From d55a30bb1db565751f5bba5c5f9c344b08771ab2 Mon Sep 17 00:00:00 2001 From: Jeff King Date: Wed, 13 Feb 2019 23:35:22 -0500 Subject: [PATCH 1/4] 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 --- builtin/prune.c | 43 ++++++++++++++++++++++++++++++++----------- t/perf/p5304-prune.sh | 24 ++++++++++++++++++++++++ t/t5304-prune.sh | 12 ++++++++++++ 3 files changed, 68 insertions(+), 11 deletions(-) create mode 100755 t/perf/p5304-prune.sh diff --git a/builtin/prune.c b/builtin/prune.c index 1ec9ddd751..04b6573945 100644 --- a/builtin/prune.c +++ b/builtin/prune.c @@ -31,16 +31,40 @@ static int prune_tmp_file(const char *fullpath) return 0; } -static int prune_object(const struct object_id *oid, const char *fullpath, - void *data) +static void perform_reachability_traversal(struct rev_info *revs) { - struct stat st; + static int initialized; + struct progress *progress = NULL; + + if (initialized) + return; + + if (show_progress) + progress = start_delayed_progress(_("Checking connectivity"), 0); + mark_reachable_objects(revs, 1, expire, progress); + stop_progress(&progress); + initialized = 1; +} + +static int is_object_reachable(const struct object_id *oid, + struct rev_info *revs) +{ + perform_reachability_traversal(revs); /* * Do we know about this object? * It must have been reachable */ - if (lookup_object(the_repository, oid->hash)) + return !!lookup_object(the_repository, oid->hash); +} + +static int prune_object(const struct object_id *oid, const char *fullpath, + void *data) +{ + struct rev_info *revs = data; + struct stat st; + + if (is_object_reachable(oid, revs)) return 0; if (lstat(fullpath, &st)) { @@ -102,7 +126,6 @@ static void remove_temporary_files(const char *path) int cmd_prune(int argc, const char **argv, const char *prefix) { struct rev_info revs; - struct progress *progress = NULL; int exclude_promisor_objects = 0; const struct option options[] = { OPT__DRY_RUN(&show_only, N_("do not remove, show only")), @@ -142,17 +165,13 @@ int cmd_prune(int argc, const char **argv, const char *prefix) if (show_progress == -1) show_progress = isatty(2); - if (show_progress) - progress = start_delayed_progress(_("Checking connectivity"), 0); if (exclude_promisor_objects) { fetch_if_missing = 0; revs.exclude_promisor_objects = 1; } - mark_reachable_objects(&revs, 1, expire, progress); - stop_progress(&progress); for_each_loose_file_in_objdir(get_object_directory(), prune_object, - prune_cruft, prune_subdir, NULL); + prune_cruft, prune_subdir, &revs); prune_packed_objects(show_only ? PRUNE_PACKED_DRY_RUN : 0); remove_temporary_files(get_object_directory()); @@ -160,8 +179,10 @@ int cmd_prune(int argc, const char **argv, const char *prefix) remove_temporary_files(s); free(s); - if (is_repository_shallow(the_repository)) + if (is_repository_shallow(the_repository)) { + perform_reachability_traversal(&revs); prune_shallow(show_only ? PRUNE_SHOW_ONLY : 0); + } return 0; } 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" && From fde67d68966e2acc4f2790d0a69991ab1f89a042 Mon Sep 17 00:00:00 2001 From: Jeff King Date: Wed, 13 Feb 2019 23:37:43 -0500 Subject: [PATCH 2/4] 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 --- reachable.c | 42 ++++++++++++++++++++++++++++++++++++++++++ t/perf/p5304-prune.sh | 11 +++++++++++ 2 files changed, 53 insertions(+) diff --git a/reachable.c b/reachable.c index 6e9b810d2a..0d00a91de4 100644 --- a/reachable.c +++ b/reachable.c @@ -12,6 +12,7 @@ #include "packfile.h" #include "worktree.h" #include "object-store.h" +#include "pack-bitmap.h" struct connectivity_progress { struct progress *progress; @@ -158,10 +159,44 @@ int add_unseen_recent_objects_to_traversal(struct rev_info *revs, FOR_EACH_OBJECT_LOCAL_ONLY); } +static void *lookup_object_by_type(struct repository *r, + const struct object_id *oid, + enum object_type type) +{ + switch (type) { + case OBJ_COMMIT: + return lookup_commit(r, oid); + case OBJ_TREE: + return lookup_tree(r, oid); + case OBJ_TAG: + return lookup_tag(r, oid); + case OBJ_BLOB: + return lookup_blob(r, oid); + default: + die("BUG: unknown object type %d", type); + } +} + +static int mark_object_seen(const struct object_id *oid, + enum object_type type, + int exclude, + uint32_t name_hash, + struct packed_git *found_pack, + off_t found_offset) +{ + struct object *obj = lookup_object_by_type(the_repository, oid, type); + if (!obj) + die("unable to create object '%s'", oid_to_hex(oid)); + + obj->flags |= SEEN; + return 0; +} + void mark_reachable_objects(struct rev_info *revs, int mark_reflog, timestamp_t mark_recent, struct progress *progress) { struct connectivity_progress cp; + struct bitmap_index *bitmap_git; /* * Set up revision parsing, and mark us as being interested @@ -188,6 +223,13 @@ void mark_reachable_objects(struct rev_info *revs, int mark_reflog, cp.progress = progress; cp.count = 0; + bitmap_git = prepare_bitmap_walk(revs); + if (bitmap_git) { + traverse_bitmap_commit_list(bitmap_git, mark_object_seen); + free_bitmap_index(bitmap_git); + return; + } + /* * Set up the revision walk - this will move all commits * from the pending list to the commit walking list. diff --git a/t/perf/p5304-prune.sh b/t/perf/p5304-prune.sh index 3c852084eb..83baedb8a4 100755 --- a/t/perf/p5304-prune.sh +++ b/t/perf/p5304-prune.sh @@ -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 +' + test_done From c2bf473d0d60f44a22a724871ada7981e51606f9 Mon Sep 17 00:00:00 2001 From: Jeff King Date: Wed, 13 Feb 2019 23:38:21 -0500 Subject: [PATCH 3/4] prune: check SEEN flag for reachability The git-prune command checks reachability by doing a traversal, and then checking whether a given object exists in the global object hash. This can yield false positives if any other part of the code had to create an object struct for some reason. It's not clear whether this is even possible, but it's more robust to rely on something a little more concrete: the SEEN flag set by our traversal. Note that there is a slight possibility of regression here, as we're relying on mark_reachable_objects() to consistently set the flag. However, it has always done so, and we're already relying on that fact in prune_shallow(), which is called as part of git-prune. So this is making these two parts of the prune operation more consistent. Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- builtin/prune.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-) diff --git a/builtin/prune.c b/builtin/prune.c index 04b6573945..97613eccb5 100644 --- a/builtin/prune.c +++ b/builtin/prune.c @@ -49,13 +49,12 @@ static void perform_reachability_traversal(struct rev_info *revs) static int is_object_reachable(const struct object_id *oid, struct rev_info *revs) { + struct object *obj; + perform_reachability_traversal(revs); - /* - * Do we know about this object? - * It must have been reachable - */ - return !!lookup_object(the_repository, oid->hash); + obj = lookup_object(the_repository, oid->hash); + return obj && (obj->flags & SEEN); } static int prune_object(const struct object_id *oid, const char *fullpath, From cc80c95f42289f6288cda5c0e62f618f585e142c Mon Sep 17 00:00:00 2001 From: Jeff King Date: Thu, 14 Feb 2019 06:07:36 -0500 Subject: [PATCH 4/4] t5304: rename "sha1" variables to "oid" Let's make the script less jarring to read in a post-sha1 world by using more hash-agnostic variable names. Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- t/t5304-prune.sh | 24 ++++++++++++------------ 1 file changed, 12 insertions(+), 12 deletions(-) diff --git a/t/t5304-prune.sh b/t/t5304-prune.sh index 2c19a790c1..1eeb828a15 100755 --- a/t/t5304-prune.sh +++ b/t/t5304-prune.sh @@ -118,10 +118,10 @@ test_expect_success 'prune: do not prune detached HEAD with no reflog' ' test_expect_success 'prune: prune former HEAD after checking out branch' ' - head_sha1=$(git rev-parse HEAD) && + head_oid=$(git rev-parse HEAD) && git checkout --quiet master && git prune -v >prune_actual && - grep "$head_sha1" prune_actual + grep "$head_oid" prune_actual ' @@ -265,24 +265,24 @@ EOF ' test_expect_success 'prune .git/shallow' ' - SHA1=$(echo hi|git commit-tree HEAD^{tree}) && - echo $SHA1 >.git/shallow && + oid=$(echo hi|git commit-tree HEAD^{tree}) && + echo $oid >.git/shallow && git prune --dry-run >out && - grep $SHA1 .git/shallow && - grep $SHA1 out && + grep $oid .git/shallow && + grep $oid out && git prune && test_path_is_missing .git/shallow ' test_expect_success 'prune .git/shallow when there are no loose objects' ' - SHA1=$(echo hi|git commit-tree HEAD^{tree}) && - echo $SHA1 >.git/shallow && - git update-ref refs/heads/shallow-tip $SHA1 && + oid=$(echo hi|git commit-tree HEAD^{tree}) && + echo $oid >.git/shallow && + git update-ref refs/heads/shallow-tip $oid && git repack -ad && # verify assumption that all loose objects are gone git count-objects | grep ^0 && git prune && - echo $SHA1 >expect && + echo $oid >expect && test_cmp expect .git/shallow ' @@ -326,8 +326,8 @@ test_expect_success 'prune: handle HEAD reflog in multiple worktrees' ' git reset --hard HEAD^ ) && git prune --expire=now && - SHA1=`git hash-object expected` && - git -C third-worktree show "$SHA1" >actual && + oid=`git hash-object expected` && + git -C third-worktree show "$oid" >actual && test_cmp expected actual '