Re: [PATCH]: Free pages from local pcp lists under tight memory conditions

[Paul Jackson <pj@sgi.com>]

Rohit wrote:
> I thought Nick et.al came up with some of the constant values like batch
> size to tackle the page coloring issue specifically. 

I think this came about on a linux-ia64 thread started by Jack Steiner:

  http://www.gelato.unsw.edu.au/archives/linux-ia64/0504/13668.html
  Subject: per_cpu_pagesets degrades MPI performance
  From: Jack Steiner <steiner_at_sgi.com>
  Date: 2005-04-05 05:28:27

Jack reported that per_cpu_pagesets were degrading some MPI benchmarks due
to adverse page coloring.  Nick responded, recommending a non-power of two
batch size.  Jack found that this helped nicely.  This thread trails off,
but seems to be the origins of the 2**n-1 batch size in:

	mm/page_alloc.c:zone_batchsize()
	 * Clamp the batch to a 2^n - 1 value. Having a power ...
        batch = (1 << fls(batch + batch/2)) - 1;

I don't see here evidence that "per_cpu_pagelist is ... one single main
reason the coloring effect is drastically reduced in 2.6 (over 2.4)
based kernels."  Rather in this case anyway a batch size not a power of
two was apparently needed to keep per_cpu_pagesets from hurting
performance due to page coloring affects on some workloads.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.925.600.0401

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