linux-kernel - Re: [00/17] Large Blocksize Support V3

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Message-ID: <20070426100652.GB27620@skynet.ie>
Date:	Thu, 26 Apr 2007 11:06:52 +0100
From:	mel@...net.ie (Mel Gorman)
To:	Christoph Lameter <clameter@....com>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>
Subject: Re: [00/17] Large Blocksize Support V3

On (25/04/07 23:46), Christoph Lameter didst pronounce:
> On Thu, 26 Apr 2007, Nick Piggin wrote:
> 
> > Yeah. IMO anti-fragmentation and defragmentation is the hack, and we
> > should stay away from higher order allocations whenever possible.
> 
> Right and we need to create series of other approaches that we then label 
> "non-hack" to replace it. 
> 

To date, there hasn't been a credible alternative to dealing with
fragmentation. Breaking the 1:1 virtual:physical mapping and defragmenting
would incur a serious performance hit.

> > Hardware is built to handle many small pages efficintly, and I don't
> > understand how it could be an SGI-only issue. Sure, you may have an
> > order of magnitude or more memory than anyone else, but even my lowly
> > desktop _already_ has orders of magnitude more pages than it has TLB
> > entries or cache -- if a workload is cache-nice for me, it probably
> > will be on a 1TB machine as well, and if it is bad for the 1TB machine,
> > it is also bad on mine.
> 
> There have been numbers of people that have argued the same point. Just 
> because we have developed a way of thinking to defend our traditional 4k 
> values does not make them right.
> 
> > If this is instead an issue of io path or reclaim efficiency, then it
> > would be really nice to see numbers... but I don't think making these
> > fundamental paths more complex and slower is a nice way to fix it
> > (larger PAGE_SIZE would be, though).
> 
> The code paths can stay the same. You can switch CONFIG_LARGE pages off
> if you do not want it and it is as it was.
> 

It may not even need that that much effort. The most stressful use of the
high order allocation paths here require the creation of a filesystem and
is a deliberate action by the user.

> If you would have a look the patches: The code is significantly cleanup 
> and easier to read.

It is easier to read all right and may be worth doing anyway.

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