Re: OOM fixes 2/5

[Andrea Arcangeli <andrea@suse.de>]

On Thu, Jan 20, 2005 at 11:00:16PM -0800, Andrew Morton wrote:
> Last time we dicsussed this you pointed out that reserving more lowmem from
> highmem-capable allocations may actually *help* things.  (Tries to remember
> why) By reducing inode/dentry eviction rates?  I asked Martin Bligh if he
> could test that on a big NUMA box but iirc the results were inconclusive.

This is correct, guaranteeing more memory to be freeable in lowmem (ptes
aren't freeable without a sigkill for example) the icache/dcache will at
least have a margin where it can grow indipendently from highmem
allocations.

> Maybe it just won't make much difference.  Hard to say.

I don't know myself if it makes a performance difference, all old
benchmarks have been run with this applied. This was applied for
correcntess (i.e.  to avoid sigkills or lockups), it wasn't applied for
performance. But I don't see how it could hurt performance (especially
given current code already does the check at runtime, which is
pratically the only fast-path cost ;).

> >  The sysctl name had to change to lowmem_reserve_ratio because its
> >  semantics are completely different now.
> 
> That reminds me.  Documentation/filesystems/proc.txt ;)

Woops, forgotten about it ;)

> I'll cook something up for that.

Thanks. If you prefer I can write it too to relieve you from this load,
it's up to you. If you want to fix it yourself go ahead of course ;)