Re: git pull takes ~8 seconds on up-to-date Linux git tree
From: Junio C Hamano <hidden>
Date: 2016-06-15 22:54:57
Junio C Hamano [off-list ref] writes:
Here is a tested (in the sense that it passes the test suite, and also in the sense that an empty pull in the kernel history gives quick turnaround) patch. As I do not think we would want to revert 5802f81 (fmt-merge-msg: discard needless merge parents, 2012-04-18) which was a correctness fix, I think we would rather want to do something like this.
OK, I think I am convinced myself that this patch is the right fix. The performance regression Markus saw is in fmt-merge-message, and it is caused by the updated remove_redundant() that is used by get_merge_bases_many() and reduce_heads(). On the topic branch, all callers of reduce_heads() were passing commits that are already parsed, but before the topic was merged to 'master', we added one more caller to reduce_heads() on the 'master' front that passed an unparsed commit, which is why the problem surfaced at that merge. It might make sense to assert or die in commit_list_insert_by_date() when a caller mistakenly pass an unparsed commit object to prevent this kind of breakages in the future.
quoted hunk
-- >8 -- Subject: paint_down_to_common(): parse commit before relying on its timestamp When refactoring the merge-base computation to reduce the pairwise O(n*(n-1)) traversals to parallel O(n) traversals, the code forgot that timestamp based heuristics needs each commit to have been parsed. This caused an empty "git pull" to spend cycles, traversing the history all the way down to 0 (because an unparsed commit object has 0 timestamp, and any other commit object with positive timestamp will be processed for its parents, all getting parsed), only to come up with a merge message to be used. Signed-off-by: Junio C Hamano <redacted> --- commit.c | 5 +++++ 1 file changed, 5 insertions(+)diff --git i/commit.c w/commit.c index 0246767..213bc98 100644 --- i/commit.c +++ w/commit.c@@ -609,6 +609,7 @@ static struct commit *interesting(struct commit_list *list) return NULL; } +/* all input commits in one and twos[] must have been parsed! */ static struct commit_list *paint_down_to_common(struct commit *one, int n, struct commit **twos) { struct commit_list *list = NULL;@@ -617,6 +618,8 @@ static struct commit_list *paint_down_to_common(struct commit *one, int n, struc one->object.flags |= PARENT1; commit_list_insert_by_date(one, &list); + if (!n) + return list; for (i = 0; i < n; i++) { twos[i]->object.flags |= PARENT2; commit_list_insert_by_date(twos[i], &list);@@ -737,6 +740,8 @@ static int remove_redundant(struct commit **array, int cnt) redundant = xcalloc(cnt, 1); filled_index = xmalloc(sizeof(*filled_index) * (cnt - 1)); + for (i = 0; i < cnt; i++) + parse_commit(array[i]); for (i = 0; i < cnt; i++) { struct commit_list *common;