From: Andi Kleen <ak@suse.de>
To: Paul Jackson <pj@sgi.com>
Subject: Re: [RFC 5/8] Make writeout during reclaim cpuset aware
Date: Wed, 17 Jan 2007 16:59:15 +1100
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On Wednesday 17 January 2007 15:36, Paul Jackson wrote:
> > With a per node dirty limit ...
>
> What would this mean?
>
> Lets say we have a simple machine with 4 nodes, cpusets disabled.

There can be always NUMA policy without cpusets for once.

> Lets say all tasks are allowed to use all nodes, no set_mempolicy
> either.

Ok.

> If a task happens to fill up 80% of one node with dirty pages, but
> we have no dirty pages yet on other nodes, and we have a dirty ratio
> of 40%, then do we throttle that task's writes?

Yes we should actually. Every node should be able to supply
memory (unless extreme circumstances like mlock) and that much dirty 
memory on a node will make that hard.

> I am surprised you are asking for this, Andi.  I would have thought
> that on no-cpuset systems, the system wide throttling served your
> needs fine.  

No actually people are fairly unhappy when one node is filled with 
file data and then they don't get local memory from it anymore.
I get regular complaints about that for Opteron.

Dirty limit wouldn't be a full solution, but a good step.

> If not, then I can only guess that is because NUMA 
> mempolicy constraints on allowed nodes are causing the same dirty page
> problems as cpuset constrained systems -- is that your concern?

That is another concern. I haven't checked recently, but it used
to be fairly simple to put a system to its knees by oversubscribing
a single node with a strict memory policy. Fixing that would be good.

-Andi
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