On Mon, Jun 28, 2021 at 07:33:15AM +0200, Patrick Steinhardt wrote: > As expected, performance doesn't change in cases where we do not have a > bitmap available given that the old code path still kicks in. In case we > do have bitmaps, this is kind of a mixed bag: while git-receive-pack(1) > is slower in a "normal" clone of linux.git, it is significantly faster > for a clone with lots of references. The slowness can potentially be > explained by the overhead of loading the bitmap. On the other hand, the > new code is faster as expected in repos which have lots of references > given that we do not have to mark all negative references anymore. Hmm. We _do_ still have to mark those negative references now, though (the bitmap code still considers each as a reachability tip for the "have" side of the traversal). It's just that we may have to do less traversal on them, if they're mentioned by other bitmaps. So in that sense I don't think your "a ref for every commit" cases are all that interesting. Any bitmap near the tip of history is going to include a bit for all those old commits, because our fake set of refs are all reachable. A much more interesting history is when you have a bunch of little unreachable spikes coming off the main history. This is common if you have a lot of branches in the repo, but also if you maintain a lot of book-keeping refs (like the refs/pull/* we do at GitHub; I assume GitLab does something similar). Here are some real-world numbers from one of the repos that gives us frequent problems with bitmaps. refs/pull/9937/head in this case is an unmerged PR with 8 commits on it. [without bitmaps, full check but with count to suppress output] $ time git rev-list --count refs/pull/9937/head --objects --not --all 0 real 0m1.280s user 0m1.131s sys 0m0.148s [same, but with bitmaps] $ time git rev-list --count refs/pull/9937/head --objects --not --all --use-bitmap-index 0 real 1m38.146s user 1m30.015s sys 0m3.443s Yikes. Now this is a pretty extreme case, as it has a lot of bookkeeping refs. If we limited ourselves to just the branches (in which case our unmerged PR will appear to have a couple new commits), though, we still get: $ time git rev-list --count refs/pull/9937/head --objects --not --branches 64 real 0m0.366s user 0m0.272s sys 0m0.093s $ time git rev-list --count refs/pull/9937/head --objects --not --branches --use-bitmap-index 61 real 0m10.372s user 0m9.633s sys 0m0.736s which is still a pretty bad regression (the difference in the output is expected; the regular traversal is not as thorough at finding objects which appear in non-contiguous sections of history). Again, this is one of the repositories that routinely gives us problems. But even on git/git, which is usually not a problematic repo, I get: $ time git rev-list refs/pull/986/head --objects --not --all real 0m0.121s user 0m0.024s sys 0m0.097s $ time git rev-list refs/pull/986/head --objects --not --all --use-bitmap-index real 0m12.406s user 0m5.843s sys 0m0.734s So even if this tradeoff might help on balance, it really makes some cases pathologically bad. -Peff
