| Message ID | 20201112161938.20494-1-szeder.dev@gmail.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | b13026a0f3560bf4bdc1687eea91ad51cd0d7193 |
| Headers | show |
| Series | [v2] bisect: loosen halfway() check for a large number of commits | expand |
SZEDER Gábor <szeder.dev@gmail.com> writes: > So let's loosen the check in the halfway() helper to consider commits > within about 0.1% of the exact halfway point as halfway as well, and > rename the function to approx_halfway() accordingly. This will allow > us to return early on a bigger good-bad range, even when there is no > commit exactly at the halfway point, thereby reducing the runtime of > the first command above considerably, from ~15s to 4.901s. The optimization presented with this change would probably offer more merge commits to be tested than a single-parent commit than the original algorithm, simply because merges are inspected first before single-parent commits so have better chance to be picked as "good enough" among commits with similar goodness. Side note: This is merely an observation---I do not know if it is a good thing, a bad thing, or a neutral thing, but it would likely affect the end-user experience. The optimization presented here gives probably more than enough improvement, but it just occured to me when writing the entry to explain the topic in the What's cooking report: "git bisect start/next" in a large span of history spends a lot of time trying to come up with exactly the half-way point; this can be optimized by stopping when we see a commit that is close enough to the half-way point. The realization is that the optimization naturally will be affected by the order the commits are visited. If a commit that is close enough to the half-way point happens to be visited earlier, it would help terminate our search early. And do_find_bisection() search counts all merge commits before any commit on the linear ancestry chain can be counted to optimize counting of commits on the linear ancestry chain, which are expected to exist more than merge commits. By sorting the "list" to somehow encourage commits near the half-way appear early on it, we may raise the likelyhood that we'd find good-enough commit early and terminate, no? Perhaps sort by the (absolute) distance between the committer timestamp of individual commit and its median value, or something? Thanks.
diff --git a/bisect.c b/bisect.c index f5b1368128..bedce28cb6 100644 --- a/bisect.c +++ b/bisect.c @@ -103,8 +103,10 @@ static int count_interesting_parents(struct commit *commit, unsigned bisect_flag return count; } -static inline int halfway(struct commit_list *p, int nr) +static inline int approx_halfway(struct commit_list *p, int nr) { + int diff; + /* * Don't short-cut something we are not going to return! */ @@ -113,13 +115,22 @@ static inline int halfway(struct commit_list *p, int nr) if (DEBUG_BISECT) return 0; /* - * 2 and 3 are halfway of 5. + * For small number of commits 2 and 3 are halfway of 5, and * 3 is halfway of 6 but 2 and 4 are not. */ - switch (2 * weight(p) - nr) { + diff = 2 * weight(p) - nr; + switch (diff) { case -1: case 0: case 1: return 1; default: + /* + * For large number of commits we are not so strict, it's + * good enough if it's within ~0.1% of the halfway point, + * e.g. 5000 is exactly halfway of 10000, but we consider + * the values [4996, 5004] as halfway as well. + */ + if (abs(diff) < nr / 1024) + return 1; return 0; } } @@ -321,8 +332,9 @@ static struct commit_list *do_find_bisection(struct commit_list *list, weight_set(p, count_distance(p)); clear_distance(list); - /* Does it happen to be at exactly half-way? */ - if (!(bisect_flags & FIND_BISECTION_ALL) && halfway(p, nr)) + /* Does it happen to be at half-way? */ + if (!(bisect_flags & FIND_BISECTION_ALL) && + approx_halfway(p, nr)) return p; counted++; } @@ -362,8 +374,9 @@ static struct commit_list *do_find_bisection(struct commit_list *list, else weight_set(p, weight(q)); - /* Does it happen to be at exactly half-way? */ - if (!(bisect_flags & FIND_BISECTION_ALL) && halfway(p, nr)) + /* Does it happen to be at half-way? */ + if (!(bisect_flags & FIND_BISECTION_ALL) && + approx_halfway(p, nr)) return p; } }
'git bisect start ...' and subsequent 'git bisect (good|bad)' commands can take quite a while when the given/remaining revision range between good and bad commits is big and contains a lot of merge commits, e.g. in git.git: $ git rev-list --count v1.6.0..v2.28.0 44284 $ time git bisect start v2.28.0 v1.6.0 Bisecting: 22141 revisions left to test after this (roughly 15 steps) [e197c21807dacadc8305250baa0b9228819189d4] unable_to_lock_die(): rename function from unable_to_lock_index_die() real 0m15.472s user 0m15.220s sys 0m0.255s The majority of the runtime is spent in do_find_bisection(), where we try to find a commit as close as possible to the halfway point between the bad and good revisions, i.e. a commit from which the number of reachable commits that are in the good-bad range is half the total number of commits in that range. So we count how many commits are reachable in the good-bad range for each commit in that range, which is quick and easy for a linear history, even over 300k commits in a linear range are handled in ~0.3s on my machine. Alas, handling merge commits is non-trivial and quite expensive as the algorithm used seems to be quadratic, causing the long runtime shown above. Interestingly, look at what a big difference one additional commit can make: $ git rev-list --count v1.6.0^..v2.28.0 44285 $ time git bisect start v2.28.0 v1.6.0^ Bisecting: 22142 revisions left to test after this (roughly 15 steps) [565301e41670825ceedf75220f2918ae76831240] Sync with 2.1.2 real 0m5.848s user 0m5.600s sys 0m0.252s The difference is caused by one of the optimizations attempting to cut down the runtime added in 1c4fea3a40 (git-rev-list --bisect: optimization, 2007-03-21): Another small optimization is whenever we find a half-way commit (that is, a commit that can reach exactly half of the commits), we stop giving counts to remaining commits, as we will not find any better commit than we just found. In this second 'git bisect start' command we happen to find a commit exactly at the halfway point and can return early, but in the first case there is no such commit, so we can't return early and end up counting the number of reachable commits from all commits in the good-bad range. However, when we have thousands of commits it's not all that important to find the _exact_ halfway point, a few commits more or less doesn't make any real difference for the bisection. So let's loosen the check in the halfway() helper to consider commits within about 0.1% of the exact halfway point as halfway as well, and rename the function to approx_halfway() accordingly. This will allow us to return early on a bigger good-bad range, even when there is no commit exactly at the halfway point, thereby reducing the runtime of the first command above considerably, from ~15s to 4.901s. Furthermore, even if there is a commit exactly at the halfway point, we might still stumble upon a commit within that 0.1% range before finding the exact halfway point, allowing us to return a bit earlier, slightly reducing the runtime of the second command from 5.848s to 5.058s. Note that this change doesn't affect good-bad ranges containing ~2000 commits or less, because that 0.1% tolerance becomes zero due to integer arithmetic; however, if the range is that small then counting the reachable commits for all commits is already fast enough anyway. Naturally, this will likely change which commits get picked at each bisection step, and, in turn, might change how many bisection steps are necessary to find the first bad commit. If the number of necessary bisection steps were to increase often, then this change could backfire, because building and testing at each step might take much longer than the time spared. OTOH, if the number of steps were to decrease, then it would be a double win. So I ran some tests to see how often that happens: picked random good and bad starting revisions at least 50k commits apart and a random first bad commit in between in git.git, and used 'git bisect run git merge-base --is-ancestor HEAD $first_bad_commit' to check the number of necessary bisection steps. After repeating all this 1000 times both with and without this patch I found that: - 146 cases needed one more bisection step than before, 149 cases needed one less step, while in the remaining 705 cases the number of steps didn't change. So the number of bisection steps does indeed change in a non-negligible number of cases, but it seems that the average number of steps doesn't change in the long run. - The first 'git bisect start' command got over 3x faster in 456 cases, so this "no commit at the exact halfway point" case seems to be common enough to care about. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> --- Range-diff: 1: 79ae48469f ! 1: 1a2f5135de bisect: loosen halfway() check for a large number of commits @@ Commit message to find the _exact_ halfway point, a few commits more or less doesn't make any real difference for the bisection. - So let's loosen the halfway check to consider commits within about - 0.1% of the exact halfway point as halfway as well. This will allow + So let's loosen the check in the halfway() helper to consider commits + within about 0.1% of the exact halfway point as halfway as well, and + rename the function to approx_halfway() accordingly. This will allow us to return early on a bigger good-bad range, even when there is no commit exactly at the halfway point, thereby reducing the runtime of the first command above considerably, from ~15s to 4.901s. @@ Commit message cases, so this "no commit at the exact halfway point" case seems to be common enough to care about. - [TODO: - - Update comments at callsites mentioning "exact halfway". - - Rename function to approx_halfway(), perhaps?] + Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> ## bisect.c ## @@ bisect.c: static int count_interesting_parents(struct commit *commit, unsigned bisect_flag + return count; + } - static inline int halfway(struct commit_list *p, int nr) +-static inline int halfway(struct commit_list *p, int nr) ++static inline int approx_halfway(struct commit_list *p, int nr) { + int diff; + @@ bisect.c: static inline int halfway(struct commit_list *p, int nr) return 0; } } +@@ bisect.c: static struct commit_list *do_find_bisection(struct commit_list *list, + weight_set(p, count_distance(p)); + clear_distance(list); + +- /* Does it happen to be at exactly half-way? */ +- if (!(bisect_flags & FIND_BISECTION_ALL) && halfway(p, nr)) ++ /* Does it happen to be at half-way? */ ++ if (!(bisect_flags & FIND_BISECTION_ALL) && ++ approx_halfway(p, nr)) + return p; + counted++; + } +@@ bisect.c: static struct commit_list *do_find_bisection(struct commit_list *list, + else + weight_set(p, weight(q)); + +- /* Does it happen to be at exactly half-way? */ +- if (!(bisect_flags & FIND_BISECTION_ALL) && halfway(p, nr)) ++ /* Does it happen to be at half-way? */ ++ if (!(bisect_flags & FIND_BISECTION_ALL) && ++ approx_halfway(p, nr)) + return p; + } + } bisect.c | 27 ++++++++++++++++++++------- 1 file changed, 20 insertions(+), 7 deletions(-)