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.

contrib: add a write microbenchmark to perf.py I'm adding some performance logging to ui.write - this benchmark lets us confirm that the cost of that logging is acceptably low. At this point, the microbenchmark on Linux over SSH shows: ! wall 3.213560 comb 0.410000 user 0.350000 sys 0.060000 (best of 4) while on the Mac locally, it shows: ! wall 0.342325 comb 0.180000 user 0.110000 sys 0.070000 (best of 20)

ui: provide a mechanism to track and log blocked 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.

mercurial: switch to util.timer for all interval timings util.timer is now the best available interval timer, at the expense of not having a known epoch. Let's use it whenever the epoch is irrelevant.