linux-kernel - Re: [RFC] Add --show-total-period for perf annotate

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Date:	Wed, 27 May 2015 11:04:39 +0200
From:	Martin Liška <mliska@...e.cz>
To:	Andi Kleen <andi@...stfloor.org>
CC:	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [RFC] Add --show-total-period for perf annotate

On 05/27/2015 10:46 AM, Martin Liška wrote:
> On 05/26/2015 07:03 PM, Andi Kleen wrote:
>>> Anyway, attached patch is capable of displaying milliseconds approximation for each instruction.
>>
>> You realize that the events perf is not counting do not directly map to
>> wall time? Even if you count cycles, the cycles are either stopping in idle
>> or changing unit as the CPU's frequencies change. For other events the
>> relationship is even more remote, think what happens when counting cache or
>> TLB misses.
>>
>> Also even if it was mapping to time somehow, it's just a hit, not a
>> duration, so it cannot say how long a individual instruction took.
>>
>> So you cannot map a sample event to time.
>>
>> To do what you want you would need to use something like processor
>> trace, which can do exact accounting.
>>
>> I think the only thing that makes sense is to account it relative to
>> the event counts.
>>
>> -Andi
>>
>
> Hello Andi.
>
> I realize all aspects and capabilities of perf infrastructure. Even though
> these numbers are not precise, I helped me a lot with debugging of a benchmark
> which heavily utilizes a single CPU and runs in magnitude of seconds.
>
> Ok, so let's convert the patch to feature that we can map an instruction
> to a percentage number of events (cycles) it takes.
>
> If I understand correctly, is it just about division of the number of events
> related to an instruction and total number of events?
>
> Thanks,
> Martin
>
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Hi.

Sample output to verify that we have the same idea in mind:

$ perf annotate --show-total-period

  Disassembly of section .text:
          :
          :      0000000000038890 <random_r>:
          :      __random_r():
     1695 :        38890:       test   %rdi,%rdi
        0 :        38893:       je     38918 <random_r+0x88>
        1 :        38899:       test   %rsi,%rsi
        0 :        3889c:       je     38918 <random_r+0x88>
        9 :        3889e:       mov    0x18(%rdi),%eax
     1833 :        388a1:       mov    0x10(%rdi),%rdx
        2 :        388a5:       test   %eax,%eax
        0 :        388a7:       je     388f8 <random_r+0x68>
      168 :        388a9:       mov    (%rdi),%rcx
        8 :        388ac:       mov    0x8(%rdi),%r8
     1325 :        388b0:       mov    0x28(%rdi),%r9

Where:
$ perf report | head

# To display the perf.data header info, please use --header/--header-only options.
#
# Samples: 44K of event 'cycles'
# Event count (approx.): 42988831618

Thank for ideas,
Martin

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