linux-kernel - Re: [PATCH 5/7] Add /proc trigger for memory compaction

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Date:	Thu, 21 Jan 2010 15:34:39 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Mel Gorman <mel@....ul.ie>
cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 5/7] Add /proc trigger for memory compaction

On Thu, 21 Jan 2010, Mel Gorman wrote:

> > It would be helpful to be able to determine what is "compactable" at the 
> > same time by adding both global and per-node "compact_order" tunables that 
> > would default to HUGETLB_PAGE_ORDER. 
> 
> Well, rather than having a separate tunable, writing a number to
> /proc/sys/vm/compact could indicate the order if that trigger is now
> working system-wide. Would that be suitable?
> 

Do you think you'll eventually find a need to call try_to_compact_pages() 
with a higher order than the one passed to the page allocator to limit 
"compaction thrashing" from fragmented frees to a zone where we're 
constantly compacting order-1 pages, for instance?  I agree that memory 
compaction should always be used before direct reclaim for higher order 
allocations, but it may be more efficient to define a compact_min_order, 
tunable from userspace, that would avoid the need for constant order-1 
compactions from subsequent page allocations.

If that's a possibility, we may find a need for "compact_order", now 
renamed "compact_min_order", outside of the explicit trigger.
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