[PATCH 0/4] midx: trace2 regions and grab-bag patches

[PATCH 0/4] midx: trace2 regions and grab-bag patches

[Taylor Blau]

Here is a small handful of MIDX and MIDX bitmap-related patches that
I've been carrying in GitHub's fork for a while now and forgot to send
upstream.

The first is a small typofix, and the second is a legitimate bug fix
which allows us to consider annotated tags as bitmap candidates during
commit selection. The final two are trace2 regions and instrumentation
that I've found helpful when rolling out MIDX bitmaps in a production
setting.

Sorry that these are so disjointed in nature ;-). I figured that it was
better to send a grab-bag series like this than to hold onto these
patches forever!

Thanks in advance for your review.

Taylor Blau (4):
  midx.c: fix whitespace typo
  midx.c: consider annotated tags during bitmap selection
  midx.c: instrument MIDX and bitmap generation with trace2 regions
  pack-bitmap-write.c: instrument number of reused bitmaps

 midx.c                        | 34 +++++++++++++++++++++++++++++++++-
 pack-bitmap-write.c           |  8 +++++++-
 t/t5326-multi-pack-bitmaps.sh | 24 ++++++++++++++++++++++++
 3 files changed, 64 insertions(+), 2 deletions(-)

-- 
2.38.0.16.g393fd4c6db

[PATCH 1/4] midx.c: fix whitespace typo

[Taylor Blau]

This was unintentionally introduced via 893b563505 (midx: inline
nth_midxed_pack_entry(), 2021-09-11) where "struct repository *r"
became "struct repository * r".

The latter does not adhere to our usual style conventions, so fix that
up to look more like our usual declarations.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/midx.c b/midx.c
index 3a8dcfe98e..a9df753220 100644
--- a/midx.c
+++ b/midx.c
@@ -278,7 +278,7 @@ uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos)
 			(off_t)pos * MIDX_CHUNK_OFFSET_WIDTH);
 }
 
-int fill_midx_entry(struct repository * r,
+int fill_midx_entry(struct repository *r,
 		    const struct object_id *oid,
 		    struct pack_entry *e,
 		    struct multi_pack_index *m)
-- 
2.38.0.16.g393fd4c6db

[PATCH 2/4] midx.c: consider annotated tags during bitmap selection

[Taylor Blau]

When generating a multi-pack bitmap without a `--refs-snapshot` (e.g.,
by running `git multi-pack-index write --bitmap` directly), we determine
the set of bitmap-able commits by enumerating each reference, and adding
the referrent as the tip of a reachability traversal when it appears
somewhere in the MIDX. (Any commit we encounter during the reachability
traversal then becomes a candidate for bitmap selection).

But we incorrectly avoid peeling the object at the tip of each
reference. So if we see some reference that points at an annotated tag
(which in turn points through zero or more additional annotated tags at
a commit), that we will not add it as a tip for the reachability
traversal. This means that if some commit C is only referenced through
one or more annotated tag(s), then C won't become a bitmap candidate.

Correct this by peeling the reference tips as we enumerate them to
ensure that we consider commits which are the targets of annotated tags,
in addition to commits which are referenced directly.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c                        |  4 ++++
 t/t5326-multi-pack-bitmaps.sh | 24 ++++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/midx.c b/midx.c
index a9df753220..d41dfd8174 100644
--- a/midx.c
+++ b/midx.c
@@ -980,6 +980,7 @@ static int add_ref_to_pending(const char *refname,
 			      int flag, void *cb_data)
 {
 	struct rev_info *revs = (struct rev_info*)cb_data;
+	struct object_id peeled;
 	struct object *object;
 
 	if ((flag & REF_ISSYMREF) && (flag & REF_ISBROKEN)) {
@@ -987,6 +988,9 @@ static int add_ref_to_pending(const char *refname,
 		return 0;
 	}
 
+	if (!peel_iterated_oid(oid, &peeled))
+		oid = &peeled;
+
 	object = parse_object_or_die(oid, refname);
 	if (object->type != OBJ_COMMIT)
 		return 0;
diff --git a/t/t5326-multi-pack-bitmaps.sh b/t/t5326-multi-pack-bitmaps.sh
index ad6eea5fa0..0882cbb6e4 100755
--- a/t/t5326-multi-pack-bitmaps.sh
+++ b/t/t5326-multi-pack-bitmaps.sh
@@ -410,4 +410,28 @@ test_expect_success 'preferred pack change with existing MIDX bitmap' '
 	)
 '
 
+test_expect_success 'tagged commits are selected for bitmapping' '
+	rm -fr repo &&
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		test_commit --annotate base &&
+		git repack -d &&
+
+		# Remove refs/heads/main which points at the commit directly,
+		# leaving only a reference to the annotated tag.
+		git branch -M main &&
+		git checkout base &&
+		git branch -d main &&
+
+		git multi-pack-index write --bitmap &&
+
+		git rev-parse HEAD >want &&
+		test-tool bitmap list-commits >actual &&
+		grep $(cat want) actual
+	)
+'
+
 test_done
-- 
2.38.0.16.g393fd4c6db

[PATCH 3/4] midx.c: instrument MIDX and bitmap generation with trace2 regions

[Taylor Blau]

When debugging MIDX and MIDX-bitmap related issues, it is useful to
figure out where Git is spending its time.

GitHub has been using the below trace2 regions to instrument various
components of generating a MIDX itself, as well time spent preparing to
build a MIDX bitmap.

These are limited to instrumenting the following functions:

  - midx.c::find_commits_for_midx_bitmap()
  - midx.c::midx_pack_order()
  - midx.c::prepare_midx_packing_data()
  - midx.c::write_midx_bitmap()
  - midx.c::write_midx_internal()
  - midx.c::write_midx_reverse_index()

to start and end with a trace2_region_enter() and trace2_region_leave(),
respectively.

The category for all of these is "midx", which matches the existing
convention. The region description matches the name of the function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/midx.c b/midx.c
index d41dfd8174..7cfad04a24 100644
--- a/midx.c
+++ b/midx.c
@@ -913,6 +913,8 @@ static uint32_t *midx_pack_order(struct write_midx_context *ctx)
 	uint32_t *pack_order;
 	uint32_t i;
 
+	trace2_region_enter("midx", "midx_pack_order", the_repository);
+
 	ALLOC_ARRAY(data, ctx->entries_nr);
 	for (i = 0; i < ctx->entries_nr; i++) {
 		struct pack_midx_entry *e = &ctx->entries[i];
@@ -930,6 +932,8 @@ static uint32_t *midx_pack_order(struct write_midx_context *ctx)
 		pack_order[i] = data[i].nr;
 	free(data);
 
+	trace2_region_leave("midx", "midx_pack_order", the_repository);
+
 	return pack_order;
 }
 
@@ -939,6 +943,8 @@ static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
 	struct strbuf buf = STRBUF_INIT;
 	const char *tmp_file;
 
+	trace2_region_enter("midx", "write_midx_reverse_index", the_repository);
+
 	strbuf_addf(&buf, "%s-%s.rev", midx_name, hash_to_hex(midx_hash));
 
 	tmp_file = write_rev_file_order(NULL, ctx->pack_order, ctx->entries_nr,
@@ -948,6 +954,8 @@ static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
 		die(_("cannot store reverse index file"));
 
 	strbuf_release(&buf);
+
+	trace2_region_leave("midx", "write_midx_reverse_index", the_repository);
 }
 
 static void clear_midx_files_ext(const char *object_dir, const char *ext,
@@ -963,6 +971,8 @@ static void prepare_midx_packing_data(struct packing_data *pdata,
 {
 	uint32_t i;
 
+	trace2_region_enter("midx", "prepare_midx_packing_data", the_repository);
+
 	memset(pdata, 0, sizeof(struct packing_data));
 	prepare_packing_data(the_repository, pdata);
 
@@ -973,6 +983,8 @@ static void prepare_midx_packing_data(struct packing_data *pdata,
 		oe_set_in_pack(pdata, to,
 			       ctx->info[ctx->pack_perm[from->pack_int_id]].p);
 	}
+
+	trace2_region_leave("midx", "prepare_midx_packing_data", the_repository);
 }
 
 static int add_ref_to_pending(const char *refname,
@@ -1070,6 +1082,9 @@ static struct commit **find_commits_for_midx_bitmap(uint32_t *indexed_commits_nr
 	struct rev_info revs;
 	struct bitmap_commit_cb cb = {0};
 
+	trace2_region_enter("midx", "find_commits_for_midx_bitmap",
+			    the_repository);
+
 	cb.ctx = ctx;
 
 	repo_init_revisions(the_repository, &revs, NULL);
@@ -1103,6 +1118,10 @@ static struct commit **find_commits_for_midx_bitmap(uint32_t *indexed_commits_nr
 		*indexed_commits_nr_p = cb.commits_nr;
 
 	release_revisions(&revs);
+
+	trace2_region_leave("midx", "find_commits_for_midx_bitmap",
+			    the_repository);
+
 	return cb.commits;
 }
 
@@ -1120,6 +1139,8 @@ static int write_midx_bitmap(const char *midx_name,
 	char *bitmap_name = xstrfmt("%s-%s.bitmap", midx_name,
 					hash_to_hex(midx_hash));
 
+	trace2_region_enter("midx", "write_midx_bitmap", the_repository);
+
 	if (flags & MIDX_WRITE_BITMAP_HASH_CACHE)
 		options |= BITMAP_OPT_HASH_CACHE;
 
@@ -1165,6 +1186,9 @@ static int write_midx_bitmap(const char *midx_name,
 cleanup:
 	free(index);
 	free(bitmap_name);
+
+	trace2_region_leave("midx", "write_midx_bitmap", the_repository);
+
 	return ret;
 }
 
@@ -1211,6 +1235,8 @@ static int write_midx_internal(const char *object_dir,
 	int result = 0;
 	struct chunkfile *cf;
 
+	trace2_region_enter("midx", "write_midx_internal", the_repository);
+
 	get_midx_filename(&midx_name, object_dir);
 	if (safe_create_leading_directories(midx_name.buf))
 		die_errno(_("unable to create leading directories of %s"),
@@ -1552,6 +1578,8 @@ static int write_midx_internal(const char *object_dir,
 	free(ctx.pack_order);
 	strbuf_release(&midx_name);
 
+	trace2_region_leave("midx", "write_midx_internal", the_repository);
+
 	return result;
 }
 
-- 
2.38.0.16.g393fd4c6db

[PATCH 4/4] pack-bitmap-write.c: instrument number of reused bitmaps

[Taylor Blau]

When debugging bitmap generation performance, it is useful to know how
many bitmaps were generated from scratch, and how many were the result
of permuting the bit-order of an existing bitmap.

Keep track of the latter, and emit the count as a trace2_data line to
aid in debugging.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 pack-bitmap-write.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
index a213f5eddc..cfa67a510f 100644
--- a/pack-bitmap-write.c
+++ b/pack-bitmap-write.c
@@ -384,6 +384,8 @@ static int fill_bitmap_tree(struct bitmap *bitmap,
 	return 0;
 }
 
+static int reused_bitmaps_nr;
+
 static int fill_bitmap_commit(struct bb_commit *ent,
 			      struct commit *commit,
 			      struct prio_queue *queue,
@@ -409,8 +411,10 @@ static int fill_bitmap_commit(struct bb_commit *ent,
 			 * bitmap and add its bits to this one. No need to walk
 			 * parents or the tree for this commit.
 			 */
-			if (old && !rebuild_bitmap(mapping, old, ent->bitmap))
+			if (old && !rebuild_bitmap(mapping, old, ent->bitmap)) {
+				reused_bitmaps_nr++;
 				continue;
+			}
 		}
 
 		/*
@@ -526,6 +530,8 @@ int bitmap_writer_build(struct packing_data *to_pack)
 
 	trace2_region_leave("pack-bitmap-write", "building_bitmaps_total",
 			    the_repository);
+	trace2_data_intmax("pack-bitmap-write", the_repository,
+			   "building_bitmaps_reused", reused_bitmaps_nr);
 
 	stop_progress(&writer.progress);
 
-- 
2.38.0.16.g393fd4c6db

Re: [PATCH 0/4] midx: trace2 regions and grab-bag patches

[Derrick Stolee]

On 10/12/2022 6:01 PM, Taylor Blau wrote:
> Here is a small handful of MIDX and MIDX bitmap-related patches that
> I've been carrying in GitHub's fork for a while now and forgot to send
> upstream.
> 
> The first is a small typofix, and the second is a legitimate bug fix
> which allows us to consider annotated tags as bitmap candidates during
> commit selection. The final two are trace2 regions and instrumentation
> that I've found helpful when rolling out MIDX bitmaps in a production
> setting.
> 
> Sorry that these are so disjointed in nature ;-). I figured that it was
> better to send a grab-bag series like this than to hold onto these
> patches forever!

As advertised, this set of patches are all nice and small. I've found
the additional tracing useful during performance investigations and
unobtrusive otherwise.

Though they looked familiar, I gave them a careful read and have no
comments. LGTM.

Thanks,
-Stolee

Re: [PATCH 0/4] midx: trace2 regions and grab-bag patches

[Junio C Hamano]

Derrick Stolee <derrickstolee@github.com> writes:

> On 10/12/2022 6:01 PM, Taylor Blau wrote:
>> Here is a small handful of MIDX and MIDX bitmap-related patches that
>> I've been carrying in GitHub's fork for a while now and forgot to send
>> upstream.
>> 
>> The first is a small typofix, and the second is a legitimate bug fix
>> which allows us to consider annotated tags as bitmap candidates during
>> commit selection. The final two are trace2 regions and instrumentation
>> that I've found helpful when rolling out MIDX bitmaps in a production
>> setting.
>> 
>> Sorry that these are so disjointed in nature ;-). I figured that it was
>> better to send a grab-bag series like this than to hold onto these
>> patches forever!
>
> As advertised, this set of patches are all nice and small. I've found
> the additional tracing useful during performance investigations and
> unobtrusive otherwise.
>
> Though they looked familiar, I gave them a careful read and have no
> comments. LGTM.

Yeah, they look good to me, too.

Thanks, both.