GCC compilation performance under RAM starvation

[Segher Boessenkool]

Hi!

On Sat, Jun 28, 2025 at 12:55:26PM +0900, Simon Richter wrote:
> On 28.06.25 10:54, Segher Boessenkool wrote:
> >How do you disable memory overcommit?  "Just" don't run problems that
> >are bigger than you have memmory for?
> 
> That is dependent on the kernel you use. On Linux, you would use the 
> "vm.overcommit_memory" sysctl:
> 
>  - a value of 0 (default) allows overcommit, but rejects some clearly 
> silly requests.

Yup.  Very conservative.  But also usually the best choice.

> So, something like
> 
>     # sysctl vm.overcommit_ratio=80
>     # sysctl vm.overcommit_memory=2
> 
> should guarantee you that your compiles will either run at full speed, 
> or fail quickly, but as mentioned, that also applies to the rest of your 
> system -- if you are using systemd or a desktop environment, all of that 
> will *also* need to fit into physical RAM and cannot be swapped out even 
> if unused, but these tools are written with the assumption that having a 
> program loaded but not doing anything costs pretty much nothing because 
> unaccessed pages will be evicted as soon as something else needs them, 
> which is normally a reasonable assumption.
> 
> So this solves the original problem, but is not optimal, because the 
> optimal behaviour would be to swap out other programs to give GCC more 
> space.

Yeah.  The only real solution is to just not have too little memoory :-)


Segher