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

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Date:	Fri, 27 Apr 2007 00:47:49 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Mel Gorman <mel@...net.ie>
CC:	Christoph Lameter <clameter@....com>,
	"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

Mel Gorman wrote:
> 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.

Depends what you mean by "dealing with", I guess.

I would say there has been no credible alternative to virtually mapping
the kernel.


>>>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.

Saying "oh this stuff may not always work quite right for everyone, but
it is OK because it is a special purpose solution for now" IMO is a big
sign saying that it is a bad design, and including it means we're lumped
with it forever.

-- 
SUSE Labs, Novell Inc.
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