[PATCH 2 of 2] ui: record and print wall clock time

[Augie Fackler]

On Thu, Feb 02, 2017 at 05:22:07PM -0800, Bryan O'Sullivan wrote:
> On Thu, Feb 2, 2017 at 4:33 PM, Bryan O'Sullivan <bos at serpentine.com> wrote:
>
> > ui: record and print wall clock time
> >
>
> This patch is a total hack, but it contains an interesting few nuggets that
> it doesn't call out explicitly.
>
> As Simon mentions in one of his patches, we log command performance so that
> we can analyse it at scale and identify, and then fix, problems. But
> today's simple measure of wallclock time is thrown into confusion by
> anything interactive: calls to ssh (might ask for a password), prompts for
> input, merges, pagers, you name it.
>
> Measuring CPU time isn't any better, because a cold disk cache can result
> in long I/O waits that aren't accounted for, but which a user will
> definitely notice. So a wallclock metric that more accurately tracks when
> Mercurial is "really doing work" offers significant value.
>
> The current infrastructure for measuring time (when you use "hg --time")
> runs as an atexit hook, so it stops counting when Mercurial is about to
> exit. Thanks to the pager infrastructure, it's possible to exit long after
> we've finished doing any real work, because we're waiting on the pager
> child process to quit.

I wonder if we shouldn't just ditch the atexit hook entirely and
instead print the recorded time as we exit the appropriate function in
dispatch?

>
> As a quantity to measure, "how long between us starting a timer and the
> process exiting" is arguably not (ever?) what we really want. Instead, I
> think it's "how long between starting and finishing meaningful work", which
> the "wall" figure in the patch above gets closer to. Being able to identify
> time spent waiting on I/O or subprocesses (the "io" figure in the patch,
> and the main focus of Simon's patches) is another aspect of accurate
> counting for doing performance analysis at scale.

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