Starting from 6b8fda2d (pack-objects: use bitmaps when packing objects)
if a repository has bitmap index, pack-objects can nicely speedup
"Counting objects" graph traversal phase. That however was done only for
case when resultant pack is sent to stdout, not written into a file.
The reason here is for on-disk repack by default we want:
- to produce good pack (with bitmap index not-yet-packed objects are
emitted to pack in suboptimal order).
- to use more robust pack-generation codepath (avoiding possible
bugs in bitmap code and possible bitmap index corruption).
Jeff King further explains:
The reason for this split is that pack-objects tries to determine how
"careful" it should be based on whether we are packing to disk or to
stdout. Packing to disk implies "git repack", and that we will likely
delete the old packs after finishing. We want to be more careful (so
as not to carry forward a corruption, and to generate a more optimal
pack), and we presumably run less frequently and can afford extra CPU.
Whereas packing to stdout implies serving a remote via "git fetch" or
"git push". This happens more frequently (e.g., a server handling many
fetching clients), and we assume the receiving end takes more
responsibility for verifying the data.
But this isn't always the case. One might want to generate on-disk
packfiles for a specialized object transfer. Just using "--stdout" and
writing to a file is not optimal, as it will not generate the matching
pack index.
So it would be useful to have some way of overriding this heuristic:
to tell pack-objects that even though it should generate on-disk
files, it is still OK to use the reachability bitmaps to do the
traversal.
So we can teach pack-objects to use bitmap index for initial object
counting phase when generating resultant pack file too:
- if we care it is not activated under git-repack:
See above about repack robustness and not forward-carrying corruption.
- if we know bitmap index generation is not enabled for resultant pack:
Current code has singleton bitmap_git so cannot work simultaneously
with two bitmap indices.
We also want to avoid (at least with current implementation)
generating bitmaps off of bitmaps. The reason here is: when generating
a pack, not-yet-packed objects will be emitted into pack in
suboptimal order and added to tail of the bitmap as "extended entries".
When the resultant pack + some new objects in associated repository
are in turn used to generate another pack with bitmap, the situation
repeats: new objects are again not emitted optimally and just added to
bitmap tail - not in recency order.
So the pack badness can grow over time when at each step we have
bitmapped pack + some other objects. That's why we want to avoid
generating bitmaps off of bitmaps, not to let pack badness grow.
- if we keep pack reuse enabled still only for "send-to-stdout" case:
Because on pack reuse raw entries are directly written out to destination
pack by write_reused_pack() bypassing needed for pack index generation
bookkeeping done by regular codepath in write_one() and friends.
This way for pack-objects -> file we get nice speedup:
erp5.git[1] (~230MB) extracted from ~ 5GB lab.nexedi.com backup
repository managed by git-backup[2] via
time echo 0186ac99 | git pack-objects --revs erp5pack
before: 37.2s
after: 26.2s
And for `git repack -adb` packed git.git
time echo 5c589a73 | git pack-objects --revs gitpack
before: 7.1s
after: 3.6s
i.e. it can be 30% - 50% speedup for pack extraction.
git-backup extracts many packs on repositories restoration. That was my
initial motivation for the patch.
[1] https://lab.nexedi.com/nexedi/erp5
[2] https://lab.nexedi.com/kirr/git-backup
NOTE
Jeff also suggests that pack.useBitmaps was probably a mistake to
introduce originally. This way we are not adding another config point,
but instead just always default to-file pack-objects not to use bitmap
index: Tools which need to generate on-disk packs with using bitmap, can
pass --use-bitmap-index explicitly. And git-repack does never pass
--use-bitmap-index, so this way we can be sure regular on-disk repacking
remains robust.
NOTE2
`git pack-objects --stdout >file.pack` + `git index-pack file.pack` is much slower
than `git pack-objects file.pack`. Extracting erp5.git pack from
lab.nexedi.com backup repository:
$ time echo 0186ac99 | git pack-objects --stdout --revs >erp5pack-stdout.pack
real 0m22.309s
user 0m21.148s
sys 0m0.932s
$ time git index-pack erp5pack-stdout.pack
real 0m50.873s <-- more than 2 times slower than time to generate pack itself!
user 0m49.300s
sys 0m1.360s
So the time for
`pack-object --stdout >file.pack` + `index-pack file.pack` is 72s,
while
`pack-objects file.pack` which does both pack and index is 27s.
And even
`pack-objects --no-use-bitmap-index file.pack` is 37s.
Jeff explains:
The packfile does not carry the sha1 of the objects. A receiving
index-pack has to compute them itself, including inflating and applying
all of the deltas.
that's why for `git-backup restore` we want to teach `git pack-objects
file.pack` to use bitmaps instead of using `git pack-objects --stdout
>file.pack` + `git index-pack file.pack`.
More context:
http://marc.info/?t=146792101400001&r=1&w=2
Cc: Vicent Marti <tanoku@xxxxxxxxx>
Helped-by: Jeff King <peff@xxxxxxxx>
Signed-off-by: Kirill Smelkov <kirr@xxxxxxxxxx>
---
builtin/pack-objects.c | 31 ++++++++++++++++++++++++-------
t/t5310-pack-bitmaps.sh | 12 ++++++++++++
2 files changed, 36 insertions(+), 7 deletions(-)
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index b1007f2..c92d7fc 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -67,7 +67,8 @@ static struct packed_git *reuse_packfile;
static uint32_t reuse_packfile_objects;
static off_t reuse_packfile_offset;
-static int use_bitmap_index = 1;
+static int use_bitmap_index_default = 1;
+static int use_bitmap_index = -1;
static int write_bitmap_index;
static uint16_t write_bitmap_options;
@@ -2270,7 +2271,7 @@ static int git_pack_config(const char *k, const char *v, void *cb)
write_bitmap_options &= ~BITMAP_OPT_HASH_CACHE;
}
if (!strcmp(k, "pack.usebitmaps")) {
- use_bitmap_index = git_config_bool(k, v);
+ use_bitmap_index_default = git_config_bool(k, v);
return 0;
}
if (!strcmp(k, "pack.threads")) {
@@ -2519,13 +2520,13 @@ static void loosen_unused_packed_objects(struct rev_info *revs)
}
/*
- * This tracks any options which a reader of the pack might
- * not understand, and which would therefore prevent blind reuse
- * of what we have on disk.
+ * This tracks any options which pack-reuse code expects to be on, or which a
+ * reader of the pack might not understand, and which would therefore prevent
+ * blind reuse of what we have on disk.
*/
static int pack_options_allow_reuse(void)
{
- return allow_ofs_delta;
+ return pack_to_stdout && allow_ofs_delta;
}
static int get_object_list_from_bitmap(struct rev_info *revs)
@@ -2818,7 +2819,23 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
if (!rev_list_all || !rev_list_reflog || !rev_list_index)
unpack_unreachable_expiration = 0;
- if (!use_internal_rev_list || !pack_to_stdout || is_repository_shallow())
+ /*
+ * "soft" reasons not to use bitmaps - for on-disk repack by default we want
+ *
+ * - to produce good pack (with bitmap index not-yet-packed objects are
+ * packed in suboptimal order).
+ *
+ * - to use more robust pack-generation codepath (avoiding possible
+ * bugs in bitmap code and possible bitmap index corruption).
+ */
+ if (!pack_to_stdout)
+ use_bitmap_index_default = 0;
+
+ if (use_bitmap_index < 0)
+ use_bitmap_index = use_bitmap_index_default;
+
+ /* "hard" reasons not to use bitmaps; these just won't work at all */
+ if (!use_internal_rev_list || (!pack_to_stdout && write_bitmap_index) || is_repository_shallow())
use_bitmap_index = 0;
if (pack_to_stdout || !rev_list_all)
diff --git a/t/t5310-pack-bitmaps.sh b/t/t5310-pack-bitmaps.sh
index a50d867..44914ac 100755
--- a/t/t5310-pack-bitmaps.sh
+++ b/t/t5310-pack-bitmaps.sh
@@ -196,6 +196,18 @@ test_expect_success 'pack-objects respects --local (non-local bitmapped pack)' '
! has_any packbitmap.objects 3b.objects
'
+test_expect_success 'pack-objects to file can use bitmap' '
+ # make sure we still have 1 bitmap index from previous tests
+ ls .git/objects/pack/ | grep bitmap >output &&
+ test_line_count = 1 output &&
+ # verify equivalent packs are generated with/without using bitmap index
+ packasha1=$(git pack-objects --no-use-bitmap-index --all packa </dev/null) &&
+ packbsha1=$(git pack-objects --use-bitmap-index --all packb </dev/null) &&
+ pack_list_objects <packa-$packasha1.idx >packa.objects &&
+ pack_list_objects <packb-$packbsha1.idx >packb.objects &&
+ test_cmp packa.objects packb.objects
+'
+
test_expect_success 'full repack, reusing previous bitmaps' '
git repack -ad &&
ls .git/objects/pack/ | grep bitmap >output &&
--
2.9.2.701.gf965a18.dirty
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