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Date: Sun, 18 Mar 2007 11:28:20 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Mel Gorman <mel@csn.ul.ie>
Cc: Mariusz Kozlowski <m.kozlowski@tuxland.pl>,
       Andy Whitcroft <apw@shadowen.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Bias the location of pages freed for min_free_kbytes in
 the same MAX_ORDER_NR_PAGES blocks
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On Sun, 18 Mar 2007 19:05:41 +0000 (GMT) Mel Gorman <mel@csn.ul.ie> wrote:

> > How much additional memory consumption are we expecting here?
> >
> 
> Short answer, about 1.5KB on a 1GB system of which 1.3KB is statically 
> defined in the 3 struct zones on a 1 node x86 system.
> 
> Longer answer that I hopefully have not made any mistakes in - There is 
> the zone overhead which is statically sized and a runtime overhead which 
> depends on the amount of memory in the system. The additional zone 
> overhead is the overhead for additional freelists (larger struct 
> free_area) and is as follows;
> 
> (MIGRATE_TYPES-1) * sizeof(list_head) * (MAX_ORDER-1)
> 
> so, on 32 bit in general, thats
> 
> 4 * 8 * 10 = 320 bytes per zone (would be 240 bytes if MIGRATE_RESERVE is
>  				sufficient for higher order allocations
>  				instead of MIGRATE_HIGHALLOC)
> 
> on x86 with DMA, Normal and HighMem, thats 1280 bytes. On a NUMA system, 
> it's 1280 bytes per node. On 64 bit, it would be double because of the 
> larger pointer size. At worst, I guess you are looking at 3KB per node.

That a very modest overhead - not worth the config option, IMO.

The runtime overhead might be a concern - is it possible to quantify
it?