Re: [PATCH] remote.c: avoid O(n^2) behavior in match_push_refs by using string_list
From: Brandon Casey <hidden>
Date: 2016-06-15 22:57:59
On Tue, Jul 2, 2013 at 11:23 PM, Jeff King [off-list ref] wrote:
On Tue, Jul 02, 2013 at 04:53:48PM -0700, Brandon Casey wrote:quoted
From: Brandon Casey <redacted> When pushing, each ref in the local repository must be paired with a ref advertised by the remote server. Currently, this is performed by first applying the refspec to the local ref to transform the local ref into the name of the remote ref, and then performing a linear search through the list of remote refs to see if the remote ref was advertised by the remote system. This has O(n) complexity and makes match_push_refs() be an O(n^2) operation.Just to be sure I understand correctly, is this actually O(m*n) where "m" is the number of local refs and "n" is the number of remote refs?
Yes, O(m*n) is more correct. I think you understand completely.
For a repository that repeatedly pushes everything it has to the remote, we end up with m=n, but it would not necessarily be the case if you are pushing a subset of your refs. But even pushing a small number of refs into a repository with a very large number of refs would be unnecessarily slow, as we would do several O(n) lookups which could be O(log n). So it may speed things up even in the case of a normal-sized repo pushing to a large one.
Right. For repos with few refs on either side, I don't think there will be any measurable difference. When pushing a single ref to a repo with a very large number of refs, we will see a very small net loss for the time required to prepare the string list (which grows linearly with the number of remote refs). After 2 or 3 refs, we should see a net gain. So we're really just improving our worst case performance here.
quoted hunk ↗ jump to hunk
quoted
Dry-run push of a repository with 121913 refs: before after real 1m40.582s 0m0.804s user 1m39.914s 0m0.515s sys 0m0.125s 0m0.106sVery nice. :)quoted
Signed-off-by: Brandon Casey <redacted> --- remote.c | 26 ++++++++++++++++++++++++-- 1 file changed, 24 insertions(+), 2 deletions(-)Patch itself looks good to me, although...quoted
@@ -1362,6 +1378,8 @@ int match_push_refs(struct ref *src, struct ref **dst, free(dst_name); } + string_list_clear(&ref_list, 0); + if (flags & MATCH_REFS_FOLLOW_TAGS) add_missing_tags(src, dst, &dst_tail);@@ -1376,11 +1394,15 @@ int match_push_refs(struct ref *src, struct ref **dst, src_name = get_ref_match(rs, nr_refspec, ref, send_mirror, FROM_DST, NULL); if (src_name) { - if (!find_ref_by_name(src, src_name)) + if (!ref_list.nr) + prepare_searchable_ref_list(src, + &ref_list); + if (!string_list_has_string(&ref_list, src_name))This hunk threw me for a bit, as it looked like we were lazily initializing ref_list in case we had not done so earlier. But we would have cleared it mid-way through the function (in the hunk above), and it is only that we are reusing the same ref_list for two different purposes. I do not feel strongly about it, but it might be a little more obvious to just declare a new variable in the block, like:diff --git a/remote.c b/remote.c index 75255af..53bef82 100644 --- a/remote.c +++ b/remote.c@@ -1399,6 +1399,7 @@ int match_push_refs(struct ref *src, struct ref **dst, add_missing_tags(src, dst, &dst_tail); if (send_prune) { + struct string_list src_ref_index = STRING_LIST_INIT_NODUP; /* check for missing refs on the remote */ for (ref = *dst; ref; ref = ref->next) { char *src_name;@@ -1409,15 +1410,15 @@ int match_push_refs(struct ref *src, struct ref **dst, src_name = get_ref_match(rs, nr_refspec, ref, send_mirror, FROM_DST, NULL); if (src_name) { - if (!ref_list.nr) + if (!src_ref_index.nr) prepare_searchable_ref_list(src, - &ref_list); - if (!string_list_has_string(&ref_list, src_name)) + &src_ref_index); + if (!string_list_has_string(&src_ref_index, src_name)) ref->peer_ref = alloc_delete_ref(); free(src_name); } } - string_list_clear(&ref_list, 0); + string_list_clear(&src_ref_index, 0); } if (errs) return -1;And similarly maybe call the outer ref_list dst_ref_index or something.
That looks/sounds reasonable.
I also note that we don't do the lazy-prepare for the other loop. I guess that is because we assume that "src" is always non-NULL?
It really wasn't a conscious decision to not do the lazy prepare in the other loop. I gave extra attention to the pruning block since I expect that deleting refs is less common than updating refs, so we should be able to avoid the cost of building the string list most of the time. But, I don't see a down side to doing the lazy prepare in the other loop too, and in fact, it looks like we may be able to avoid building the string list when only explicit refspecs are used. So, yeah, we should lazy build in both loops. -Brandon