Junio C Hamano <gitster@xxxxxxxxx> 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.
> -- >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 <gitster@xxxxxxxxx>
> ---
> 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;
>
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