GNU bug report logs - #79074
31.0.50; Increase gc-cons-percentage in Makefiles

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#79074: closed (31.0.50; Increase gc-cons-percentage in Makefiles)
Date: Sat, 02 Aug 2025 14:23:02 +0000

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Your message dated Sat, 02 Aug 2025 17:22:18 +0300
with message-id <86ikj5reg5.fsf <at> gnu.org>
and subject line Re: bug#79074: 31.0.50; Increase gc-cons-percentage in Makefiles
has caused the debbugs.gnu.org bug report #79074,
regarding 31.0.50; Increase gc-cons-percentage in Makefiles
to be marked as done.

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From: Helmut Eller <eller.helmut <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 31.0.50; Increase gc-cons-percentage in Makefiles
Date: Tue, 22 Jul 2025 17:27:11 +0200

It seems that the default value for gc-cons-percentage (10%) is very
conservative and hurts performance.

I did the following experiment: first, I built the master branch (with
-O2 and without checking); second, I executed this script:

for percentage in 0.1 0.75 1.5 ; do
    find lisp/ -name '*.elc' -exec rm -v {} +
    /usr/bin/time -o make-$percentage.time make -l3.2 -j4 \
BYTE_COMPILE_EXTRA_FLAGS="--eval '(setq gc-cons-percentage $percentage)'"\
lisp
done

The intention is to measure the time it takes to compile all the .el
files in the lisp directory for different values of gc-cons-percentage.
I get these results:

* for gc-cons-percentage = 0.1:
1697.81user 57.67system 12:08.57elapsed 240%CPU (0avgtext+0avgdata 202268maxresident)k
12424inputs+173840outputs (29major+9304710minor)pagefaults 0swaps

* for gc-cons-percentage = 0.75
1034.16user 61.77system 7:55.26elapsed 230%CPU (0avgtext+0avgdata 216888maxresident)k
42864inputs+172280outputs (157major+10152739minor)pagefaults 0swaps

* for gc-cons-percentage = 1.5
823.91user 65.62system 6:27.09elapsed 229%CPU (0avgtext+0avgdata 246948maxresident)k
118640inputs+171184outputs (106major+12651774minor)pagefaults 0swaps

We see that increasing gc-cons-percentage to 0.75 reduces the elapsed
real time considerably (from ~12 minutes to ~8); it increases maxrss from
203MB to 216MB, which seems quite modest.

So I think we should add something to BYTE_COMPILE_FLAGS to increase
gc-cons-percentage; or perhaps increase it in batch-byte-compile.

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Helmut Eller <eller.helmut <at> gmail.com>
Cc: 79074-done <at> debbugs.gnu.org, monnier <at> iro.umontreal.ca
Subject: Re: bug#79074: 31.0.50; Increase gc-cons-percentage in Makefiles
Date: Sat, 02 Aug 2025 17:22:18 +0300

> Cc: 79074 <at> debbugs.gnu.org
> From: Helmut Eller <eller.helmut <at> gmail.com>
> Date: Mon, 28 Jul 2025 13:19:12 +0200
> 
> On Sat, Jul 26 2025, Stefan Monnier wrote:
> 
> >> It seems that the default value for gc-cons-percentage (10%) is very
> >> conservative and hurts performance.
> >
> > It is definitely very conservative.  But it's used only in
> > interactive mode, not in batch mode:
> >
> >     % src/emacs -Q --batch --eval '(message "%S" gc-cons-percentage)'
> >     1.0
> >     %
> 
> Ah, I didn't know that; but it makes sense.  Which means that the bug
> can be closed.

Thanks, closing.

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