Commands started by logtoprocess are running asynchronously. To be able to
test the output, we need to block and wait for the output.
The patch uses "| cat" to wait for such "asynchronous" outputs, to make the
test more reliable. I have also written a short notice at the top, hopefully
people would be aware of the pitfall when changing the test.
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.
We want to log the time Mercurial spends trapped in things outside
programmatic control. Provide a mechanism to give us both command runtime
and as many different sources of blocking as we deem useful.
This extension lets you direct specific ui.log() information to shell
commands, spawned in the background. This can be used to, say, capture
timings, commandfailure tracebacks, etc., making the output available
to other command-line tools.
