From 59f0d5073fd9f0497b9ff9a48fb3a7a8d82d1f9d Mon Sep 17 00:00:00 2001 From: Taylor Blau Date: Thu, 17 Sep 2020 22:59:44 -0400 Subject: [PATCH] bloom: encode out-of-bounds filters as non-empty MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit When a changed-path Bloom filter has either zero, or more than a certain number (commonly 512) of entries, the commit-graph machinery encodes it as "missing". More specifically, it sets the indices adjacent in the BIDX chunk as equal to each other to indicate a "length 0" filter; that is, that the filter occupies zero bytes on disk. This has heretofore been fine, since the commit-graph machinery has no need to care about these filters with too few or too many changed paths. Both cases act like no filter has been generated at all, and so there is no need to store them. In a subsequent commit, however, the commit-graph machinery will learn to only compute Bloom filters for some commits in the current commit-graph layer. This is a change from the current implementation which computes Bloom filters for all commits that are in the layer being written. Critically for this patch, only computing some of the Bloom filters means adding a third state for length 0 Bloom filters: zero entries, too many entries, or "hasn't been computed". It will be important for that future patch to distinguish between "not representable" (i.e., zero or too-many changed paths), and "hasn't been computed". In particular, we don't want to waste time recomputing filters that have already been computed. To that end, change how we store Bloom filters in the "computed but not representable" category: - Bloom filters with no entries are stored as a single byte with all bits low (i.e., all queries to that Bloom filter will return "definitely not") - Bloom filters with too many entries are stored as a single byte with all bits set high (i.e., all queries to that Bloom filter will return "maybe"). These rules are sufficient to not incur a behavior change by changing the on-disk representation of these two classes. Likewise, no specification changes are necessary for the commit-graph format, either: - Filters that were previously empty will be recomputed and stored according to the new rules, and - old clients reading filters generated by new clients will interpret the filters correctly and be none the wiser to how they were generated. Clients will invoke the Bloom machinery in more cases than before, but this can be addressed by returning a NULL filter when all bits are set high. This can be addressed in a future patch. Note that this does increase the size of on-disk commit-graphs, but far less than other proposals. In particular, this is generally more efficient than storing a bitmap for which commits haven't computed their Bloom filters. Storing a bitmap incurs a penalty of one bit per commit, whereas storing explicit filters as above incurs a penalty of one byte per too-large or empty commit. In practice, these boundary commits likely occupy a small proportion of the overall number of commits, and so the size penalty is likely smaller than storing a bitmap for all commits. See, for example, these relative proportions of such boundary commits (collected by SZEDER Gábor): | Percentage of | commit-graph | | | commits modifying | file size | | ├────────┬──────────────┼───────────────────┤ pct. | | 0 path | >= 512 paths | before | after | change | ┌────────────────┼────────┼──────────────┼─────────┼─────────┼───────────┤ | android-base | 13.20% | 0.13% | 37.468M | 37.534M | +0.1741 % | | cmssw | 0.15% | 0.23% | 17.118M | 17.119M | +0.0091 % | | cpython | 3.07% | 0.01% | 7.967M | 7.971M | +0.0423 % | | elasticsearch | 0.70% | 1.00% | 8.833M | 8.835M | +0.0128 % | | gcc | 0.00% | 0.08% | 16.073M | 16.074M | +0.0030 % | | gecko-dev | 0.14% | 0.64% | 59.868M | 59.874M | +0.0105 % | | git | 0.11% | 0.02% | 3.895M | 3.895M | +0.0020 % | | glibc | 0.02% | 0.10% | 3.555M | 3.555M | +0.0021 % | | go | 0.00% | 0.07% | 3.186M | 3.186M | +0.0018 % | | homebrew-cask | 0.40% | 0.02% | 7.035M | 7.035M | +0.0065 % | | homebrew-core | 0.01% | 0.01% | 11.611M | 11.611M | +0.0002 % | | jdk | 0.26% | 5.64% | 5.537M | 5.540M | +0.0590 % | | linux | 0.01% | 0.51% | 63.735M | 63.740M | +0.0073 % | | llvm-project | 0.12% | 0.03% | 25.515M | 25.516M | +0.0050 % | | rails | 0.10% | 0.10% | 6.252M | 6.252M | +0.0027 % | | rust | 0.07% | 0.17% | 9.364M | 9.364M | +0.0033 % | | tensorflow | 0.09% | 1.02% | 7.009M | 7.010M | +0.0158 % | | webkit | 0.05% | 0.31% | 17.405M | 17.406M | +0.0047 % | (where the above increase is determined by computing a non-split commit-graph before and after this patch). Given that these projects are all "large" by commit count, the storage cost by writing these filters explicitly is negligible. In the most extreme example, android-base (which has 494,848 commits at the time of writing) would have its commit-graph increase by a modest 68.4 KB. Finally, a test to exercise filters which contain too many changed path entries will be introduced in a subsequent patch. Suggested-by: SZEDER Gábor Suggested-by: Jakub Narębski Helped-by: Derrick Stolee Helped-by: SZEDER Gábor Helped-by: Junio C Hamano Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano --- .../technical/commit-graph-format.txt | 2 +- bloom.c | 16 ++++++++-- bloom.h | 1 + commit-graph.c | 5 +++ t/t0095-bloom.sh | 8 ++--- t/t4216-log-bloom.sh | 31 +++++++++++++++++-- 6 files changed, 54 insertions(+), 9 deletions(-) diff --git a/Documentation/technical/commit-graph-format.txt b/Documentation/technical/commit-graph-format.txt index 440541045d..814ef810a3 100644 --- a/Documentation/technical/commit-graph-format.txt +++ b/Documentation/technical/commit-graph-format.txt @@ -120,7 +120,7 @@ CHUNK DATA: * The rest of the chunk is the concatenation of all the computed Bloom filters for the commits in lexicographic order. * Note: Commits with no changes or more than 512 changes have Bloom filters - of length zero. + of length one, with either all bits set to zero or one respectively. * The BDAT chunk is present if and only if BIDX is present. Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional] diff --git a/bloom.c b/bloom.c index db9fb82437..d234551ce0 100644 --- a/bloom.c +++ b/bloom.c @@ -177,6 +177,13 @@ static int pathmap_cmp(const void *hashmap_cmp_fn_data, return strcmp(e1->path, e2->path); } +static void init_truncated_large_filter(struct bloom_filter *filter) +{ + filter->data = xmalloc(1); + filter->data[0] = 0xFF; + filter->len = 1; +} + struct bloom_filter *get_or_compute_bloom_filter(struct repository *r, struct commit *c, int compute_if_not_present, @@ -260,12 +267,18 @@ struct bloom_filter *get_or_compute_bloom_filter(struct repository *r, } if (hashmap_get_size(&pathmap) > settings->max_changed_paths) { + init_truncated_large_filter(filter); if (computed) *computed |= BLOOM_TRUNC_LARGE; goto cleanup; } filter->len = (hashmap_get_size(&pathmap) * settings->bits_per_entry + BITS_PER_WORD - 1) / BITS_PER_WORD; + if (!filter->len) { + if (computed) + *computed |= BLOOM_TRUNC_EMPTY; + filter->len = 1; + } filter->data = xcalloc(filter->len, sizeof(unsigned char)); hashmap_for_each_entry(&pathmap, &iter, e, entry) { @@ -279,8 +292,7 @@ struct bloom_filter *get_or_compute_bloom_filter(struct repository *r, } else { for (i = 0; i < diff_queued_diff.nr; i++) diff_free_filepair(diff_queued_diff.queue[i]); - filter->data = NULL; - filter->len = 0; + init_truncated_large_filter(filter); if (computed) *computed |= BLOOM_TRUNC_LARGE; diff --git a/bloom.h b/bloom.h index c6d77e8393..adde6dfe21 100644 --- a/bloom.h +++ b/bloom.h @@ -93,6 +93,7 @@ enum bloom_filter_computed { BLOOM_NOT_COMPUTED = (1 << 0), BLOOM_COMPUTED = (1 << 1), BLOOM_TRUNC_LARGE = (1 << 2), + BLOOM_TRUNC_EMPTY = (1 << 3), }; struct bloom_filter *get_or_compute_bloom_filter(struct repository *r, diff --git a/commit-graph.c b/commit-graph.c index 4d6ce2967e..0a9ace06fb 100644 --- a/commit-graph.c +++ b/commit-graph.c @@ -968,6 +968,7 @@ struct write_commit_graph_context { int count_bloom_filter_computed; int count_bloom_filter_not_computed; + int count_bloom_filter_trunc_empty; int count_bloom_filter_trunc_large; }; @@ -1396,6 +1397,8 @@ static void trace2_bloom_filter_write_statistics(struct write_commit_graph_conte ctx->count_bloom_filter_computed); trace2_data_intmax("commit-graph", ctx->r, "filter-not-computed", ctx->count_bloom_filter_not_computed); + trace2_data_intmax("commit-graph", ctx->r, "filter-trunc-empty", + ctx->count_bloom_filter_trunc_empty); trace2_data_intmax("commit-graph", ctx->r, "filter-trunc-large", ctx->count_bloom_filter_trunc_large); } @@ -1432,6 +1435,8 @@ static void compute_bloom_filters(struct write_commit_graph_context *ctx) &computed); if (computed & BLOOM_COMPUTED) { ctx->count_bloom_filter_computed++; + if (computed & BLOOM_TRUNC_EMPTY) + ctx->count_bloom_filter_trunc_empty++; if (computed & BLOOM_TRUNC_LARGE) ctx->count_bloom_filter_trunc_large++; } else if (computed & BLOOM_NOT_COMPUTED) diff --git a/t/t0095-bloom.sh b/t/t0095-bloom.sh index 232ba2c485..7e4ab1795f 100755 --- a/t/t0095-bloom.sh +++ b/t/t0095-bloom.sh @@ -71,8 +71,8 @@ test_expect_success 'get bloom filters for commit with no changes' ' git init && git commit --allow-empty -m "c0" && cat >expect <<-\EOF && - Filter_Length:0 - Filter_Data: + Filter_Length:1 + Filter_Data:00| EOF test-tool bloom get_filter_for_commit "$(git rev-parse HEAD)" >actual && test_cmp expect actual @@ -107,8 +107,8 @@ test_expect_success EXPENSIVE 'get bloom filter for commit with 513 changes' ' git add bigDir && git commit -m "commit with 513 changes" && cat >expect <<-\EOF && - Filter_Length:0 - Filter_Data: + Filter_Length:1 + Filter_Data:ff| EOF test-tool bloom get_filter_for_commit "$(git rev-parse HEAD)" >actual && test_cmp expect actual diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh index 6535a3c5d5..ec9845c9be 100755 --- a/t/t4216-log-bloom.sh +++ b/t/t4216-log-bloom.sh @@ -30,8 +30,10 @@ test_expect_success 'setup test - repo, commits, commit graph, log outputs' ' rm file_to_be_deleted && git add . && git commit -m "file removed" && + git commit --allow-empty -m "empty" && git commit-graph write --reachable --changed-paths ' + graph_read_expect () { NUM_CHUNKS=5 cat >expect <<- EOF @@ -44,7 +46,7 @@ graph_read_expect () { } test_expect_success 'commit-graph write wrote out the bloom chunks' ' - graph_read_expect 15 + graph_read_expect 16 ' # Turn off any inherited trace2 settings for this test. @@ -151,7 +153,7 @@ test_expect_success 'setup - add commit-graph to the chain with Bloom filters' ' test_bloom_filters_used_when_some_filters_are_missing () { log_args=$1 - bloom_trace_prefix="statistics:{\"filter_not_present\":3,\"maybe\":6,\"definitely_not\":8" + bloom_trace_prefix="statistics:{\"filter_not_present\":3,\"maybe\":6,\"definitely_not\":9" setup "$log_args" && grep -q "$bloom_trace_prefix" "$TRASH_DIRECTORY/trace.perf" && test_cmp log_wo_bloom log_w_bloom @@ -180,10 +182,18 @@ test_max_changed_paths () { grep "\"max_changed_paths\":$1" $2 } +test_filter_not_computed () { + grep "\"key\":\"filter-not-computed\",\"value\":\"$1\"" $2 +} + test_filter_computed () { grep "\"key\":\"filter-computed\",\"value\":\"$1\"" $2 } +test_filter_trunc_empty () { + grep "\"key\":\"filter-trunc-empty\",\"value\":\"$1\"" $2 +} + test_filter_trunc_large () { grep "\"key\":\"filter-trunc-large\",\"value\":\"$1\"" $2 } @@ -278,4 +288,21 @@ test_expect_success 'correctly report changes over limit' ' ) ' +test_expect_success 'correctly report commits with no changed paths' ' + git init empty && + test_when_finished "rm -fr empty" && + ( + cd empty && + + git commit --allow-empty -m "initial commit" && + + GIT_TRACE2_EVENT="$(pwd)/trace.event" \ + git commit-graph write --reachable --changed-paths && + test_filter_computed 1 trace.event && + test_filter_not_computed 0 trace.event && + test_filter_trunc_empty 1 trace.event && + test_filter_trunc_large 0 trace.event + ) +' + test_done