AutoFDO profile toolchain is open-sourced

[Dehao Chen]

On Tue, Apr 21, 2015 at 6:42 AM, Ilya Palachev <i.palachev@samsung.com> wrote:
> On 21.04.2015 14:57, Diego Novillo wrote:
>>
>> >From the autofdo page: https://github.com/google/autofdo
>>
>> [ ... ]
>> Inputs:
>>
>> --profile: PERF_PROFILE collected using linux perf (with last branch
>> record).
>> In order to collect this profile, you will need to have an Intel CPU that
>> have last branch record (LBR) support. You also need to have your linux
>> kernel configured with LBR support. To profile:
>> # perf record -c PERIOD -e EVENT -b -o perf.data -- ./command
>> EVENT is refering to BR_INST_RETIRED:TAKEN if available. For some
>> architectures, BR_INST_EXEC:TAKEN also works.
>> [ ... ]
>>
>> The important one for autofdo is -b. It asks perf to use LBR registers
>> for branch tracking (assuming your architecture supports it).
>
>
> Thanks! It worked. Now big programs produce big gcov files. Sorry for this
> confusing message.
>
> But why create_gcov does not inform about that (no branch events were
> found)? It creates empty gcov file and says nothing :(
>
> Moreover, in the mentioned README it is said that perf should also be
> executed with option -e BR_INST_RETIRED:TAKEN.
> I tried to add it but perf said that
>
>    invalid or unsupported event: 'BR_INST_RETIRED:TAKEN'
>    Run 'perf list' for a list of valid events
>
> For my architecture x86_64 the perf list contains
>
>    $ sudo perf list | grep -i br
>       branch-instructions OR branches                    [Hardware event]
>       branch-misses                                      [Hardware event]
>       branch-loads                                       [Hardware
>    cache event]
>       branch-load-misses                                 [Hardware
>    cache event]
>       branch-instructions OR cpu/branch-instructions/    [Kernel PMU event]
>       branch-misses OR cpu/branch-misses/                [Kernel PMU event]
>       mem:<addr>[:access] [Hardware breakpoint]
>       syscalls:sys_enter_brk                             [Tracepoint event]
>       syscalls:sys_exit_brk                              [Tracepoint event]
>
> There is no BR_INST_RETIRED:TAKEN there. Do you use some specific
> configuration of perf for that?
>
> However, I tried to use option "-e branch-instructions". Before that the
> following error was obtained:
>
>    E0421 15:57:39.308374 11551 perf_parser.cc:210] Mapped 50% of
>    samples, expected at least 95%
>
> and now it disappeared (because of option "-e branch-instructions").
>
> Though, the performance decreases after adding option
> "-fauto-profile=file.gcov" or "-fprofile-use=file.gcov" to the list of
> compiler options.
> The program becomes 10% slower than before.
> Can you explain that? Maybe I should configure perf so that it will be able
> to collect events BR_INST_RETIRED:TAKEN ? How can it be done?

You can use dump_gcov to show a text version of the profile dump and
check if the profile data makes sense. If your program is just a very
tight single loop, the current implementation in trunk may not yield
good results because it does not have discriminator support. Try the
google-4_9 branch instead.

Dehao

>
>
> --
> Best regards,
> Ilya Palachev