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[PATCH v7 15/16] commit-graph: reuse existing Bloom filters where possible

From
Taylor Blau <me@ttaylorr.com>
Date
Jun 25, 2024, 17:40 UTC
Message-ID
<1b80023e57cc61e6d7f9bffbc0426c703ddce31a.1719333276.git.me@ttaylorr.com>
In-Reply-To
<cover.1719333276.git.me@ttaylorr.com>

In an earlier commit, a bug was described where it's possible for Git to produce non-murmur3 hashes when the platform's "char" type is signed, and there are paths with characters whose highest bit is set (i.e. all characters >= 0x80).

That patch allows the caller to control which version of Bloom filters are read and written. However, even on platforms with a signed "char" type, it is possible to reuse existing Bloom filters if and only if there are no changed paths in any commit's first parent tree-diff whose characters have their highest bit set.

When this is the case, we can reuse the existing filter without having to compute a new one. This is done by marking trees which are known to have (or not have) any such paths. When a commit's root tree is verified to not have any such paths, we mark it as such and declare that the commit's Bloom filter is reusable.

Note that this heuristic only goes in one direction. If neither a commit nor its first parent have any paths in their trees with non-ASCII characters, then we know for certain that a path with non-ASCII characters will not appear in a tree-diff against that commit's first parent. The reverse isn't necessarily true: just because the tree-diff doesn't contain any such paths does not imply that no such paths exist in either tree.

So we end up recomputing some Bloom filters that we don't strictly have to (i.e. their bits are the same no matter which version of murmur3 we use). But culling these out is impossible, since we'd have to perform the full tree-diff, which is the same effort as computing the Bloom filter from scratch.

But because we can cache our results in each tree's flag bits, we can often avoid recomputing many filters, thereby reducing the time it takes to run

    $ git commit-graph write --changed-paths --reachable
when upgrading from v1 to v2 Bloom filters.

To benchmark this, let's generate a commit-graph in linux.git with v1 changed-paths in generation order[^1]:

    $ git clone git@github.com:torvalds/linux.git
    $ cd linux
    $ git commit-graph write --reachable --changed-paths
    $ graph=".git/objects/info/commit-graph"
    $ mv $graph{,.bak}

Then let's time how long it takes to go from v1 to v2 filters (with and without the upgrade path enabled), resetting the state of the commit-graph each time:

    $ git config commitGraph.changedPathsVersion 2
    $ hyperfine -p 'cp -f $graph.bak $graph' -L v 0,1 \
        'GIT_TEST_UPGRADE_BLOOM_FILTERS={v} git.compile commit-graph write --reachable --changed-paths'

On linux.git (where there aren't any non-ASCII paths), the timings indicate that this patch represents a speed-up over recomputing all Bloom filters from scratch:

    Benchmark 1: GIT_TEST_UPGRADE_BLOOM_FILTERS=0 git.compile commit-graph write --reachable --changed-paths
      Time (mean ± σ):     124.873 s ±  0.316 s    [User: 124.081 s, System: 0.643 s]
      Range (min … max):   124.621 s … 125.227 s    3 runs
    Benchmark 2: GIT_TEST_UPGRADE_BLOOM_FILTERS=1 git.compile commit-graph write --reachable --changed-paths
      Time (mean ± σ):     79.271 s ±  0.163 s    [User: 74.611 s, System: 4.521 s]
      Range (min … max):   79.112 s … 79.437 s    3 runs
    Summary
      'GIT_TEST_UPGRADE_BLOOM_FILTERS=1 git.compile commit-graph write --reachable --changed-paths' ran
        1.58 ± 0.01 times faster than 'GIT_TEST_UPGRADE_BLOOM_FILTERS=0 git.compile commit-graph write --reachable --changed-paths'

On git.git, we do have some non-ASCII paths, giving us a more modest improvement from 4.163 seconds to 3.348 seconds, for a 1.24x speed-up. On my machine, the stats for git.git are:

  - 8,285 Bloom filters computed from scratch
  - 10 Bloom filters generated as empty
  - 4 Bloom filters generated as truncated due to too many changed paths
  - 65,114 Bloom filters were reused when transitioning from v1 to v2.
[^1]: Note that this is is important, since `--stdin-packs` or
  `--stdin-commits` orders commits in the commit-graph by their pack
  position (with `--stdin-packs`) or in the raw input (with
  `--stdin-commits`).
  Since we compute Bloom filters in the same order that commits appear
  in the graph, we must see a commit's (first) parent before we process
  the commit itself. This is only guaranteed to happen when sorting
  commits by their generation number.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 bloom.c              | 90 ++++++++++++++++++++++++++++++++++++++++++--
 bloom.h              |  1 +
 commit-graph.c       |  5 +++
 object.h             |  1 +
 t/t4216-log-bloom.sh | 39 ++++++++++++++++++-
 5 files changed, 132 insertions(+), 4 deletions(-)
diff --git a/bloom.c b/bloom.c
index 323d8012b8..740c1767ea 100644
--- a/bloom.c
+++ b/bloom.c
@@ -6,6 +6,9 @@
 #include "commit-graph.h"
 #include "commit.h"
 #include "commit-slab.h"
+#include "tree.h"
+#include "tree-walk.h"
+#include "config.h"
 
 define_commit_slab(bloom_filter_slab, struct bloom_filter);
 
@@ -283,6 +286,73 @@ static void init_truncated_large_filter(struct bloom_filter *filter,
 	filter->version = version;
 }
 
+#define VISITED   (1u<<21)
+#define HIGH_BITS (1u<<22)
+
+static int has_entries_with_high_bit(struct repository *r, struct tree *t)
+{
+	if (parse_tree(t))
+		return 1;
+
+	if (!(t->object.flags & VISITED)) {
+		struct tree_desc desc;
+		struct name_entry entry;
+
+		init_tree_desc(&desc, &t->object.oid, t->buffer, t->size);
+		while (tree_entry(&desc, &entry)) {
+			size_t i;
+			for (i = 0; i < entry.pathlen; i++) {
+				if (entry.path[i] & 0x80) {
+					t->object.flags |= HIGH_BITS;
+					goto done;
+				}
+			}
+
+			if (S_ISDIR(entry.mode)) {
+				struct tree *sub = lookup_tree(r, &entry.oid);
+				if (sub && has_entries_with_high_bit(r, sub)) {
+					t->object.flags |= HIGH_BITS;
+					goto done;
+				}
+			}
+
+		}
+
+done:
+		t->object.flags |= VISITED;
+	}
+
+	return !!(t->object.flags & HIGH_BITS);
+}
+
+static int commit_tree_has_high_bit_paths(struct repository *r,
+					  struct commit *c)
+{
+	struct tree *t;
+	if (repo_parse_commit(r, c))
+		return 1;
+	t = repo_get_commit_tree(r, c);
+	if (!t)
+		return 1;
+	return has_entries_with_high_bit(r, t);
+}
+
+static struct bloom_filter *upgrade_filter(struct repository *r, struct commit *c,
+					   struct bloom_filter *filter,
+					   int hash_version)
+{
+	struct commit_list *p = c->parents;
+	if (commit_tree_has_high_bit_paths(r, c))
+		return NULL;
+
+	if (p && commit_tree_has_high_bit_paths(r, p->item))
+		return NULL;
+
+	filter->version = hash_version;
+
+	return filter;
+}
+
 struct bloom_filter *get_bloom_filter(struct repository *r, struct commit *c)
 {
 	struct bloom_filter *filter;
@@ -325,9 +395,23 @@ struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
 						     filter, graph_pos);
 	}
 
-	if ((filter->data && filter->len) &&
-	    (!settings || settings->hash_version == filter->version))
-		return filter;
+	if (filter->data && filter->len) {
+		struct bloom_filter *upgrade;
+		if (!settings || settings->hash_version == filter->version)
+			return filter;
+
+		/* version mismatch, see if we can upgrade */
+		if (compute_if_not_present &&
+		    git_env_bool("GIT_TEST_UPGRADE_BLOOM_FILTERS", 1)) {
+			upgrade = upgrade_filter(r, c, filter,
+						 settings->hash_version);
+			if (upgrade) {
+				if (computed)
+					*computed |= BLOOM_UPGRADED;
+				return upgrade;
+			}
+		}
+	}
 	if (!compute_if_not_present)
 		return NULL;
 
diff --git a/bloom.h b/bloom.h
index bfe389e29c..e3a9b68905 100644
--- a/bloom.h
+++ b/bloom.h
@@ -102,6 +102,7 @@ enum bloom_filter_computed {
 	BLOOM_COMPUTED     = (1 << 1),
 	BLOOM_TRUNC_LARGE  = (1 << 2),
 	BLOOM_TRUNC_EMPTY  = (1 << 3),
+	BLOOM_UPGRADED     = (1 << 4),
 };
 
 struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,
diff --git a/commit-graph.c b/commit-graph.c
index c1177631bb..2f0a08f302 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1168,6 +1168,7 @@ struct write_commit_graph_context {
 	int count_bloom_filter_not_computed;
 	int count_bloom_filter_trunc_empty;
 	int count_bloom_filter_trunc_large;
+	int count_bloom_filter_upgraded;
 };
 
 static int write_graph_chunk_fanout(struct hashfile *f,
@@ -1775,6 +1776,8 @@ static void trace2_bloom_filter_write_statistics(struct write_commit_graph_conte
 			   ctx->count_bloom_filter_trunc_empty);
 	trace2_data_intmax("commit-graph", ctx->r, "filter-trunc-large",
 			   ctx->count_bloom_filter_trunc_large);
+	trace2_data_intmax("commit-graph", ctx->r, "filter-upgraded",
+			   ctx->count_bloom_filter_upgraded);
 }
 
 static void compute_bloom_filters(struct write_commit_graph_context *ctx)
@@ -1816,6 +1819,8 @@ static void compute_bloom_filters(struct write_commit_graph_context *ctx)
 				ctx->count_bloom_filter_trunc_empty++;
 			if (computed & BLOOM_TRUNC_LARGE)
 				ctx->count_bloom_filter_trunc_large++;
+		} else if (computed & BLOOM_UPGRADED) {
+			ctx->count_bloom_filter_upgraded++;
 		} else if (computed & BLOOM_NOT_COMPUTED)
 			ctx->count_bloom_filter_not_computed++;
 		ctx->total_bloom_filter_data_size += filter
diff --git a/object.h b/object.h
index d2f9de6bb5..0a04661a34 100644
--- a/object.h
+++ b/object.h
@@ -75,6 +75,7 @@ void object_array_init(struct object_array *array);
  * commit-reach.c:                                  16-----19
  * sha1-name.c:                                              20
  * list-objects-filter.c:                                      21
+ * bloom.c:                                                    2122
  * builtin/fsck.c:           0--3
  * builtin/gc.c:             0
  * builtin/index-pack.c:                                     2021
diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
index cc6e5733f6..3f163dc396 100755
--- a/t/t4216-log-bloom.sh
+++ b/t/t4216-log-bloom.sh
@@ -222,6 +222,10 @@ test_filter_trunc_large () {
 	grep "\"key\":\"filter-trunc-large\",\"value\":\"$1\"" $2
 }
 
+test_filter_upgraded () {
+	grep "\"key\":\"filter-upgraded\",\"value\":\"$1\"" $2
+}
+
 test_expect_success 'correctly report changes over limit' '
 	git init limits &&
 	(
@@ -667,7 +671,14 @@ test_expect_success 'when writing another commit graph, preserve existing versio
 test_expect_success 'when writing commit graph, do not reuse changed-path of another version' '
 	git init doublewrite &&
 	test_commit -C doublewrite c "$CENT" &&
+
 	git -C doublewrite config --add commitGraph.changedPathsVersion 1 &&
+	>trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+		git -C doublewrite commit-graph write --reachable --changed-paths &&
+	test_filter_computed 1 trace2.txt &&
+	test_filter_upgraded 0 trace2.txt &&
+
 	git -C doublewrite commit-graph write --reachable --changed-paths &&
 	for v in -2 3
 	do
@@ -678,8 +689,14 @@ test_expect_success 'when writing commit graph, do not reuse changed-path of ano
 		EOF
 		test_cmp expect err || return 1
 	done &&
+
 	git -C doublewrite config --add commitGraph.changedPathsVersion 2 &&
-	git -C doublewrite commit-graph write --reachable --changed-paths &&
+	>trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+		git -C doublewrite commit-graph write --reachable --changed-paths &&
+	test_filter_computed 1 trace2.txt &&
+	test_filter_upgraded 0 trace2.txt &&
+
 	(
 		cd doublewrite &&
 		echo "c01f" >expect &&
@@ -688,6 +705,26 @@ test_expect_success 'when writing commit graph, do not reuse changed-path of ano
 	)
 '
 
+test_expect_success 'when writing commit graph, reuse changed-path of another version where possible' '
+	git init upgrade &&
+
+	test_commit -C upgrade base no-high-bits &&
+
+	git -C upgrade config --add commitGraph.changedPathsVersion 1 &&
+	>trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+		git -C upgrade commit-graph write --reachable --changed-paths &&
+	test_filter_computed 1 trace2.txt &&
+	test_filter_upgraded 0 trace2.txt &&
+
+	git -C upgrade config --add commitGraph.changedPathsVersion 2 &&
+	>trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \
+		git -C upgrade commit-graph write --reachable --changed-paths &&
+	test_filter_computed 0 trace2.txt &&
+	test_filter_upgraded 1 trace2.txt
+'
+
 corrupt_graph () {
 	test_when_finished "rm -rf $graph" &&
 	git commit-graph write --reachable --changed-paths &&

-- 
2.45.2.664.g446e6a2b1f
Previous: Taylor BlauNext: Taylor Blau
Message 35 of 36 in “bloom: changed-path Bloom filters v2 (& sundries)”
  1. 00/16 bloom: changed-path Bloom filters v2 (& sundries)Taylor Blau, Jan 31, 2024
  2. 01/16 t/t4216-log-bloom.sh: harden `test_bloom_filters_not_used()`Taylor Blau, Jan 31, 2024
  3. 02/16 revision.c: consult Bloom filters for root commitsTaylor Blau, Jan 31, 2024
  4. 03/16 commit-graph: ensure Bloom filters are read with consistent settingsTaylor Blau, Jan 31, 2024
  5. 04/16 gitformat-commit-graph: describe version 2 of BDATTaylor Blau, Jan 31, 2024
  6. 05/16 t/helper/test-read-graph.c: extract `dump_graph_info()`Taylor Blau, Jan 31, 2024
  7. 06/16 bloom.h: make `load_bloom_filter_from_graph()` publicTaylor Blau, Jan 31, 2024
  8. 07/16 t/helper/test-read-graph: implement `bloom-filters` modeTaylor Blau, Jan 31, 2024
  9. 08/16 t4216: test changed path filters with high bit pathsTaylor Blau, Jan 31, 2024
  10. 09/16 repo-settings: introduce commitgraph.changedPathsVersionTaylor Blau, Jan 31, 2024
  11. 10/16 bloom: annotate filters with hash versionTaylor Blau, Jan 31, 2024
  12. 11/16 bloom: prepare to discard incompatible Bloom filtersTaylor Blau, Jan 31, 2024
  13. 12/16 commit-graph: unconditionally load Bloom filtersTaylor Blau, Jan 31, 2024
  14. 13/16 commit-graph: new Bloom filter version that fixes murmur3Taylor Blau, Jan 31, 2024
  15. SZEDER GáborFeb 25, 2024
  16. Jiang XinFeb 28, 2024
  17. 14/16 object.h: fix mis-aligned flag bits tableTaylor Blau, Jan 31, 2024
  18. 15/16 commit-graph: reuse existing Bloom filters where possibleTaylor Blau, Jan 31, 2024
  19. 16/16 bloom: introduce `deinit_bloom_filters()`Taylor Blau, Jan 31, 2024
  20. 00/16 bloom: changed-path Bloom filters v2 (& sundries)Taylor Blau, Jun 25, 2024
  21. 01/16 t/t4216-log-bloom.sh: harden `test_bloom_filters_not_used()`Taylor Blau, Jun 25, 2024
  22. 02/16 revision.c: consult Bloom filters for root commitsTaylor Blau, Jun 25, 2024
  23. 03/16 commit-graph: ensure Bloom filters are read with consistent settingsTaylor Blau, Jun 25, 2024
  24. 04/16 gitformat-commit-graph: describe version 2 of BDATTaylor Blau, Jun 25, 2024
  25. 05/16 t/helper/test-read-graph.c: extract `dump_graph_info()`Taylor Blau, Jun 25, 2024
  26. 06/16 bloom.h: make `load_bloom_filter_from_graph()` publicTaylor Blau, Jun 25, 2024
  27. 07/16 t/helper/test-read-graph: implement `bloom-filters` modeTaylor Blau, Jun 25, 2024
  28. 08/16 t4216: test changed path filters with high bit pathsTaylor Blau, Jun 25, 2024
  29. 09/16 repo-settings: introduce commitgraph.changedPathsVersionTaylor Blau, Jun 25, 2024
  30. 10/16 bloom: annotate filters with hash versionTaylor Blau, Jun 25, 2024
  31. 11/16 bloom: prepare to discard incompatible Bloom filtersTaylor Blau, Jun 25, 2024
  32. 12/16 commit-graph: unconditionally load Bloom filtersTaylor Blau, Jun 25, 2024
  33. 13/16 commit-graph: new Bloom filter version that fixes murmur3Taylor Blau, Jun 25, 2024
  34. 14/16 object.h: fix mis-aligned flag bits tableTaylor Blau, Jun 25, 2024
  35. 15/16 commit-graph: reuse existing Bloom filters where possibleTaylor Blau, Jun 25, 2024
  36. 16/16 bloom: introduce `deinit_bloom_filters()`Taylor Blau, Jun 25, 2024

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