commit-graph: check all leading directories in changed path Bloom filters

The file 'dir/subdir/file' can only be modified if its leading
directories 'dir' and 'dir/subdir' are modified as well.

So when checking modified path Bloom filters looking for commits
modifying a path with multiple path components, then check not only
the full path in the Bloom filters, but all its leading directories as
well.  Take care to check these paths in "deepest first" order,
because it's the full path that is least likely to be modified, and
the Bloom filter queries can short circuit sooner.

This can significantly reduce the average false positive rate, by
about an order of magnitude or three(!), and can further speed up
pathspec-limited revision walks.  The table below compares the average
false positive rate and runtime of

  git rev-list HEAD -- "$path"

before and after this change for 5000+ randomly* selected paths from
each repository:

                    Average false           Average        Average
                    positive rate           runtime        runtime
                  before     after     before     after   difference
  ------------------------------------------------------------------
  git             3.220%   0.7853%     0.0558s   0.0387s   -30.6%
  linux           2.453%   0.0296%     0.1046s   0.0766s   -26.8%
  tensorflow      2.536%   0.6977%     0.0594s   0.0420s   -29.2%

*Path selection was done with the following pipeline:

	git ls-tree -r --name-only HEAD | sort -R | head -n 5000

The improvements in runtime are much smaller than the improvements in
average false positive rate, as we are clearly reaching diminishing
returns here.  However, all these timings depend on that accessing
tree objects is reasonably fast (warm caches).  If we had a partial
clone and the tree objects had to be fetched from a promisor remote,
e.g.:

  $ git clone --filter=tree:0 --bare file://.../webkit.git webkit.notrees.git
  $ git -C webkit.git -c core.modifiedPathBloomFilters=1 \
        commit-graph write --reachable
  $ cp webkit.git/objects/info/commit-graph webkit.notrees.git/objects/info/
  $ git -C webkit.notrees.git -c core.modifiedPathBloomFilters=1 \
        rev-list HEAD -- "$path"

then checking all leading path component can reduce the runtime from
over an hour to a few seconds (and this is with the clone and the
promisor on the same machine).

This adjusts the tracing values in t4216-log-bloom.sh, which provides a
concrete way to notice the improvement.

Helped-by: Taylor Blau <me@ttaylorr.com>
Helped-by: René Scharfe <l.s.r@web.de>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
index 29338f36bf..4892364e74 100755
--- a/t/t4216-log-bloom.sh
+++ b/t/t4216-log-bloom.sh
@@ -146,7 +146,7 @@ test_expect_success 'setup - add commit-graph to the chain with Bloom filters' '
 
 test_bloom_filters_used_when_some_filters_are_missing () {
 	log_args=$1
-	bloom_trace_prefix="statistics:{\"filter_not_present\":3,\"maybe\":8,\"definitely_not\":6"
+	bloom_trace_prefix="statistics:{\"filter_not_present\":3,\"maybe\":6,\"definitely_not\":8"
 	setup "$log_args" &&
 	grep -q "$bloom_trace_prefix" "$TRASH_DIRECTORY/trace.perf" &&
 	test_cmp log_wo_bloom log_w_bloom