branchcache: lazily validate nodes from the branchmap On my personal hg-repository with 365 entries in .hg/cache/branch2, following are the numbers for perfbranchmapload. Before this patch: ! wall 0.000866 comb 0.000000 user 0.000000 sys 0.000000 (best of 2680) ! wall 0.001525 comb 0.000000 user 0.000000 sys 0.000000 (max of 2680) ! wall 0.001107 comb 0.001097 user 0.001086 sys 0.000011 (avg of 2680) ! wall 0.001104 comb 0.000000 user 0.000000 sys 0.000000 (median of 2680) With this patch: ! wall 0.000530 comb 0.000000 user 0.000000 sys 0.000000 (best of 4240) ! wall 0.001078 comb 0.000000 user 0.000000 sys 0.000000 (max of 4240) ! wall 0.000696 comb 0.000693 user 0.000677 sys 0.000017 (avg of 4240) ! wall 0.000690 comb 0.000000 user 0.000000 sys 0.000000 (median of 4240) On our internal repository with ~20k entries in branchcache, I see improvement from 0.125 sec to 0.066 sec which is 47% speed up. The above are the numbers of perfbranchmapload which shows how much time we saved by not validating the nodes. But we need to validate some nodes. Following are timings of some mercurial operations which have speed up because of this lazy validation of nodes: No-op `hg update` on our internal repository (Avg on 4 runs): Before: 0.540 secs After: 0.430 secs Setting a branch name which already exists without --force (Avg of 4 runs): Before: 0.510 secs After: 0.250 secs I ran the ASV performance suite and was unable to see any improvements except there was improvement of perfdirstatewrite() on netbeans which I think was not related. I looked into the commit code, the command which I am trying to speedup, it looks like it uses revbranchcache to update the branchcache. Differential Revision: https://phab.mercurial-scm.org/D6208