Richard Eisenberg rae at richarde.dev
Wed Mar 24 12:08:45 UTC 2021

What about the case where the rebase *lessens* the improvement? That is, you're expecting these 10 cases to improve, but after a rebase, only 1 improves. That's news! But a blanket "accept improvements" won't tell you.

I'm not hard against this proposal, because I know precise tracking has its own costs. Just wanted to bring up another scenario that might be factored in.


> On Mar 24, 2021, at 7:44 AM, Andreas Klebinger <klebinger.andreas at gmx.at> wrote:
> After the idea of letting marge accept unexpected perf improvements and
> looking at https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4759
> which failed because of a single test, for a single build flavour
> crossing the
> improvement threshold where CI fails after rebasing I wondered.
> When would accepting a unexpected perf improvement ever backfire?
> In practice I either have a patch that I expect to improve performance
> for some things
> so I want to accept whatever gains I get. Or I don't expect improvements
> so it's *maybe*
> worth failing CI for in case I optimized away some code I shouldn't or
> something of that
> sort.
> How could this be actionable? Perhaps having a set of indicator for CI of
> "Accept allocation decreases"
> "Accept residency decreases"
> Would be saner. I have personally *never* gotten value out of the
> requirement
> to list the indivial tests that improve. Usually a whole lot of them do.
> Some cross
> the threshold so I add them. If I'm unlucky I have to rebase and a new
> one might
> make it across the threshold.
> Being able to accept improvements (but not regressions) wholesale might be a
> reasonable alternative.
> Opinions?
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