Re: [PATCH] refs: remove lookup cache for reference-transaction hook

[Patrick Steinhardt <ps@pks.im>]

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On Fri, Aug 21, 2020 at 10:37:27AM -0400, Jeff King wrote:
> On Fri, Aug 21, 2020 at 10:29:18AM +0200, Patrick Steinhardt wrote:
> 
> > One case notably absent from those benchmarks is a single executable
> > searching for the hook hundreds of times, which is exactly the case for
> > which the negative cache was added. p1400.2 will spawn a new update-ref
> > for each transaction and p1400.3 only has a single reference-transaction
> > for all reference updates. So this commit adds a third benchmark, which
> > performs an non-atomic push of a thousand references. This will create a
> > new reference transaction per reference. But even for this case, the
> > negative cache doesn't consistently improve performance:
> 
> Ah, right, I forgot that update-ref would use one single transaction. So
> what we were testing in our earlier discussion was not even useful. :)
> 
> >  test_expect_success "setup" '
> > +	git init --bare target-repo.git &&
> >  	test_commit PRE &&
> >  	test_commit POST &&
> >  	printf "create refs/heads/%d PRE\n" $(test_seq 1000) >create &&
> >  	printf "update refs/heads/%d POST PRE\n" $(test_seq 1000) >update &&
> > -	printf "delete refs/heads/%d POST\n" $(test_seq 1000) >delete
> > +	printf "delete refs/heads/%d POST\n" $(test_seq 1000) >delete &&
> > +	printf "create refs/heads/branch-%d PRE\n" $(test_seq 1000) | git update-ref --stdin
> >  '
> 
> OK, we need these new branches to have something to push into and delete
> from the remote. They might impact the timings of the other tests,
> though (since we now have 1000 entries in .git/refs/heads/, which might
> affect filesystem performance). But it should do so uniformly, so I
> don't think it invalidates their results.
> 
> However, I wondered...
> 
> > +test_perf "nonatomic push" '
> > +	git push ./target-repo.git branch-{1..1000} &&
> > +	git push --delete ./target-repo.git branch-{1..1000}
> > +'
> 
> ...if it might make the test more consistent (not to mention isolated
> from the cost of other parts of the push) if we used update-ref here, as
> well. You added the code necessary to control individual transactions,
> so I thought that:
> 
>   printf 'start\ncreate refs/heads/%d PRE\ncommit\n' \
>     $(test_seq 1000) >create-transaction
> 
> might work. But it doesn't, because after the first transaction is
> closed, we refuse to accept any other commands. That makes sense for
> "prepare", etc, but there's no reason we couldn't start a new one.
> 
> Is that worth supporting? It would allow a caller to use a single
> update-ref to make a series of non-atomic updates, which is something
> that can't currently be done. And we're so close.

Yeah, I had the exact same thought and I do think it's useful to be able
to create multiple reference transactions per git-update-ref(1) session.
I might whip something up as soon as I find the time to do so, it really
shouldn't be a lot of work.

Patrick

> Even if it is, though, that's definitely outside the scope of this
> patch, and I think we should take it as-is with "push".
> 
> -Peff

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