Re: [PATCH 1/5] cpuset memory spread basic implementation

[Ingo Molnar <mingo@elte.hu>]

* Andi Kleen <ak@suse.de> wrote:

> > > If you have a much worse worst case NUMA factor it might be different,
> > > but even there it would be a good idea to at least spread it out
> > > to nearby nodes.
> > 
> > I dont understand you here. What would be the benefit of selecting more 
> > distant memory over local? I can only imagine that this would be 
> > beneficial if we know that the data would be used later by other 
> > processes.
> 
> The benefit would be to not fill up the local node as quickly when you 
> do something IO (or dcache intensive).  And on contrary when you do 
> something local memory intensive on that node then you won't need to 
> throw away all the IO caches if they are already spread out.
> 
> The kernel uses of these cached objects are not really _that_ latency 
> sensitive and not that frequent so it makes sense to spread it out a 
> bit to nearby nodes.

I'm not sure i agree. If a cache isnt that important, then there wont be 
that much of them (hence they cannot interact with user pages that 
much), and it wont be used that frequently -> the VM will discard it 
faster. If there's tons of dentries and inodes and pagecache around, 
then there must be a reason it's around: it was actively used. In that 
case we should spread them out only if we know in advance that their use 
is global, not local - and we should default to local.

	Ingo