Re: Timings for 'make check' with and without symbols with position

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sat, Feb 26, 2022 at 18:55:06 +0200, Eli Zaretskii wrote:
> > Date: Sat, 26 Feb 2022 16:39:05 +0000
> > Cc: David Engster <deng@randomsample.de>, Eli Zaretskii <eliz@gnu.org>,
> >   larsi@gnus.org, mattiase@acm.org, gregory@heytings.org,
> >   Óscar Fuentes <ofv@wanadoo.es>
> > From: Alan Mackenzie <acm@muc.de>
> > 
> > Short summary: The new build is slower than the old by 12.6%.  perf
> > counted 419k samples on the new, 372k samples on the old.
> > 
> > What is taking the extra time is largely the garbage collection:  I
> > filtered perf's output successively for the strings "mark" and "sweep",
> > and include those functions with at least 0.1% of the run time.

> Thanks.  Any idea why there will be more GC in the new version?

Char-tables seem to have something to do with it.  The perf output for
mark_char_table increase from 2.98% to 3.79%.  Adding the 12.6% factor
onto the 3.79% gives us 4.27%.  That's a factor of ~1.5 increase.

I'm still slogging through the detailed output for mark_char_table,
which is not easy in the optimised build.  I suspect that somehow,
somewhere, symbols with position have got themselves into a char-table,
and they take (a lot) longer to process than bare symbols (for which
mark_char_table is optimised).  I've not yet managed to find such a
char-table though.

> Or are you saying the number of GC calls was the same, but each GC
> cycle took a bit longer?

I've tried to get an output of gcs-done at the end of the batch emacs
invocation in make check, but not yet managed it.  Maybe later.

> If so, how does symbols-with-pos affect GC?

I don't yet have a hypothesis for this.

-- 
Alan Mackenzie (Nuremberg, Germany).