@matheusbsbs / matheus-bagliano / commits / 5421e7c3a1

commit-graph: reuse existing Bloom filters where possible

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> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Taylor Blau committed Jun 25, 2024 at 13:40 UTC 5421e7c3a119bc869c25e2e3d5692c86bb8aa5d9

5 files changed +132 -4

bloom.c

+87 -3

@@ -6,6 +6,9 @@

6 #include "commit-graph.h"

7 #include "commit.h"

8 #include "commit-slab.h"

9 +#include "tree.h"

10 +#include "tree-walk.h"

11 +#include "config.h"

13 define_commit_slab(bloom_filter_slab, struct bloom_filter);

@@ -283,6 +286,73 @@ static void init_truncated_large_filter(struct bloom_filter *filter,

286 filter->version = version;

287 }

288

289 +#define VISITED (1u<<21)

290 +#define HIGH_BITS (1u<<22)

291 +

292 +static int has_entries_with_high_bit(struct repository *r, struct tree *t)

293 +{

294 + if (parse_tree(t))

295 + return 1;

296 +

297 + if (!(t->object.flags & VISITED)) {

298 + struct tree_desc desc;

299 + struct name_entry entry;

300 +

301 + init_tree_desc(&desc, &t->object.oid, t->buffer, t->size);

302 + while (tree_entry(&desc, &entry)) {

303 + size_t i;

304 + for (i = 0; i < entry.pathlen; i++) {

305 + if (entry.path[i] & 0x80) {

306 + t->object.flags |= HIGH_BITS;

307 + goto done;

308 + }

309 + }

310 +

311 + if (S_ISDIR(entry.mode)) {

312 + struct tree *sub = lookup_tree(r, &entry.oid);

313 + if (sub && has_entries_with_high_bit(r, sub)) {

314 + t->object.flags |= HIGH_BITS;

315 + goto done;

316 + }

317 + }

318 +

319 + }

320 +

321 +done:

322 + t->object.flags |= VISITED;

323 + }

324 +

325 + return !!(t->object.flags & HIGH_BITS);

326 +}

327 +

328 +static int commit_tree_has_high_bit_paths(struct repository *r,

329 + struct commit *c)

330 +{

331 + struct tree *t;

332 + if (repo_parse_commit(r, c))

333 + return 1;

334 + t = repo_get_commit_tree(r, c);

335 + if (!t)

336 + return 1;

337 + return has_entries_with_high_bit(r, t);

338 +}

339 +

340 +static struct bloom_filter *upgrade_filter(struct repository *r, struct commit *c,

341 + struct bloom_filter *filter,

342 + int hash_version)

343 +{

344 + struct commit_list *p = c->parents;

345 + if (commit_tree_has_high_bit_paths(r, c))

346 + return NULL;

347 +

348 + if (p && commit_tree_has_high_bit_paths(r, p->item))

349 + return NULL;

350 +

351 + filter->version = hash_version;

352 +

353 + return filter;

354 +}

355 +

356 struct bloom_filter *get_bloom_filter(struct repository *r, struct commit *c)

357 {

358 struct bloom_filter *filter;

@@ -325,9 +395,23 @@ struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,

395 filter, graph_pos);

396 }

397

328 - if ((filter->data && filter->len) &&

329 - (!settings || settings->hash_version == filter->version))

330 - return filter;

398 + if (filter->data && filter->len) {

399 + struct bloom_filter *upgrade;

400 + if (!settings || settings->hash_version == filter->version)

401 + return filter;

402 +

403 + /* version mismatch, see if we can upgrade */

404 + if (compute_if_not_present &&

405 + git_env_bool("GIT_TEST_UPGRADE_BLOOM_FILTERS", 1)) {

406 + upgrade = upgrade_filter(r, c, filter,

407 + settings->hash_version);

408 + if (upgrade) {

409 + if (computed)

410 + *computed |= BLOOM_UPGRADED;

411 + return upgrade;

412 + }

413 + }

414 + }

415 if (!compute_if_not_present)

416 return NULL;

417

bloom.h

@@ -102,6 +102,7 @@ enum bloom_filter_computed {

102 BLOOM_COMPUTED = (1 << 1),

103 BLOOM_TRUNC_LARGE = (1 << 2),

104 BLOOM_TRUNC_EMPTY = (1 << 3),

105 + BLOOM_UPGRADED = (1 << 4),

106 };

107

108 struct bloom_filter *get_or_compute_bloom_filter(struct repository *r,

commit-graph.c

@@ -1165,6 +1165,7 @@ struct write_commit_graph_context {

1165 int count_bloom_filter_not_computed;

1166 int count_bloom_filter_trunc_empty;

1167 int count_bloom_filter_trunc_large;

1168 + int count_bloom_filter_upgraded;

1169 };

1170

1171 static int write_graph_chunk_fanout(struct hashfile *f,

@@ -1772,6 +1773,8 @@ static void trace2_bloom_filter_write_statistics(struct write_commit_graph_conte

1773 ctx->count_bloom_filter_trunc_empty);

1774 trace2_data_intmax("commit-graph", ctx->r, "filter-trunc-large",

1775 ctx->count_bloom_filter_trunc_large);

1776 + trace2_data_intmax("commit-graph", ctx->r, "filter-upgraded",

1777 + ctx->count_bloom_filter_upgraded);

1778 }

1779

1780 static void compute_bloom_filters(struct write_commit_graph_context *ctx)

@@ -1813,6 +1816,8 @@ static void compute_bloom_filters(struct write_commit_graph_context *ctx)

1816 ctx->count_bloom_filter_trunc_empty++;

1817 if (computed & BLOOM_TRUNC_LARGE)

1818 ctx->count_bloom_filter_trunc_large++;

1819 + } else if (computed & BLOOM_UPGRADED) {

1820 + ctx->count_bloom_filter_upgraded++;

1821 } else if (computed & BLOOM_NOT_COMPUTED)

1822 ctx->count_bloom_filter_not_computed++;

1823 ctx->total_bloom_filter_data_size += filter

object.h

@@ -75,6 +75,7 @@ void object_array_init(struct object_array *array);

75 * commit-reach.c: 16-----19

76 * sha1-name.c: 20

77 * list-objects-filter.c: 21

78 + * bloom.c: 2122

79 * builtin/fsck.c: 0--3

80 * builtin/gc.c: 0

81 * builtin/index-pack.c: 2021

t/t4216-log-bloom.sh

+38 -1

@@ -222,6 +222,10 @@ test_filter_trunc_large () {

222 grep "\"key\":\"filter-trunc-large\",\"value\":\"$1\"" $2

223 }

224

225 +test_filter_upgraded () {

226 + grep "\"key\":\"filter-upgraded\",\"value\":\"$1\"" $2

227 +}

228 +

229 test_expect_success 'correctly report changes over limit' '

230 git init limits &&

231 (

@@ -667,7 +671,14 @@ test_expect_success 'when writing another commit graph, preserve existing versio

671 test_expect_success 'when writing commit graph, do not reuse changed-path of another version' '

672 git init doublewrite &&

673 test_commit -C doublewrite c "$CENT" &&

674 +

675 git -C doublewrite config --add commitGraph.changedPathsVersion 1 &&

676 + >trace2.txt &&

677 + GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \

678 + git -C doublewrite commit-graph write --reachable --changed-paths &&

679 + test_filter_computed 1 trace2.txt &&

680 + test_filter_upgraded 0 trace2.txt &&

681 +

682 git -C doublewrite commit-graph write --reachable --changed-paths &&

683 for v in -2 3

684 do

@@ -678,8 +689,14 @@ test_expect_success 'when writing commit graph, do not reuse changed-path of ano

689 EOF

690 test_cmp expect err || return 1

691 done &&

692 +

693 git -C doublewrite config --add commitGraph.changedPathsVersion 2 &&

682 - git -C doublewrite commit-graph write --reachable --changed-paths &&

694 + >trace2.txt &&

695 + GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \

696 + git -C doublewrite commit-graph write --reachable --changed-paths &&

697 + test_filter_computed 1 trace2.txt &&

698 + test_filter_upgraded 0 trace2.txt &&

699 +

700 (

701 cd doublewrite &&

702 echo "c01f" >expect &&

@@ -688,6 +705,26 @@ test_expect_success 'when writing commit graph, do not reuse changed-path of ano

705 )

706 '

707

708 +test_expect_success 'when writing commit graph, reuse changed-path of another version where possible' '

709 + git init upgrade &&

710 +

711 + test_commit -C upgrade base no-high-bits &&

712 +

713 + git -C upgrade config --add commitGraph.changedPathsVersion 1 &&

714 + >trace2.txt &&

715 + GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \

716 + git -C upgrade commit-graph write --reachable --changed-paths &&

717 + test_filter_computed 1 trace2.txt &&

718 + test_filter_upgraded 0 trace2.txt &&

719 +

720 + git -C upgrade config --add commitGraph.changedPathsVersion 2 &&

721 + >trace2.txt &&

722 + GIT_TRACE2_EVENT="$(pwd)/trace2.txt" \

723 + git -C upgrade commit-graph write --reachable --changed-paths &&

724 + test_filter_computed 0 trace2.txt &&

725 + test_filter_upgraded 1 trace2.txt

726 +'

727 +

728 corrupt_graph () {

729 test_when_finished "rm -rf $graph" &&

730 git commit-graph write --reachable --changed-paths &&