linux-kernel - Re: [PATCH] Add __GFP_MOVABLE for callers to flag allocations that may be migrated

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Message-ID: <20061205214721.GE20614@skynet.ie>
Date:	Tue, 5 Dec 2006 21:47:21 +0000
From:	mel@...net.ie (Mel Gorman)
To:	Christoph Lameter <clameter@....com>
Cc:	Andrew Morton <akpm@...l.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Add __GFP_MOVABLE for callers to flag allocations that may be migrated

On (05/12/06 12:01), Christoph Lameter didst pronounce:
> On Tue, 5 Dec 2006, Andrew Morton wrote:
> 
> > > We always run reclaim against the whole zone not against parts. Why 
> > > would we start running reclaim against a portion of a zone?
> > 
> > Oh for gawd's sake.
> 
> Yes indeed. Another failure to answer a simple question.
>  

There are times you want to reclaim just part of a zone - specifically
satisfying a high-order allocations. See sitations 1 and 2 from elsewhere
in this thread. On a similar vein, there will be times when you want to
migrate a PFN range for similar reasons.

> > If you want to allocate a page from within the first 1/4 of a zone, and if
> > all those pages are in use for something else then you'll need to run
> > reclaim against the first 1/4 of that zone.  Or fail the allocation.  Or
> > run reclaim against the entire zone.  The second two options are
> > self-evidently dumb.
> 
> Why would one want to allocate from the 1/4th of a zone? (Are we still 
> discussing Mel's antifrag scheme or what is this about?)
> 

Because you wanted contiguous blocks of pages.  This is related to anti-frag
because with anti-frag, reclaiming memory or migration memory will free up
contiguous blocks. Without it, you're probably wasting your time.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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