On Wed, 30 Apr 2025, Krystian Kazmierczak (Nokia) via Gcc-help wrote:
I'm observing that gcc compilation slows down significantly when the
system experiences RAM starvation. I understand that some slowdown
is expected under memory pressure, but I'm trying to determine
whether this level of degradation is expected GCC behavior or is
primarily due to Linux kernel memory management.
What I observe:
* During large builds (make -jN), when total RAM is nearly or fully
exhausted, compilation slows drastically. Same source file
compile ~20s when we have free memory comparing to 40s-60s when
we reach the threshold of memory (if not killed by OOM
meanwhile).
* Sometimes the Linux kernel's OOM killer is triggered, mostly by
other processes not gcc itself, then gcc is selected to be
killed e.g.:
[Fri Apr 18 16:14:49 2025] python invoked oom-killer: gfp_mask=0xcc0(GFP_KERNEL), order=0, oom_score_adj=815
[Fri Apr 18 16:14:49 2025] Memory cgroup out of memory: Killed process 108353 (cc1plus) total-vm:5550480kB, anon-rss:5421224kB, file-rss:0kB, shmem-rss:0kB, UID:1000 pgtables:10876kB oom_score_adj:815
* The system has swap disabled.
Enabling swap might help. Try it and see.
At the least it would reduce the number of oom-kills.
Questions:
* Is this performance degradation considered normal for gcc under memory pressure?
Seems normal to me.
* Is there any GCC-specific tuning or flags that can help in
constrained memory environments? (I'm aware of ggc-min-expand
and ggc-min-heapsize but these in most cases leads to lower
memory consumption at the cost of higher user time).
Sounds like a typical trade off.
* I want to understand whether this observation is only an outcome
of kernel's memory management or gcc itself is aware of
reaching limit of RAM and then somehow "slows down".
System Info:
* GCC version: gcc (GCC) 13.1.0
* OS: AlmaLinux release 9.3
Any insights into whether this is GCC-internal behaviour or if it's
more likely tied to Linux's memory management would be appreciated.
I would try gcc-link-option first.
After that I would point the finger at make. It has --load-average but
nothing similar to inhibit starting new processes when memory is low.
https://savannah.gnu.org/bugs/index.php?52697
is a patch from 2017 for gmake to do this.
Not sure how much work it would be to make it apply to a curent version.
--
Andrew C. Aitchison Kendal, UK
andrew@xxxxxxxxxxxxxxx
