From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,LOTS_OF_MONEY, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by dcvr.yhbt.net (Postfix) with ESMTP id DE6D11F66E for ; Tue, 11 Aug 2020 22:05:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726430AbgHKWFG (ORCPT ); Tue, 11 Aug 2020 18:05:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36518 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726023AbgHKWFG (ORCPT ); Tue, 11 Aug 2020 18:05:06 -0400 Received: from mail-qv1-xf42.google.com (mail-qv1-xf42.google.com [IPv6:2607:f8b0:4864:20::f42]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 419D7C06174A for ; Tue, 11 Aug 2020 15:05:06 -0700 (PDT) Received: by mail-qv1-xf42.google.com with SMTP id s15so135904qvv.7 for ; Tue, 11 Aug 2020 15:05:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ttaylorr-com.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=Hzwe8WZ5iPRCkdvl6EPf7PxdrH7roNS0cSBklE00TEU=; b=aht48WVBkcU0gKSL0mlsy4jUNRWpK1NwK5CkU+7b/TQqaeshkFppj2mbjO2vAf4aPN OGTK3v/xvVxkJxKEtLD7zR79QhtbOJeYGRfgFYlLJWJhVhY0z76fvdiCjY6ELATl/3SB zrqmWj8kpNGaVxDn3jK/0J5wtn1nHdKuPg7TdUGkz0GzL+IQwyYCeFqVLqNJqH1GVqes lhK7H1crkWZ3hz6Ic5AhIrsgagKMzqtg11UKWrwtKKyvA/8VW5eVv5lirfbrZQVtTded dWCBkzlZ3IUGdC6fOrQIK0G0wqtPZi7mENW49V0dYqPzWrfULN+vm+o7JmgaODURfCie KtJA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=Hzwe8WZ5iPRCkdvl6EPf7PxdrH7roNS0cSBklE00TEU=; b=gbkirvdipnbz+V64MhSqlJG/brIN96OiuJl8+L3l049l3ADOQyHte9kPpF6G5BN7lM Ad8oMaVE1c2Iq3zis7fW8zt0Bd7i9r5bq70S3/5V+sGBSweSilFWjK7j15kibumojRkD 3N5/HRJtOiaIF1OV2g/YgsLmiN7j6HzxaXFHwbrTXEWHfZvGkG5sE/Wfn+1LL+xzPxaa 1QiAaKgHn/EaeibcY4sXFPLGIBrGGJiQmOP2remOI1rg6WPQi4qWOHSb4mZsL3Vdu6IH b/iyXwcSozALKx1nCeGnnYiDl+K6Cy8aAxalpPbsdM7EMmMVN8g4YuW1QUVL+0Ssil16 ZOFA== X-Gm-Message-State: AOAM5306BsiKc7G7QaTHc5jMO14DZlYRQdfyyQbkYOT3fHjwVw6R+fDo 4yZ9wGsi9/jp9ouZxrP73crOxw== X-Google-Smtp-Source: ABdhPJytZe66ZHG2JQFx1qsd2Y75YQSw9vpWF8IQLqZsJQ1heapXeIO5lmxSvt90yVWqseu4ErCivw== X-Received: by 2002:ad4:46e7:: with SMTP id h7mr3439813qvw.153.1597183505089; Tue, 11 Aug 2020 15:05:05 -0700 (PDT) Received: from localhost ([2605:9480:22e:ff10:a92f:57be:59a6:7cb2]) by smtp.gmail.com with ESMTPSA id f189sm210688qke.15.2020.08.11.15.05.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 11 Aug 2020 15:05:04 -0700 (PDT) Date: Tue, 11 Aug 2020 18:05:03 -0400 From: Taylor Blau To: Taylor Blau Cc: Derrick Stolee , git@vger.kernel.org, peff@peff.net, dstolee@microsoft.com, szeder.dev@gmail.com, gitster@pobox.com Subject: Re: [PATCH v3 12/14] commit-graph: add large-filters bitmap chunk Message-ID: <20200811220503.GC66656@syl.lan> References: <619e0c619dd12e2bea2b80c7d249ba66fe2a315c.1597178915.git.me@ttaylorr.com> <4f487dca-a938-b4c4-8b77-a202209459cb@gmail.com> <20200811211815.GA65971@syl.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200811211815.GA65971@syl.lan> Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Tue, Aug 11, 2020 at 05:18:15PM -0400, Taylor Blau wrote: > On Tue, Aug 11, 2020 at 05:11:49PM -0400, Derrick Stolee wrote: > > On 8/11/20 4:52 PM, Taylor Blau wrote: > > > To allow using the existing bitmap code with 64-bit words, we write the > > > data in network byte order from the 64-bit words. This means we also > > > need to read the array from the commit-graph file by translating each > > > word from network byte order using get_be64() when loading the commit > > > graph. (Note that this *could* be delayed until first-use, but a later > > > patch will rely on this being initialized early, so we assume the > > > up-front cost when parsing instead of delaying initialization). > > > > I don't think this is correct to do. This means that every commit-graph > > load will load the entire large bloom filter chunk into memory before > > allowing a single commit to be read from the graph. > > > > In the case of a very large commit-graph (> 1 million commits), this > > would cause a significant initial cost that is not necessarily needed > > for anything. For example, "git log -1" will be delayed by this action. > > > > If I understand correctly, this bloom_large bitmap is only used when > > writing Bloom filters. At that point, reading the entire bitmap from > > disk into memory is inexpensive compared to the time saved by the > > feature. > > That's not quite correct. By this point in the patch series, we only use > this bitmap for writing, but in the final patch, we will use it in > 'load_bloom_filter_from_graph()' (see that patch for the details of why, > there is an explanatory comment to that effect). > > So, the way that this lazy initialization was written was subtly > incorrect, since calling 'load_bloom_filter_from_graph' would return the > wrong value depending on what was or wasn't called before it (namely > whether or not we had initialized the bitmap). Ack, this is totally wrong. I had the wrong impression of where you were actually initializing the bitmaps, which is what motivated the change. Whoops, totally my mistake. Here's a version of the patch that does what it says. There is a small adjustment that needs to be made to the final patch, so I can send a version of that, too, if we decide that this is the direction we want to go (and that there are no other changes to the series). The change to the final patch basically boils down to using 'get_bloom_filter_large_in_graph()' instead of manually checking the bitmap (so that the aforementioned function can do its initialization ahead of time). Thanks for catching this subtle issue, I'm glad that we got it resolved before going further. --- >8 --- Subject: [PATCH] commit-graph: add large-filters bitmap chunk When a commit has more than a certain number of changed paths (commonly 512), the commit-graph machinery represents it as a zero-length filter. This is done since having many entries in the Bloom filter has undesirable effects on the false positivity rate. In addition to these too-large filters, the commit-graph machinery also represents commits with no filter and commits with no changed paths in the same way. When writing a commit-graph that aggregates several incremental commit-graph layers (eg., with '--split=replace'), the commit-graph machinery first computes all of the Bloom filters that it wants to write but does not already know about from existing graph layers. Because we overload the zero-length filter in the above fashion, this leads to recomputing large filters over and over again. This is already undesirable, since it means that we are wasting considerable effort to discover that a commit with too many changed paths, only to throw that effort away (and then repeat the process the next time a roll-up is performed). In a subsequent patch, we will add a '--max-new-filters=' option, which specifies an upper-bound on the number of new filters we are willing to compute from scratch. Suppose that there are 'N' too-large filters, and we specify '--max-new-filters=M'. If 'N >= M', it is unlikely that any filters will be generated, since we'll spend most of our effort on filters that we ultimately throw away. If 'N < M', filters will trickle in over time, but only at most 'M - N' per-write. To address this, add a new chunk which encodes a bitmap where the ith bit is on iff the ith commit has zero or at least 512 changed paths. Likewise store the maximum number of changed paths we are willing to store in order to prepare for eventually making this value more easily customizable. When computing Bloom filters, first consult the relevant bitmap (in the case that we are rolling up existing layers) to see if computing the Bloom filter from scratch would be a waste of time. This patch implements a new chunk instead of extending the existing BIDX and BDAT chunks because modifying these chunks would confuse old clients. (Eg., setting the most-significant bit in the BIDX chunk would confuse old clients and require a version bump). To allow using the existing bitmap code with 64-bit words, we write the data in network byte order from the 64-bit words. This means we also need to read the array from the commit-graph file by translating each word from network byte order using get_be64() when loading the commit graph. Initialize this bitmap lazily to avoid paying a linear-time cost upon each commit-graph load even if we do not need the bitmaps themselves. By avoiding the need to move to new versions of the BDAT and BIDX chunk, we can give ourselves more time to consider whether or not other modifications to these chunks are worthwhile without holding up this change. Another approach would be to introduce a new BIDX chunk (say, one identified by 'BID2') which is identical to the existing BIDX chunk, except the most-significant bit of each offset is interpreted as "this filter is too big" iff looking at a BID2 chunk. This avoids having to write a bitmap, but forces older clients to rewrite their commit-graphs (as well as reduces the theoretical largest Bloom filters we could write, and forces us to maintain the code necessary to translate BIDX chunks to BID2 ones). Separately from this patch, I implemented this alternate approach and did not find it to be advantageous. Signed-off-by: Taylor Blau --- .../technical/commit-graph-format.txt | 12 +++ bloom.h | 2 +- commit-graph.c | 100 +++++++++++++++--- commit-graph.h | 10 ++ t/t4216-log-bloom.sh | 25 ++++- 5 files changed, 134 insertions(+), 15 deletions(-) diff --git a/Documentation/technical/commit-graph-format.txt b/Documentation/technical/commit-graph-format.txt index 440541045d..5f2d9ab4d7 100644 --- a/Documentation/technical/commit-graph-format.txt +++ b/Documentation/technical/commit-graph-format.txt @@ -123,6 +123,18 @@ CHUNK DATA: of length zero. * The BDAT chunk is present if and only if BIDX is present. + Large Bloom Filters (ID: {'B', 'F', 'X', 'L'}) [Optional] + * It starts with a 32-bit unsigned integer specifying the maximum number of + changed-paths that can be stored in a single Bloom filter. + * It then contains a list of 64-bit words (the length of this list is + determined by the width of the chunk) which is a bitmap. The 'i'th bit is + set exactly when the 'i'th commit in the graph has a changed-path Bloom + filter with zero entries (either because the commit is empty, or because + it contains more than 512 changed paths). + * The BFXL chunk is present only when the BIDX and BDAT chunks are + also present. + + Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional] This list of H-byte hashes describe a set of B commit-graph files that form a commit-graph chain. The graph position for the ith commit in this diff --git a/bloom.h b/bloom.h index 3f19e3fca4..464d9b57de 100644 --- a/bloom.h +++ b/bloom.h @@ -33,7 +33,7 @@ struct bloom_filter_settings { * The maximum number of changed paths per commit * before declaring a Bloom filter to be too-large. * - * Not written to the commit-graph file. + * Written to the 'BFXL' chunk (instead of 'BDAT'). */ uint32_t max_changed_paths; }; diff --git a/commit-graph.c b/commit-graph.c index 8964453433..4ccc7a3e56 100644 --- a/commit-graph.c +++ b/commit-graph.c @@ -41,8 +41,9 @@ void git_test_write_commit_graph_or_die(void) #define GRAPH_CHUNKID_EXTRAEDGES 0x45444745 /* "EDGE" */ #define GRAPH_CHUNKID_BLOOMINDEXES 0x42494458 /* "BIDX" */ #define GRAPH_CHUNKID_BLOOMDATA 0x42444154 /* "BDAT" */ +#define GRAPH_CHUNKID_BLOOMLARGE 0x4246584c /* "BFXL" */ #define GRAPH_CHUNKID_BASE 0x42415345 /* "BASE" */ -#define MAX_NUM_CHUNKS 7 +#define MAX_NUM_CHUNKS 8 #define GRAPH_DATA_WIDTH (the_hash_algo->rawsz + 16) @@ -429,6 +430,16 @@ struct commit_graph *parse_commit_graph(struct repository *r, graph->bloom_filter_settings->bits_per_entry = get_be32(data + chunk_offset + 8); } break; + + case GRAPH_CHUNKID_BLOOMLARGE: + if (graph->chunk_bloom_large_filters) + chunk_repeated = 1; + else if (r->settings.commit_graph_read_changed_paths) { + graph->chunk_bloom_large_filters = data + chunk_offset + sizeof(uint32_t); + graph->bloom_large_alloc = get_be64(chunk_lookup + 4) - chunk_offset - sizeof(uint32_t); + graph->bloom_filter_settings->max_changed_paths = get_be32(data + chunk_offset); + } + break; } if (chunk_repeated) { @@ -443,6 +454,8 @@ struct commit_graph *parse_commit_graph(struct repository *r, /* We need both the bloom chunks to exist together. Else ignore the data */ graph->chunk_bloom_indexes = NULL; graph->chunk_bloom_data = NULL; + graph->chunk_bloom_large_filters = NULL; + graph->bloom_large_alloc = 0; FREE_AND_NULL(graph->bloom_filter_settings); } @@ -932,6 +945,32 @@ struct tree *get_commit_tree_in_graph(struct repository *r, const struct commit return get_commit_tree_in_graph_one(r, r->objects->commit_graph, c); } +int get_bloom_filter_large_in_graph(struct commit_graph *g, + const struct commit *c) +{ + uint32_t graph_pos = commit_graph_position(c); + if (graph_pos == COMMIT_NOT_FROM_GRAPH) + return 0; + + while (g && graph_pos < g->num_commits_in_base) + g = g->base_graph; + + if (!g) + return 0; + + if (!g->bloom_large && g->bloom_large_alloc) { + size_t i; + g->bloom_large = bitmap_word_alloc(g->bloom_large_alloc); + + for (i = 0; i < g->bloom_large->word_alloc; i++) + g->bloom_large->words[i] = get_be64( + g->chunk_bloom_large_filters + i * sizeof(eword_t)); + } + + if (!g->bloom_large) + return 0; + return bitmap_get(g->bloom_large, graph_pos - g->num_commits_in_base); +} struct packed_oid_list { struct object_id *list; @@ -970,8 +1009,10 @@ struct write_commit_graph_context { size_t total_bloom_filter_data_size; const struct bloom_filter_settings *bloom_settings; + int count_bloom_filter_known_large; int count_bloom_filter_found_large; int count_bloom_filter_computed; + struct bitmap *bloom_large; }; static int write_graph_chunk_fanout(struct hashfile *f, @@ -1235,6 +1276,23 @@ static int write_graph_chunk_bloom_data(struct hashfile *f, return 0; } +static int write_graph_chunk_bloom_large(struct hashfile *f, + struct write_commit_graph_context *ctx) +{ + size_t i, alloc = ctx->commits.nr / BITS_IN_EWORD; + if (ctx->commits.nr % BITS_IN_EWORD) + alloc++; + if (alloc > ctx->bloom_large->word_alloc) + BUG("write_graph_chunk_bloom_large: bitmap not large enough"); + + trace2_region_enter("commit-graph", "bloom_large", ctx->r); + hashwrite_be32(f, ctx->bloom_settings->max_changed_paths); + for (i = 0; i < ctx->bloom_large->word_alloc; i++) + hashwrite_be64(f, ctx->bloom_large->words[i]); + trace2_region_leave("commit-graph", "bloom_large", ctx->r); + return 0; +} + static int oid_compare(const void *_a, const void *_b) { const struct object_id *a = (const struct object_id *)_a; @@ -1398,6 +1456,8 @@ static void trace2_bloom_filter_write_statistics(struct write_commit_graph_conte struct json_writer jw = JSON_WRITER_INIT; jw_object_begin(&jw, 0); + jw_object_intmax(&jw, "filter_known_large", + ctx->count_bloom_filter_known_large); jw_object_intmax(&jw, "filter_found_large", ctx->count_bloom_filter_found_large); jw_object_intmax(&jw, "filter_computed", @@ -1416,6 +1476,7 @@ static void compute_bloom_filters(struct write_commit_graph_context *ctx) int *sorted_commits; init_bloom_filters(); + ctx->bloom_large = bitmap_word_alloc(ctx->commits.nr / BITS_IN_EWORD + 1); if (ctx->report_progress) progress = start_delayed_progress( @@ -1430,21 +1491,28 @@ static void compute_bloom_filters(struct write_commit_graph_context *ctx) &ctx->commits); for (i = 0; i < ctx->commits.nr; i++) { - int computed = 0; int pos = sorted_commits[i]; struct commit *c = ctx->commits.list[pos]; - struct bloom_filter *filter = get_or_compute_bloom_filter( - ctx->r, - c, - 1, - ctx->bloom_settings, - &computed); - if (computed) { - ctx->count_bloom_filter_computed++; - if (filter && !filter->len) - ctx->count_bloom_filter_found_large++; + if (get_bloom_filter_large_in_graph(ctx->r->objects->commit_graph, c)) { + bitmap_set(ctx->bloom_large, pos); + ctx->count_bloom_filter_known_large++; + } else { + int computed = 0; + struct bloom_filter *filter = get_or_compute_bloom_filter( + ctx->r, + c, + 1, + ctx->bloom_settings, + &computed); + if (computed) { + ctx->count_bloom_filter_computed++; + if (filter && !filter->len) { + bitmap_set(ctx->bloom_large, pos); + ctx->count_bloom_filter_found_large++; + } + } + ctx->total_bloom_filter_data_size += sizeof(unsigned char) * filter->len; } - ctx->total_bloom_filter_data_size += sizeof(unsigned char) * filter->len; display_progress(progress, i + 1); } @@ -1764,6 +1832,11 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx) + ctx->total_bloom_filter_data_size; chunks[num_chunks].write_fn = write_graph_chunk_bloom_data; num_chunks++; + chunks[num_chunks].id = GRAPH_CHUNKID_BLOOMLARGE; + chunks[num_chunks].size = sizeof(eword_t) * ctx->bloom_large->word_alloc + + sizeof(uint32_t); + chunks[num_chunks].write_fn = write_graph_chunk_bloom_large; + num_chunks++; } if (ctx->num_commit_graphs_after > 1) { chunks[num_chunks].id = GRAPH_CHUNKID_BASE; @@ -2503,6 +2576,7 @@ void free_commit_graph(struct commit_graph *g) } free(g->filename); free(g->bloom_filter_settings); + bitmap_free(g->bloom_large); free(g); } diff --git a/commit-graph.h b/commit-graph.h index d9acb22bac..9afb1477d5 100644 --- a/commit-graph.h +++ b/commit-graph.h @@ -4,6 +4,7 @@ #include "git-compat-util.h" #include "object-store.h" #include "oidset.h" +#include "ewah/ewok.h" #define GIT_TEST_COMMIT_GRAPH "GIT_TEST_COMMIT_GRAPH" #define GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE "GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE" @@ -50,6 +51,9 @@ void load_commit_graph_info(struct repository *r, struct commit *item); struct tree *get_commit_tree_in_graph(struct repository *r, const struct commit *c); +int get_bloom_filter_large_in_graph(struct commit_graph *g, + const struct commit *c); + struct commit_graph { const unsigned char *data; size_t data_len; @@ -71,6 +75,10 @@ struct commit_graph { const unsigned char *chunk_base_graphs; const unsigned char *chunk_bloom_indexes; const unsigned char *chunk_bloom_data; + const unsigned char *chunk_bloom_large_filters; + + struct bitmap *bloom_large; + size_t bloom_large_alloc; struct bloom_filter_settings *bloom_filter_settings; }; @@ -83,6 +91,8 @@ struct commit_graph *read_commit_graph_one(struct repository *r, struct commit_graph *parse_commit_graph(struct repository *r, void *graph_map, size_t graph_size); +void prepare_commit_graph_bloom_large(struct commit_graph *g); + /* * Return 1 if and only if the repository has a commit-graph * file and generation numbers are computed in that file. diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh index 21b67677ef..6859d85369 100755 --- a/t/t4216-log-bloom.sh +++ b/t/t4216-log-bloom.sh @@ -33,7 +33,7 @@ test_expect_success 'setup test - repo, commits, commit graph, log outputs' ' git commit-graph write --reachable --changed-paths ' graph_read_expect () { - NUM_CHUNKS=5 + NUM_CHUNKS=6 cat >expect <<- EOF header: 43475048 1 1 $NUM_CHUNKS 0 num_commits: $1 @@ -262,5 +262,28 @@ test_expect_success 'correctly report changes over limit' ' done ) ' +test_bloom_filters_computed () { + commit_graph_args=$1 + bloom_trace_prefix="{\"filter_known_large\":$2,\"filter_found_large\":$3,\"filter_computed\":$4" + rm -f "$TRASH_DIRECTORY/trace.event" && + GIT_TRACE2_EVENT="$TRASH_DIRECTORY/trace.event" git commit-graph write $commit_graph_args && + grep "$bloom_trace_prefix" "$TRASH_DIRECTORY/trace.event" +} + +test_expect_success 'Bloom generation does not recompute too-large filters' ' + ( + cd limits && + + # start from scratch and rebuild + rm -f .git/objects/info/commit-graph && + GIT_TEST_BLOOM_SETTINGS_MAX_CHANGED_PATHS=10 \ + git commit-graph write --reachable --changed-paths \ + --split=replace && + test_commit c1 filter && + + test_bloom_filters_computed "--reachable --changed-paths --split=replace" \ + 2 0 1 + ) +' test_done -- 2.28.0.rc1.13.ge78abce653