Problem with compiler perf tests

[Andreas Klebinger]

Ömer Sinan Ağacan schrieb am 17.11.2019 um 09:22:
> I think what we should do instead is that once it's clear that the
> patch did not
> introduce *accidental* increases in numbers (e.g. in !2100 I checked and
> explained the increase in average residency, and showed that the increase makes
> sense and is not a leak) and it's the right thing to do, we should merge it
But that's what we do already isn't it? We don't expect all changes to
have no performance implications
if they can be argued for.

However it's easy for "insignificant" changes to compound to a
significant slowdown so I don't think we are too careful currently. I've
never seen anyone care about "a few bytes".
Assuming we get 6 MR's who regresses a metric by 1% per year that adds
up quickly. Three years and we will be about 20% worse! So I think we
are right to be cautions with those things.

It's just that people sometimes (as in !2100 initially) disagree on what
the right thing to do is.
But I don't see a way around that no matter where we set the thresholds.
That can only be resolved by discourse.

What I don't agree with is pushing that discussion into separate tickets
in general.
That would just mean we get a bunch of performance regression, and a
bunch of tickets documenting them.

Which is better than not documenting them! And sometimes that will be
the best course of action.
But if there is a chance to resolve performance issues while a patch is
still being worked on that
will in general always be a better solution.

At least that's my opinion on the general case.

Cheers,
Andreas

> Hi,
>
> Currently we have a bunch of tests in testsuite/tests/perf/compiler for keeping
> compile time allocations, max residency etc. in the expected ranges and avoid
> introducing accidental compile time performance regressions.
>
> This has a problem: we expect every MR to keep the compile time stats in the
> specified ranges, but sometimes a patch fixes an issue, or does something right
> (removes hacks/refactors bad code etc.) but also increases the numbers because
> sometimes doing it right means doing more work or keeping more things in memory
> (e.g. !1747, !2100 which is required by !1304).
>
> We then spend hours/days trying to shave a few bytes off in those patches,
> because the previous hacky/buggy code set the standards. It doesn't make sense
> to compare bad/buggy code with good code and expect them to do the same thing.
>
> Second problem is that it forces the developer to focus on a tiny part of the
> compiler to reduce the numbers to the where they were. If they looked at the big
> picture instead it might be possible to see rooms of improvements in other
> places that could be possibly lead to much more efficient use of the developer
> time.
>
> I think what we should do instead is that once it's clear that the patch did not
> introduce *accidental* increases in numbers (e.g. in !2100 I checked and
> explained the increase in average residency, and showed that the increase makes
> sense and is not a leak) and it's the right thing to do, we should merge it, and
> track the performance issues in another issue. The CI should still run perf
> tests, but those should be allowed to fail.
>
> Any opinions on this?
>
> Ömer
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