf` or libpfm4 support this?
The example I showed was ran on a x86 machine, it seems `perf` does not use PEBS
feature.
 
> In summary, except for cycles you should do not expect profiles to point at
> instructions that generated the occurrences of the sampling event.

  That means we cannot trace back a few instructions and say some instruction
generates the event? Take "BRANCH_INSTRUCTIONS_RETIRED" for example,

0.00 :        401d56:       eb 12                   jmp    401d6a <fallbackSort+0x1aa>
2.23 :        401d58:       83 c0 01                add    $0x1,%eax

  Can I say the jmp before add generate the BRANCH_INSTRUCTIONS_RETIRED
event?

  Thanks! 

Regards,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Parallel Processing Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667