On 11/9/2022 5:18 PM, Victoria Dye wrote: > 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." Thanks for pointing out my misunderstanding. I missed the repeat counts because 2-3 seconds "seemed right" based on performance tests of large monorepos, but clearly that's not right when using the Git repository for performance tests. >> 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. Getting unit-level performance tests is always tricky. Sometimes the best way to do it is to collect a sample using GIT_TRACE2_PERF and then manually collect the region times. It could be a fun project to integrate region measurements into the performance test suite instead of only end-to-end timings. > 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. Taking a look now. Thanks! -Stolee
