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. 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? > 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, -Stolee
