linux-kernel - Re: [PATCH 6/7] mm: page_alloc: Only account batch allocations requests that are eligible

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Date:	Tue, 17 Dec 2013 11:20:07 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/7] mm: page_alloc: Only account batch allocations
 requests that are eligible

On Mon, Dec 16, 2013 at 03:52:37PM -0500, Johannes Weiner wrote:
> On Fri, Dec 13, 2013 at 02:10:06PM +0000, Mel Gorman wrote:
> > Not signed off. Johannes, was the intent really to decrement the batch
> > counts regardless of whether the policy was being enforced or not?
> 
> Yes.  Bursts of allocations for which the policy does not get enforced
> will still create memory pressure and affect cache aging on a given
> node.  So even if we only distribute page cache, we want to distribute
> it in a way that all allocations on the eligible zones equal out.

This means that allocations for page table pages affects the distribution of
page cache pages. An adverse workload could time when it faults anonymous
pages (to allocate anon and page table pages) in batch sequences and then
access files to force page cache pages to be allocated from a single node.

I think I know what your response will be. It will be that the utilisation of
the zone for page table pages and anon pages means that you want more page
cache pages to be allocated from the other zones so the reclaim pressure
is still more or less even. If this is the case or there is another reason
then it could have done with a comment because it's a subtle detail.

-- 
Mel Gorman
SUSE Labs
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