view CONTRIBUTING @ 30978:fdecd24ca4dc

ui: log time spent blocked on stdio

We use a wrapper around Mercurial at Facebook that logs key statistics (like
elpased time) to our standard performance tooling.

This is less useful than it could be, because we currently can't tell when a
command is slow because we need to fix Mercurial versus when a command is
slow because the user isn't interacting quickly.

Teach Mercurial to log the time it spends blocked, so that our tooling can
pick it up and submit it with the elapsed time - we can then do the math in
our tooling to see if Mercurial is slow, or if the user simply failed to
interact.

Combining this with the command duration log means that we can ensure that
we concentrate performance efforts on the things that bite Facebook users.

The perfwrite microbenchmark shifts from:
Linux:
! wall 3.213560 comb 0.410000 user 0.350000 sys 0.060000 (best of 4)
Mac:
! wall 0.342325 comb 0.180000 user 0.110000 sys 0.070000 (best of 20)

before this change to:
! wall 3.478070 comb 0.500000 user 0.420000 sys 0.080000 (best of 3)
Mac:
! wall 0.218112 comb 0.220000 user 0.150000 sys 0.070000 (best of 15)

showing a small hit in comb time, but firmly in the noise on wall time.

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).