changegroup: store old heads as a set
Previously, the "oldheads" variable was a list. On a repository at
Mozilla with 46,492 heads, profiling revealed that list membership
testing was dominating execution time of applying small changegroups.
This patch converts the list of old heads to a set. This makes
membership testing significantly faster. On the aforementioned
repository with 46,492 heads:
$ hg unbundle <file with 1 changeset>
before: 18.535s wall
after: 1.303s
Consumers of this variable only check for truthiness (`if oldheads`),
length (`len(oldheads)`), and (most importantly) item membership
(`h not in oldheads` - which occurs twice). So, the change to a set
should be safe and suitable for stable.
The practical effect of this change is that changegroup application
and related operations (like `hg push`) no longer exhibit an O(n^2)
CPU explosion as the number of heads grows.
--- a/mercurial/changegroup.py Mon Mar 20 04:36:55 2017 -0700
+++ b/mercurial/changegroup.py Thu Mar 23 19:54:59 2017 -0700
@@ -293,7 +293,7 @@
# will not see an inconsistent view
cl = repo.changelog
cl.delayupdate(tr)
- oldheads = cl.heads()
+ oldheads = set(cl.heads())
trp = weakref.proxy(tr)
# pull off the changeset group