From d55a30bb1db565751f5bba5c5f9c344b08771ab2 Mon Sep 17 00:00:00 2001 From: Jeff King Date: Wed, 13 Feb 2019 23:35:22 -0500 Subject: prune: lazily perform reachability traversal The general strategy of "git prune" is to do a full reachability walk, then for each loose object see if we found it in our walk. But if we don't have any loose objects, we don't need to do the expensive walk in the first place. This patch postpones that walk until the first time we need to see its results. Note that this is really a specific case of a more general optimization, which is that we could traverse only far enough to find the object under consideration (i.e., stop the traversal when we find it, then pick up again when asked about the next object, etc). That could save us in some instances from having to do a full walk. But it's actually a bit tricky to do with our traversal code, and you'd need to do a full walk anyway if you have even a single unreachable object (which you generally do, if any objects are actually left after running git-repack). So in practice this lazy-load of the full walk catches one easy but common case (i.e., you've just repacked via git-gc, and there's nothing unreachable). The perf script is fairly contrived, but it does show off the improvement: Test HEAD^ HEAD ------------------------------------------------------------------------- 5304.4: prune with no objects 3.66(3.60+0.05) 0.00(0.00+0.00) -100.0% and would let us know if we accidentally regress this optimization. Note also that we need to take special care with prune_shallow(), which relies on us having performed the traversal. So this optimization can only kick in for a non-shallow repository. Since this is easy to get wrong and is not covered by existing tests, let's add an extra test to t5304 that covers this case explicitly. Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- t/perf/p5304-prune.sh | 24 ++++++++++++++++++++++++ t/t5304-prune.sh | 12 ++++++++++++ 2 files changed, 36 insertions(+) create mode 100755 t/perf/p5304-prune.sh (limited to 't') diff --git a/t/perf/p5304-prune.sh b/t/perf/p5304-prune.sh new file mode 100755 index 0000000000..3c852084eb --- /dev/null +++ b/t/perf/p5304-prune.sh @@ -0,0 +1,24 @@ +#!/bin/sh + +test_description='performance tests of prune' +. ./perf-lib.sh + +test_perf_default_repo + +test_expect_success 'remove reachable loose objects' ' + git repack -ad +' + +test_expect_success 'remove unreachable loose objects' ' + git prune +' + +test_expect_success 'confirm there are no loose objects' ' + git count-objects | grep ^0 +' + +test_perf 'prune with no objects' ' + git prune +' + +test_done diff --git a/t/t5304-prune.sh b/t/t5304-prune.sh index 270da21ac3..2c19a790c1 100755 --- a/t/t5304-prune.sh +++ b/t/t5304-prune.sh @@ -274,6 +274,18 @@ test_expect_success 'prune .git/shallow' ' test_path_is_missing .git/shallow ' +test_expect_success 'prune .git/shallow when there are no loose objects' ' + SHA1=$(echo hi|git commit-tree HEAD^{tree}) && + echo $SHA1 >.git/shallow && + git update-ref refs/heads/shallow-tip $SHA1 && + git repack -ad && + # verify assumption that all loose objects are gone + git count-objects | grep ^0 && + git prune && + echo $SHA1 >expect && + test_cmp expect .git/shallow +' + test_expect_success 'prune: handle alternate object database' ' test_create_repo A && git -C A commit --allow-empty -m "initial commit" && -- cgit v1.2.3