Derrick Stolee wrote: > On 11/8/2022 5:44 PM, Victoria Dye via GitGitGadget wrote: >> Following up on a discussion [1] around cache tree refreshes in 'git reset', >> this series updates callers of 'unpack_trees()' to skip its internal >> invocation of 'cache_tree_update()' when 'prime_cache_tree()' is called >> immediately after 'unpack_trees()'. 'cache_tree_update()' can be an >> expensive operation, and it is redundant when 'prime_cache_tree()' clears >> and rebuilds the cache tree from scratch immediately after. >> >> The first patch adds a test directly comparing the execution time of >> 'prime_cache_tree()' with that of 'cache_tree_update()'. The results show >> that on a fully-valid cache tree, they perform the same, but on a >> fully-invalid cache tree, 'prime_cache_tree()' is multiple times faster >> (although both are so fast that the total execution time of 100 invocations >> is needed to compare the results in the default perf repo). > > One thing I found interesting is how you needed 200 iterations to show > a meaningful change in this test script, but in the case of 'git reset' > we can see sizeable improvements even with a single iteration. All of the new performance tests run with multiple iterations: 20 for reset (10 iterations of two resets each), 100 for read-tree, 200 for the comparison of 'cache_tree_update()' & 'prime_cache_tree()'. Those counts were picked mostly by trial-and-error, to strike a balance of "the test doesn't take too long to run" and "the change in execution time is clearly visible in the results." > > Is there something about this test that is artificially speeding up > these iterations? Perhaps the index has up-to-date filesystem information > that allows these methods to avoid filesystem interactions that are > necessary in the 'git reset' case? I would expect the "cache_tree_update, invalid" test's execution time, when scaled to the iterations of 'read-tree' and 'reset', to match the change in timing of those commands, but the command tests are reporting *much* larger improvements (e.g., I'd expect a 0.27s improvement in 'git read-tree', but the results are *consistently* >=0.9s). Per trace2 logs, a single invocation of 'read-tree' matching the one added in 'p0006' spent 0.010108s in 'cache_tree_update()'. Over 100 iterations, the total time would be ~1.01s, which lines up with the 'p0006' test results. However, the trace2 results for "test-tool cache-tree --count 3 --fresh --update" show the first iteration taking 0.013060s (looks good), then the next taking 0.003755s, then 0.004026s (_much_ faster than expected). To be honest, I can't figure out what's going on there. It might be some kind of runtime/memory optimization with repeatedly rebuilding the same cache tree (doesn't seem to be compiler optimization, since the speedup still happens with '-O0'). The only sure-fire way to avoid it seems to be moving the iteration outside of 'test-cache-tree.c' and into 'p0090'. Unfortunately, the command initialization overhead *really* slows things down, but I can add a "control" test (with no cache tree refresh) to quantify how long that initialization takes. While looking into this, I found a few other things I'd like to add to/fix in that test (add a "partially-invalidated" cache tree case, use the original cache tree OID in 'prime_cache_tree()' rather than the OID at HEAD), so I'll re-roll with those + the updated iteration logic. Thanks for bringing this up! > >> The second patch introduces the 'skip_cache_tree_update' option for >> 'unpack_trees()', but does not use it yet. >> >> The remaining three patches update callers that make the aforementioned >> redundant cache tree updates. The performance impact on these callers ranges >> from "negligible" (in 'rebase') to "substantial" (in 'read-tree') - more >> details can be found in the commit messages of the patch associated with the >> affected code path. > > I found these patches well motivated and the code change to be so > unobtrusive that the benefits are well worth the new options. Thanks! > > Thanks, > -Stolee
