On Fri, Aug 07, 2020 at 05:32:39AM -0400, Jeff King wrote: > On Fri, Aug 07, 2020 at 11:04:12AM +0200, Patrick Steinhardt wrote: > > > > It's perhaps more justifiable when there isn't a hook (we're still just > > > paying that one access(), but it's proportionally bigger). I kind of > > > doubt it's measurable, though, since a ref write requires a bunch of > > > syscalls anyway. > > > > Yeah, this really was done to not have to pay a performance penalty if > > updating thousands of refs if no reference-transaction hook exists. E.g. > > if doing a non-atomic push of n reference, we'd have n calls to > > access(3P). See [1] for reference. > > > > I've just did another quick benchmark without the cache, and it still > > consistently shows a non-zero performance hit without it: > > > > Test pks-reftx-hook-interleaving no-cache > > -------------------------------------------------------------------------------- > > 1400.2: update-ref 2.82(2.13+0.81) 2.86(2.19+0.78) +1.4% > > 1400.3: update-ref --stdin 0.22(0.07+0.15) 0.22(0.07+0.15) +0.0% > > I'm skeptical that those results are useful. In the first test, we're > running update-ref 1000 times, so: > > - the cache shouldn't be helping at all, since we only have one ref to > update (well, I guess once for "prepare" and once for "commit", so > really it's saving one syscall total per process). > > - I'd expect a lot of noise because we're spending most of our time in > starting up the process > > In the second test, we run 1000 ref operations per update-ref process. > So we should be cutting down on our hook-lookup overhead by a factor of > 1000. Yet it shows no improvement. > > That implies you're just seeing noise. And indeed, with the patch below > I get: > > Test HEAD^ HEAD > -------------------------------------------------------------------- > 1400.2: update-ref 1.93(1.57+0.42) 1.91(1.55+0.42) -1.0% > 1400.3: update-ref --stdin 0.07(0.02+0.05) 0.07(0.02+0.05) +0.0% > > Running it a second time gets me +0.5%. :) Yeah, it's also been my take that OS-level overhead is probably going to matter more than those access calls, and I argued such back when I proposed the hook. So I'm perfectly happy to see this caching mechanism go. Should I re-post a v2 with your patch and my test? Patrick
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