From 89a1ab8fb5766d0474ff72f176804508b0558f8a Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Mon, 26 Sep 2022 13:17:57 +0000 Subject: pack-bitmap: remove trace2 region from hot path The trace2 region around the call to lazy_bitmap_for_commit() in bitmap_for_commit() was added in 28cd730680d (pack-bitmap: prepare to read lookup table extension, 2022-08-14). While adding trace2 regions is typically helpful for tracking performance, this method is called possibly thousands of times as a commit walk explores commit history looking for a matching bitmap. When trace2 output is enabled, this region is emitted many times and performance is throttled by that output. For now, remove these regions entirely. This is a critical path, and it would be valuable to measure that the time spent in bitmap_for_commit() does not increase when using the commit lookup table. The best way to do that would be to use a mechanism that sums the time spent in a region and reports a single value at the end of the process. This technique was introduced but not merged by [1] so maybe this example presents some justification to revisit that approach. [1] https://lore.kernel.org/git/pull.1099.v2.git.1640720202.gitgitgadget@gmail.com/ To help with the 'git blame' output in this region, add a comment that warns against adding a trace2 region. Delete a test from t5310 that used that trace output to check that this lookup optimization was activated. To create this kind of test again in the future, the stopwatch traces mentioned earlier could be used as a signal that we activated this code path. Helpedy-by: Junio C Hamano Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano diff --git a/pack-bitmap.c b/pack-bitmap.c index 9a208ab..a2bbbbd 100644 --- a/pack-bitmap.c +++ b/pack-bitmap.c @@ -830,10 +830,9 @@ struct ewah_bitmap *bitmap_for_commit(struct bitmap_index *bitmap_git, if (!bitmap_git->table_lookup) return NULL; - trace2_region_enter("pack-bitmap", "reading_lookup_table", the_repository); + /* this is a fairly hot codepath - no trace2_region please */ /* NEEDSWORK: cache misses aren't recorded */ bitmap = lazy_bitmap_for_commit(bitmap_git, commit); - trace2_region_leave("pack-bitmap", "reading_lookup_table", the_repository); if (!bitmap) return NULL; return lookup_stored_bitmap(bitmap); diff --git a/t/t5310-pack-bitmaps.sh b/t/t5310-pack-bitmaps.sh index 7e50f8e..6d693ee 100755 --- a/t/t5310-pack-bitmaps.sh +++ b/t/t5310-pack-bitmaps.sh @@ -455,13 +455,6 @@ test_expect_success 'verify writing bitmap lookup table when enabled' ' grep "\"label\":\"writing_lookup_table\"" trace2 ' -test_expect_success 'lookup table is actually used to traverse objects' ' - git repack -adb && - GIT_TRACE2_EVENT="$(pwd)/trace3" \ - git rev-list --use-bitmap-index --count --all && - grep "\"label\":\"reading_lookup_table\"" trace3 -' - test_expect_success 'truncated bitmap fails gracefully (lookup table)' ' test_config pack.writebitmaphashcache false && git repack -adb && -- cgit v0.10.2-6-g49f6