Re: [gentoo-amd64] Re: System becomes almost unusable when compling c++ code

[Richard Freeman]

Volker Armin Hemmann wrote:
> emm, from my very personal point of view: no
>
> swap is horrible slow. Its use must be avoided at any cost. Swap sucks.
> Everything is faster than accessing swap. So hitting the disk to read or
> write some files is IMHO better than hitting the disk to shove X into swap.
>
> X in swap is another problem. You can be sure, if X is forced into swap,
> because gcc uses up all ram for itself, everything sucks. Mouse is jerky,
> windows need ages to get displayed.
>

Depends - if you're not using X then you won't mind X getting swapped 
(running emerge from ssh/etc).  If you are using X then chance are it 
won't get swapped in the first place.  If the issue is gcc using all ram 
for itself, then the presence/absence of tmpfs probably won't make much 
difference - gcc still has to run.

I don't see why swap should be any better/worse than any other form of 
disk access.  If anything it is superior as it allows the kernel to 
manage it like any other form of RAM, and the kernel isn't forced to 
flush it out to disk within some time (writes to a filesystem are forced 
to sync within some time to prevent data loss - this hurts performance 
and is totally unnecessary for temporary build files that will get 
deleted when you re-run emerge anyway).

Even if you don't have a tmpfs writing to disk will tend to drive unused 
ram into swap - the system will swap idle memory to make room for 
cache/buffers - where the recently-written files will reside in ram.

Now, in an extreme case where you have less RAM than the resident size 
of the apps you have running then swap will be horrible - but that is 
because you're continuously swapping in and out.  And I doubt a tmpfs 
will make much difference either way.

Now, if somebody has empirical data I'll certainly pay attention, or at 
least a lot of expertise.  However, I wouldn't jump to the conclusion 
that swap is worse than ordinary disk writes - linux manages swap fairly 
well all things considered.  If you don't like how it is being managed 
there are kernel settings that can be used to tweak it (swappiness, etc).