linux-kernel - Re: [PATCH 6/8] Add __GFP_OTHER

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Date:	Tue, 22 Feb 2011 13:42:00 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrea Arcangeli <aarcange@...hat.com>, lwoodman@...hat.com,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 6/8] Add __GFP_OTHER_NODE flag

On Mon, 21 Feb 2011, Andi Kleen wrote:

> From: Andi Kleen <ak@...ux.intel.com>
> 
> Add a new __GFP_OTHER_NODE flag to tell the low level numa statistics
> in zone_statistics() that an allocation is on behalf of another thread.
> This way the local and remote counters can be still correct, even
> when background daemons like khugepaged are changing memory
> mappings.
> 
> This only affects the accounting, but I think it's worth doing that
> right to avoid confusing users.
> 

This makes the accounting worse, NUMA_LOCAL is defined as "allocation from 
local node," meaning it's local to the allocating cpu, not local to the 
node being targeted.

Further, preferred_zone has taken on a much more significant meaning other 
than just statistics: it impacts the behavior of memory compaction and how 
long congestion timeouts are, if a timeout is taken at all, depending on 
the I/O being done on behalf of the zone.

A better way to address the issue is by making sure preferred_zone is 
actually correct by using the appropriate zonelist to be passed into the 
allocator in the first place.

> I first tried to just pass down the right node, but this required
> a lot of changes to pass down this parameter and at least one
> addition of a 10th argument to a 9 argument function. Using
> the flag is a lot less intrusive.
> 

And adding a branch to every successful page allocation for statistics 
isn't intrusive?
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