[PATCH v3 3/3] blame: use changed-path Bloom filters
- From
Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
- Date
- Apr 16, 2020, 20:14 UTC
- Message-ID
- <463d6bf5033cc6c3d22913c72535d094ffb5b41d.1587068044.git.gitgitgadget@gmail.com>
- In-Reply-To
- <pull.609.v3.git.1587068044.gitgitgadget@gmail.com>
From: Derrick Stolee <dstolee@microsoft.com>
The changed-path Bloom filters help reduce the amount of tree parsing required during history queries. Before calculating a diff, we can ask the filter if a path changed between a commit and its first parent. If the filter says "no" then we can move on without parsing trees. If the filter says "maybe" then we parse trees to discover if the answer is actually "yes" or "no".
When computing a blame, there is a section in find_origin() that computes a diff between a commit and one of its parents. When this is the first parent, we can check the Bloom filters before calling diff_tree_oid().
In order to make this work with the blame machinery, we need to initialize a struct bloom_key with the initial path. But also, we need to add more keys to a list if a rename is detected. We then check to see if _any_ of these keys answer "maybe" in the diff.
During development, I purposefully left out this "add a new key when a rename is detected" to see if the test suite would catch my error. That is how I discovered the issues with GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS from the previous change. With that change, we can feel some confidence in the coverage of this change.
If a user requests copy detection using "git blame -C", then there are more places where the set of "important" files can expand. I do not know enough about how this happens in the blame machinery. Thus, the Bloom filter integration is explicitly disabled in this mode. A later change could expand the bloom_key data with an appropriate call (or calls) to add_bloom_key().
If we did not disable this mode, then the following tests would fail:
t8003-blame-corner-cases.sh t8011-blame-split-file.sh
Generally, this is a performance enhancement and should not change the behavior of 'git blame' in any way. If a repo has a commit-graph file with computed changed-path Bloom filters, then they should notice improved performance for their 'git blame' commands.
Here are some example timings that I found by blaming some paths in the Linux kernel repository:
git blame arch/x86/kernel/topology.c >/dev/null
Before: 0.83s After: 0.24s
git blame kernel/time/time.c >/dev/null
Before: 0.72s After: 0.24s
git blame tools/perf/ui/stdio/hist.c >/dev/null
Before: 0.27s After: 0.11s
I specifically looked for "deep" paths that were also edited many times. As a counterpoint, the MAINTAINERS file was edited many times but is located in the root tree. This means that the cost of computing a diff relative to the pathspec is very small. Here are the timings for that command:
git blame MAINTAINERS >/dev/null
Before: 20.1s After: 18.0s
These timings are the best of five. The worst-case runs were on the order of 2.5 minutes for both cases. Note that the MAINTAINERS file has 18,740 lines across 17,000+ commits. This happens to be one of the cases where this change provides the least improvement.
The lack of improvement for the MAINTAINERS file and the relatively modest improvement for the other examples can be easily explained. The blame machinery needs to compute line-level diffs to determine which lines were changed by each commit. That makes up a large proportion of the computation time, and this change does not attempt to improve on that section of the algorithm. The MAINTAINERS file is large and changed often, so it takes time to determine which lines were updated by which commit. In contrast, the code files are much smaller, and it takes longer to comute the line-by-line diff for a single patch on the Linux mailing lists.
Outside of the "-C" integration, I believe there is little more to gain from the changed-path Bloom filters for 'git blame' after this patch.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com> --- blame.c | 139 ++++++++++++++++++++++++++++++++++++++++++++---- blame.h | 6 +++ builtin/blame.c | 10 ++++ 3 files changed, 146 insertions(+), 9 deletions(-)
diff --git a/blame.c b/blame.c index 29770e5c81c..9fbf79e47c3 100644 --- a/blame.c +++ b/blame.c @@ -9,6 +9,8 @@ #include "blame.h" #include "alloc.h" #include "commit-slab.h" +#include "bloom.h" +#include "commit-graph.h" define_commit_slab(blame_suspects, struct blame_origin *); static struct blame_suspects blame_suspects; @@ -1246,13 +1248,75 @@ static int fill_blob_sha1_and_mode(struct repository *r, return -1; } +struct blame_bloom_data { + /* + * Changed-path Bloom filter keys. These can help prevent + * computing diffs against first parents, but we need to + * expand the list as code is moved or files are renamed. + */ + struct bloom_filter_settings *settings; + struct bloom_key **keys; + int nr; + int alloc; +}; + +static int bloom_count_queries = 0; +static int bloom_count_no = 0; +static int maybe_changed_path(struct repository *r, + struct commit *parent, + struct blame_origin *origin, + struct blame_bloom_data *bd) +{ + int i; + struct bloom_filter *filter; + + if (!bd) + return 1; + + if (origin->commit->generation == GENERATION_NUMBER_INFINITY) + return 1; + + filter = get_bloom_filter(r, origin->commit, 0); + + if (!filter) + return 1; + + bloom_count_queries++; + for (i = 0; i < bd->nr; i++) { + if (bloom_filter_contains(filter, + bd->keys[i], + bd->settings)) + return 1; + } + + bloom_count_no++; + return 0; +} + +static void add_bloom_key(struct blame_bloom_data *bd, + const char *path) +{ + if (!bd) + return; + + if (bd->nr >= bd->alloc) { + bd->alloc *= 2; + REALLOC_ARRAY(bd->keys, bd->alloc); + } + + bd->keys[bd->nr] = xmalloc(sizeof(struct bloom_key)); + fill_bloom_key(path, strlen(path), bd->keys[bd->nr], bd->settings); + bd->nr++; +} + /* * We have an origin -- check if the same path exists in the * parent and return an origin structure to represent it. */ static struct blame_origin *find_origin(struct repository *r, struct commit *parent, - struct blame_origin *origin) + struct blame_origin *origin, + struct blame_bloom_data *bd) { struct blame_origin *porigin; struct diff_options diff_opts; @@ -1286,10 +1350,19 @@ static struct blame_origin *find_origin(struct repository *r, if (is_null_oid(&origin->commit->object.oid)) do_diff_cache(get_commit_tree_oid(parent), &diff_opts); - else - diff_tree_oid(get_commit_tree_oid(parent), - get_commit_tree_oid(origin->commit), - "", &diff_opts); + else { + int compute_diff = 1; + if (origin->commit->parents && + !oidcmp(&parent->object.oid, + &origin->commit->parents->item->object.oid)) + compute_diff = maybe_changed_path(r, parent, + origin, bd); + + if (compute_diff) + diff_tree_oid(get_commit_tree_oid(parent), + get_commit_tree_oid(origin->commit), + "", &diff_opts); + } diffcore_std(&diff_opts); if (!diff_queued_diff.nr) { @@ -1341,7 +1414,8 @@ static struct blame_origin *find_origin(struct repository *r, */ static struct blame_origin *find_rename(struct repository *r, struct commit *parent, - struct blame_origin *origin) + struct blame_origin *origin, + struct blame_bloom_data *bd) { struct blame_origin *porigin = NULL; struct diff_options diff_opts; @@ -1366,6 +1440,7 @@ static struct blame_origin *find_rename(struct repository *r, struct diff_filepair *p = diff_queued_diff.queue[i]; if ((p->status == 'R' || p->status == 'C') && !strcmp(p->two->path, origin->path)) { + add_bloom_key(bd, p->one->path); porigin = get_origin(parent, p->one->path); oidcpy(&porigin->blob_oid, &p->one->oid); porigin->mode = p->one->mode; @@ -2332,6 +2407,11 @@ static void distribute_blame(struct blame_scoreboard *sb, struct blame_entry *bl #define MAXSG 16 +typedef struct blame_origin *(*blame_find_alg)(struct repository *, + struct commit *, + struct blame_origin *, + struct blame_bloom_data *); + static void pass_blame(struct blame_scoreboard *sb, struct blame_origin *origin, int opt) { struct rev_info *revs = sb->revs; @@ -2356,8 +2436,7 @@ static void pass_blame(struct blame_scoreboard *sb, struct blame_origin *origin, * common cases, then we look for renames in the second pass. */ for (pass = 0; pass < 2 - sb->no_whole_file_rename; pass++) { - struct blame_origin *(*find)(struct repository *, struct commit *, struct blame_origin *); - find = pass ? find_rename : find_origin; + blame_find_alg find = pass ? find_rename : find_origin; for (i = 0, sg = first_scapegoat(revs, commit, sb->reverse); i < num_sg && sg; @@ -2369,7 +2448,7 @@ static void pass_blame(struct blame_scoreboard *sb, struct blame_origin *origin, continue; if (parse_commit(p)) continue; - porigin = find(sb->repo, p, origin); + porigin = find(sb->repo, p, origin, sb->bloom_data); if (!porigin) continue; if (oideq(&porigin->blob_oid, &origin->blob_oid)) { @@ -2809,3 +2888,45 @@ struct blame_entry *blame_entry_prepend(struct blame_entry *head, blame_origin_incref(o); return new_head; } + +void setup_blame_bloom_data(struct blame_scoreboard *sb, + const char *path) +{ + struct blame_bloom_data *bd; + + if (!sb->repo->objects->commit_graph) + return; + + if (!sb->repo->objects->commit_graph->bloom_filter_settings) + return; + + bd = xmalloc(sizeof(struct blame_bloom_data)); + + bd->settings = sb->repo->objects->commit_graph->bloom_filter_settings; + + bd->alloc = 4; + bd->nr = 0; + ALLOC_ARRAY(bd->keys, bd->alloc); + + add_bloom_key(bd, path); + + sb->bloom_data = bd; +} + +void cleanup_scoreboard(struct blame_scoreboard *sb) +{ + if (sb->bloom_data) { + int i; + for (i = 0; i < sb->bloom_data->nr; i++) { + free(sb->bloom_data->keys[i]->hashes); + free(sb->bloom_data->keys[i]); + } + free(sb->bloom_data->keys); + FREE_AND_NULL(sb->bloom_data); + + trace2_data_intmax("blame", sb->repo, + "bloom/queries", bloom_count_queries); + trace2_data_intmax("blame", sb->repo, + "bloom/response-no", bloom_count_no); + } +} diff --git a/blame.h b/blame.h index 089b181ff27..b6bbee41472 100644 --- a/blame.h +++ b/blame.h @@ -100,6 +100,8 @@ struct blame_entry { int unblamable; }; +struct blame_bloom_data; + /* * The current state of the blame assignment. */ @@ -156,6 +158,7 @@ struct blame_scoreboard { void(*found_guilty_entry)(struct blame_entry *, void *); void *found_guilty_entry_data; + struct blame_bloom_data *bloom_data; }; /* @@ -180,6 +183,9 @@ void init_scoreboard(struct blame_scoreboard *sb); void setup_scoreboard(struct blame_scoreboard *sb, const char *path, struct blame_origin **orig); +void setup_blame_bloom_data(struct blame_scoreboard *sb, + const char *path); +void cleanup_scoreboard(struct blame_scoreboard *sb); struct blame_entry *blame_entry_prepend(struct blame_entry *head, long start, long end, diff --git a/builtin/blame.c b/builtin/blame.c index bf1cecdf3f9..3c13634f279 100644 --- a/builtin/blame.c +++ b/builtin/blame.c @@ -1061,6 +1061,14 @@ int cmd_blame(int argc, const char **argv, const char *prefix) string_list_clear(&ignore_revs_file_list, 0); string_list_clear(&ignore_rev_list, 0); setup_scoreboard(&sb, path, &o); + + /* + * Changed-path Bloom filters are disabled when looking + * for copies. + */ + if (!(opt & PICKAXE_BLAME_COPY)) + setup_blame_bloom_data(&sb, path); + lno = sb.num_lines; if (lno && !range_list.nr) @@ -1164,5 +1172,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix) printf("num get patch: %d\n", sb.num_get_patch); printf("num commits: %d\n", sb.num_commits); } + + cleanup_scoreboard(&sb); return 0; }
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