bug#51734: 29.0.50; got slow

[Ken Brown]

On 11/12/2021 2:26 PM, Eli Zaretskii wrote:
> > Date: Fri, 12 Nov 2021 13:22:28 -0500
> > Cc: yamaoka@jpl.org, 51734@debbugs.gnu.org
> > From: Ken Brown <kbrown@cornell.edu>
> >
> > The only thing I can think of is that Cygwin is probably the only system that
> > has timerfd and that also has to use timers to poll for input.  (The others all
> > use SIGIO, if I'm not mistaken.)  By using two different kinds of timers
> > simultaneously, we're getting timers expiring twice as often and not at regular
> > intervals.  Could this account for the slowdown?
>
> At what frequency does the other timer tick, the one used to poll for
> input?

They're both used to poll for input, and both at the same frequency (I think 
every second).  For systems without SIGIO, keyboard.c:start_polling creates an 
atimer "poll_timer" via a call to start_atimer.  The latter calls set_alarm, 
which now (after commit 858868e3) calls both timerfd_settime and timer_settime. 
 So we have both a timerfd and a POSIX timer, both serving the same purpose AFAICT.

When I said "not at regular intervals" above, I meant that there will sometimes 
be delays before Emacs notices a timer expiring, so the two timers will get out 
of phase with another.

I hope this makes some sense.  I find keyboard.c and the whole alarm setup 
confusing, so I could easily have gotten some things wrong.

> > In any case, I think I should go ahead and install the patch to make the master
> > branch usable again on Cygwin.
>
> Feel free.

Done, and I'm closing the bug.

Ken