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From: "marxin at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug bootstrap/96794] --with-build-config=bootstrap-lto-lean with
 --enable-link-mutex leads to poor LTRANS utilization
Date: Wed, 26 Aug 2020 09:56:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96794

--- Comment #2 from Martin Li=C5=A1ka <marxin at gcc dot gnu.org> ---
(In reply to Jan Hubicka from comment #1)
> > As seen
> > here:https://gist.githubusercontent.com/marxin/223890df4d8d8e490b6b2918=
b77dacad/raw/7e0363da60dcddbfde4ab68fa3be755515166297/gcc-10-with-zstd.svg
> >=20
> > each blocking linking of a GCC front-end leads to a wasted jobserver wo=
rker.
> Hmm, I am not sure how to interpret the graph. I can see that there is a
> funny staircase of ltranses but how that relates to jobserver workers?

Yes, I mean the staircase of LTRANS because at the beginning N-1 links are
waiting for lock:

[  299s] lock-and-run.sh: (PID 7351) waiting 0 sec to acquire linkfe.lck fr=
om
PID 7347
...

For jobserver they are still running even though they sleep.


> We limit makefile to link a binary at a time to avoid Richi's box getting
> out of memory, right?

No. It's because we want to have a reasonable contrains which is right now =
8GB.
Without --enable-link-mutex, we would consume ~ 10 x 1.3 GB (plus WPA paral=
lel
streaming peak), which is probably not desired.

>=20
> NUmber of partitions is currently 128 what is 100% of CPU usage for you?

It's written in the graph footnote "CPU count: 16".=