2 Commits

Author	SHA1	Message	Date
Patrick Steinhardt	0a0fbbe3ff	refs: remove lookup cache for reference-transaction hook ... When adding the reference-transaction hook, there were concerns about the performance impact it may have on setups which do not make use of the new hook at all. After all, it gets executed every time a reftx is prepared, committed or aborted, which linearly scales with the number of reference-transactions created per session. And as there are code paths like `git push` which create a new transaction for each reference to be updated, this may translate to calling `find_hook()` quite a lot. To address this concern, a cache was added with the intention to not repeatedly do negative hook lookups. Turns out this cache caused a regression, which was fixed via e5256c82e5 (refs: fix interleaving hook calls with reference-transaction hook, 2020-08-07). In the process of discussing the fix, we realized that the cache doesn't really help even in the negative-lookup case. While performance tests added to benchmark this did show a slight improvement in the 1% range, this really doesn't warrent having a cache. Furthermore, it's quite flaky, too. E.g. running it twice in succession produces the following results: Test master pks-reftx-hook-remove-cache -------------------------------------------------------------------------- 1400.2: update-ref 2.79(2.16+0.74) 2.73(2.12+0.71) -2.2% 1400.3: update-ref --stdin 0.22(0.08+0.14) 0.21(0.08+0.12) -4.5% Test master pks-reftx-hook-remove-cache -------------------------------------------------------------------------- 1400.2: update-ref 2.70(2.09+0.72) 2.74(2.13+0.71) +1.5% 1400.3: update-ref --stdin 0.21(0.10+0.10) 0.21(0.08+0.13) +0.0% 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: Test master pks-reftx-hook-remove-cache -------------------------------------------------------------------------- 1400.4: nonatomic push 6.63(6.50+0.13) 6.81(6.67+0.14) +2.7% 1400.4: nonatomic push 6.35(6.21+0.14) 6.39(6.23+0.16) +0.6% 1400.4: nonatomic push 6.43(6.31+0.13) 6.42(6.28+0.15) -0.2% So let's just remove the cache altogether to simplify the code. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>	2020-08-25 15:34:42 -07:00
Patrick Steinhardt	6754159767	refs: implement reference transaction hook ... The low-level reference transactions used to update references are currently completely opaque to the user. While certainly desirable in most usecases, there are some which might want to hook into the transaction to observe all queued reference updates as well as observing the abortion or commit of a prepared transaction. One such usecase would be to have a set of replicas of a given Git repository, where we perform Git operations on all of the repositories at once and expect the outcome to be the same in all of them. While there exist hooks already for a certain subset of Git commands that could be used to implement a voting mechanism for this, many others currently don't have any mechanism for this. The above scenario is the motivation for the new "reference-transaction" hook that reaches directly into Git's reference transaction mechanism. The hook receives as parameter the current state the transaction was moved to ("prepared", "committed" or "aborted") and gets via its standard input all queued reference updates. While the exit code gets ignored in the "committed" and "aborted" states, a non-zero exit code in the "prepared" state will cause the transaction to be aborted prematurely. Given the usecase described above, a voting mechanism can now be implemented via this hook: as soon as it gets called, it will take all of stdin and use it to cast a vote to a central service. When all replicas of the repository agree, the hook will exit with zero, otherwise it will abort the transaction by returning non-zero. The most important upside is that this will catch _all_ commands writing references at once, allowing to implement strong consistency for reference updates via a single mechanism. In order to test the impact on the case where we don't have any "reference-transaction" hook installed in the repository, this commit introduce two new performance tests for git-update-refs(1). Run against an empty repository, it produces the following results: Test origin/master HEAD -------------------------------------------------------------------- 1400.2: update-ref 2.70(2.10+0.71) 2.71(2.10+0.73) +0.4% 1400.3: update-ref --stdin 0.21(0.09+0.11) 0.21(0.07+0.14) +0.0% The performance test p1400.2 creates, updates and deletes a branch a thousand times, thus averaging runtime of git-update-refs over 3000 invocations. p1400.3 instead calls `git-update-refs --stdin` three times and queues a thousand creations, updates and deletes respectively. As expected, p1400.3 consistently shows no noticeable impact, as for each batch of updates there's a single call to access(3P) for the negative hook lookup. On the other hand, for p1400.2, one can see an impact caused by this patchset. But doing five runs of the performance tests where each one was run with GIT_PERF_REPEAT_COUNT=10, the overhead ranged from -1.5% to +1.1%. These inconsistent performance numbers can be explained by the overhead of spawning 3000 processes. This shows that the overhead of assembling the hook path and executing access(3P) once to check if it's there is mostly outweighed by the operating system's overhead. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>	2020-06-19 10:46:13 -07:00