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Subject: Re: [PATCH 0/5] make slab gfp fair
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Christoph Lameter <clameter@sgi.com>
Cc: Matt Mackall <mpm@selenic.com>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, Thomas Graf <tgraf@suug.ch>,
       David Miller <davem@davemloft.net>,
       Andrew Morton <akpm@linux-foundation.org>,
       Daniel Phillips <phillips@google.com>,
       Pekka Enberg <penberg@cs.helsinki.fi>
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Date: Thu, 17 May 2007 23:26:49 +0200
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On Thu, 2007-05-17 at 12:24 -0700, Christoph Lameter wrote:
> On Thu, 17 May 2007, Peter Zijlstra wrote:
> 
> > The proposed patch doesn't change how the kernel functions at this
> > point; it just enforces an existing rule better.
> 
> Well I'd say it controls the allocation failures. And that only works if 
> one can consider the system having a single zone.
> 
> Lets say the system has two cpusets A and B. A allocs from node 1 and B 
> allocs from node 2. Two processes one in A and one in B run on the same 
> processor.
> 
> Node 1 gets very low in memory so your patch kicks in and sets up the 
> global memory emergency situation with the reserve slab.
> 
> Now the process in B will either fail although it has plenty of memory on 
> node 2.
> 
> Or it may just clear the emergency slab and then the next critical alloc 
> of the process in A that is low on memory will fail.

The way I read the cpuset page allocator, it will only respect the
cpuset if there is memory aplenty. Otherwise it will grab whatever. So
still, it will only ever use ALLOC_NO_WATERMARKS if the whole system is
in distress.