[RFC/PATCH 2/4] limit "contains" traversals based on commit timestamp
- From
Jeff King <peff@peff.net>
- Date
- Jul 5, 2010, 12:34 UTC
- Message-ID
- <20100705123419.GB25699@sigill.intra.peff.net>
- In-Reply-To
- <20100705122723.GB21146@sigill.intra.peff.net>
When looking for commits that contain other commits (e.g., via "git tag --contains"), we can end up traversing useless portions of the graph. For example, if I am looking for a tag that contains a commit made last week, there is not much point in traversing portions of the history graph made five years ago.
This optimization can provide massive speedups. For example, doing "git tag --contains HEAD~200" in the linux-2.6 repository goes from:
real 0m5.302s user 0m5.116s sys 0m0.184s
to:
real 0m0.030s user 0m0.020s sys 0m0.008s
The downside is that we will no longer find some answers in the face of extreme clock skew, as we will stop the traversal early when seeing commits skewed too far into the past.
Name-rev already implements a similar optimization, using a "slop" of one day to allow for a certain amount of clock skew in commit timestamps. This patch introduces a "core.clockskew" variable, which allows specifying the allowable amount of clock skew in seconds. For safety, it defaults to "none", causing a full traversal (i.e., no change in behavior from previous versions).
Signed-off-by: Jeff King <peff@peff.net> --- cache.h | 1 + commit.c | 27 ++++++++++++++++++++++++--- config.c | 8 ++++++++ 3 files changed, 33 insertions(+), 3 deletions(-)
diff --git a/cache.h b/cache.h index c9fa3df..2dfb636 100644 --- a/cache.h +++ b/cache.h @@ -551,6 +551,7 @@ extern int read_replace_refs; extern int fsync_object_files; extern int core_preload_index; extern int core_apply_sparse_checkout; +extern int core_clock_skew; enum safe_crlf { SAFE_CRLF_FALSE = 0, diff --git a/commit.c b/commit.c index 20354c6..c849b50 100644 --- a/commit.c +++ b/commit.c @@ -7,6 +7,7 @@ #include "revision.h" #include "notes.h" +int core_clock_skew = -1; int save_commit_buffer = 1; const char *commit_type = "commit"; @@ -855,7 +856,8 @@ static int in_commit_list(const struct commit_list *want, struct commit *c) } static int contains_recurse(struct commit *candidate, - const struct commit_list *want) + const struct commit_list *want, + unsigned long cutoff) { struct commit_list *p; @@ -872,9 +874,13 @@ static int contains_recurse(struct commit *candidate, if (parse_commit(candidate) < 0) return 0; + /* stop searching if we go too far back in time */ + if (candidate->date < cutoff) + return 0; + /* Otherwise recurse and mark ourselves for future traversals. */ for (p = candidate->parents; p; p = p->next) { - if (contains_recurse(p->item, want)) { + if (contains_recurse(p->item, want, cutoff)) { candidate->object.flags |= TMP_MARK; return 1; } @@ -885,5 +891,20 @@ static int contains_recurse(struct commit *candidate, int contains(struct commit *candidate, const struct commit_list *want) { - return contains_recurse(candidate, want); + unsigned long cutoff = 0; + + if (core_clock_skew >= 0) { + const struct commit_list *c; + unsigned long min_date = ULONG_MAX; + for (c = want; c; c = c->next) { + if (parse_commit(c->item) < 0) + continue; + if (c->item->date < min_date) + min_date = c->item->date; + } + if (min_date > core_clock_skew) + cutoff = min_date - core_clock_skew; + } + + return contains_recurse(candidate, want, cutoff); } diff --git a/config.c b/config.c index cdcf583..7a18bc9 100644 --- a/config.c +++ b/config.c @@ -595,6 +595,14 @@ static int git_default_core_config(const char *var, const char *value) return 0; } + if (!strcmp(var, "core.clockskew")) { + if (!value || !strcmp(value, "none")) + core_clock_skew = -1; + else + core_clock_skew = git_config_int(var, value); + return 0; + } + /* Add other config variables here and to Documentation/config.txt. */ return 0; }
-- 1.7.2.rc1.209.g2a36c