[llvm-dev] CTMark - regular LLVM and CLANG compile-time tracking

Matthias Braun via llvm-dev
2016-11-17T23:00:20Z

> On Nov 17, 2016, at 2:55 PM, Mehdi Amini <mehdi.amini at apple.com>
wrote:
>
> Hi Gerolf,
>
> This is really cool!
> I’m very excited about this initiative and I hope we’ll be able to get to a
stage where compile time regression are handled like other regression: if they
are not expected / justified by the commit author promptly, the commit should be
reverted in the meantime!
>
> I’d like to suggest adding to CTMark the “empty” compile test (and maybe
“empty + one empty function”), unless it is too noisy to measure.
> It is an interesting test to complete the existing ones because it measures
the general overhead of setting up all the “infrastructure” (static
initializers, creating a pass pipeline, etc.)
That would indeed be a very interesting test, however this will be way too short
to measure predictably on its own.
I could see it working if we had a flag that artifically runs the compilation
pipeline hundreds of times or alternatively puts the whole compiler into
something like googlebenchmark.

- Matthias

>
> —
> Mehdi
>
>
>
>
>> On Nov 15, 2016, at 1:27 PM, Gerolf Hoflehner via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>>
>> Hi,
>>
>> this is about kicking-off regular compile-time tracking for LLVM and
CLANG on the green dragon: http://lab.llvm.org:8080/green/view/Compile%20Time/
<http://lab.llvm.org:8080/green/view/Compile%20Time/>. The goal is to stay
on top of compile-time issues immediately when they occur so they can be
assessed rather than creeping in unnoticed. The methodology is simple: form a
CTMark suite out of 10 “long” compiling tests of the LLVM test suite and track
it closely for ARM64 O0g and Os. When there is a jump in compile-time of more
than 2.5% in one of the tests an email notification will be sent to the
committer and a bug will be filed in bugzilla. The 2.5% threshold is large
enough to be above the noise level and should be motivating enough to root cause
the issue. Also watch for spikes of >10% in O3 LTO.
>>
>> There will be process and effort. Chris Matthews put together CTMark
together with Michael and Matthias, and keeps the servers running and constantly
improves tooling. Michael stepped up helping with the ongoing process of
watching for issues and filing PRs until that part is automated. It will require
the effort of everyone to control and possibly improve compile-time over time.
And everybody is invited to root cause past regressions.
>>
>> The need for this work has been recognized by the communities and also
externally. It has been observed more than once that llvm and clang are getting
slower over time, for example in this thread:
http://lists.llvm.org/pipermail/llvm-dev/2016-March/096491.html
<http://lists.llvm.org/pipermail/llvm-dev/2016-March/096491.html>.
Phoronix reported recently compile-time slow downs in clang 3.9 vs 3.8:
http://www.phoronix.com/scan.php?page=article&item=llvm-clang-39&num=4
<http://www.phoronix.com/scan.php?page=article&item=llvm-clang-39&num=4>.
At the LLVM Developer Meeting Michael showed double digit compile-time increases
in clang 3.7 and 3.8 in Os/O3 and O0 in his Loop Passes presentation
(https://llvmdevelopersmeetingbay2016.sched.org/event/8Z0B/loop-passes-adding-new-features-while-reducing-technical-debt
<https://llvmdevelopersmeetingbay2016.sched.org/event/8Z0B/loop-passes-adding-new-features-while-reducing-technical-debt>):
>> <PastedGraphic-3.png>
>>
>>
>> The selection of 10 tests out of the LLVM tests invites criticism. The
motivation here is to pick something “reasonable” and adjust as needed moving
forward. Specifically this means running a wider set of tests from time to time
and adjust the tests in CTMark either by adding more tests or by removing
in-effective tests. Internally we also track a set of benchmarks and check for
correlations of compile-time regressions to CTMark. And most importantly I think
the shared interest and commitment of the community to compile-time will carry
this forward.
>>
>> Special thanks to Chris and Michael to get this started!
>>
>> -Gerolf
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>

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