> I'd be quite happy to accept a 25% regression on T9872c if it yielded a 1% improvement on compiling Cabal. T9872 is very very very strange! (Maybe if *all* the T9872 tests regressed, I'd be more worried.)

While I fully agree with this. We should *always* want to know if a small syntetic benchmark regresses by a lot.
Or in other words we don't want CI to accept such a regression for us ever, but the developer of a patch should need to explicitly ok it.

Now that isn't really an issue anyway I think. The question is rather is 2% a large enough regression to worry about? 5%? 10%?

On Mar 17, 2021, at 6:18 AM, Moritz Angermann <moritz.angermann@gmail.com> wrote:

But what do we expect of patch authors? Right now if five people write patches to GHC, and each of them eventually manage to get their MRs green, after a long review, they finally see it assigned to marge, and then it starts failing? Their patch on its own was fine, but their aggregate with other people's code leads to regressions? So we now expect all patch authors together to try to figure out what happened? Figuring out why something regressed is hard enough, and we only have a very few people who are actually capable of debugging this. Thus I believe it would end up with Ben, Andreas, Matthiew, Simon, ... or someone else from GHC HQ anyway to figure out why it regressed, be it in the Review Stage, or dissecting a marge aggregate, or on master.

I have previously posted against the idea of allowing Marge to accept regressions... but the paragraph above is sadly convincing. Maybe Simon is right about opening up the windows to, say, be 100% (which would catch a 10x regression) instead of infinite, but I'm now convinced that Marge should be very generous in allowing regressions -- provided we also have some way of monitoring drift over time.

Separately, I've been concerned for some time about the peculiarity of our perf tests. For example, I'd be quite happy to accept a 25% regression on T9872c if it yielded a 1% improvement on compiling Cabal. T9872 is very very very strange! (Maybe if *all* the T9872 tests regressed, I'd be more worried.) I would be very happy to learn that some more general, representative tests are included in our examinations.

Richard

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