path: root/t/perf/
diff options
authorJeff King <>2019-02-14 04:37:43 (GMT)
committerJunio C Hamano <>2019-02-14 23:25:33 (GMT)
commitfde67d68966e2acc4f2790d0a69991ab1f89a042 (patch)
treee564f24043dfb4463fdce811ace00b12dc6f1b66 /t/perf/
parentd55a30bb1db565751f5bba5c5f9c344b08771ab2 (diff)
prune: use bitmaps for reachability traversal
Pruning generally has to traverse the whole commit graph in order to see which objects are reachable. This is the exact problem that reachability bitmaps were meant to solve, so let's use them (if they're available, of course). Here are timings on git.git: Test HEAD^ HEAD ------------------------------------------------------------------------ 5304.6: prune with bitmaps 3.65(3.56+0.09) 1.01(0.92+0.08) -72.3% And on linux.git: Test HEAD^ HEAD -------------------------------------------------------------------------- 5304.6: prune with bitmaps 35.05(34.79+0.23) 3.00(2.78+0.21) -91.4% The tests show a pretty optimal case, as we'll have just repacked and should have pretty good coverage of all refs with our bitmaps. But that's actually pretty realistic: normally prune is run via "gc" right after repacking. A few notes on the implementation: - the change is actually in reachable.c, so it would improve reachability traversals by "reflog expire --stale-fix", as well. Those aren't performed regularly, though (a normal "git gc" doesn't use --stale-fix), so they're not really worth measuring. There's a low chance of regressing that caller, since the use of bitmaps is totally transparent from the caller's perspective. - The bitmap case could actually get away without creating a "struct object", and instead the caller could just look up each object id in the bitmap result. However, this would be a marginal improvement in runtime, and it would make the callers much more complicated. They'd have to handle both the bitmap and non-bitmap cases separately, and in the case of git-prune, we'd also have to tweak prune_shallow(), which relies on our SEEN flags. - Because we do create real object structs, we go through a few contortions to create ones of the right type. This isn't strictly necessary (lookup_unknown_object() would suffice), but it's more memory efficient to use the correct types, since we already know them. Signed-off-by: Jeff King <> Signed-off-by: Junio C Hamano <>
Diffstat (limited to 't/perf/')
1 files changed, 11 insertions, 0 deletions
diff --git a/t/perf/ b/t/perf/
index 3c85208..83baedb 100755
--- a/t/perf/
+++ b/t/perf/
@@ -21,4 +21,15 @@ test_perf 'prune with no objects' '
git prune
+test_expect_success 'repack with bitmaps' '
+ git repack -adb
+# We have to create the object in each trial run, since otherwise
+# runs after the first see no object and just skip the traversal entirely!
+test_perf 'prune with bitmaps' '
+ echo "probably not present in repo" | git hash-object -w --stdin &&
+ git prune