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

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Date:	Fri, 27 Apr 2007 22:25:44 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Christoph Lameter <clameter@....com>
CC:	Paul Mackerras <paulus@...ba.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Chinner <dgc@....com>, linux-kernel@...r.kernel.org,
	Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.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

Christoph Lameter wrote:
> On Fri, 27 Apr 2007, Nick Piggin wrote:
> 
> 
>>For the TLB issue, higher order pagecache doesn't help. If distros
>>ship with a 4K page size on powerpc, and use some larger pages in
>>the pagecache, some people are still going to get angry because
>>they wanted to use 64K pages... But I agree 64K pages is too big
>>for most things anyway, and 16 would be better as a default (which
>>hopefully x86-64 will get one day).
> 
> 
> Powerpc supports multiple pagesizes. Maybe we could make mmap use those 
> page sizes some day if we had a variable order page cache. Your stands on 
> the issue means that powerpc will be forever crippled and not be able to 
> use its full potential.

Linus's favourite jokes about powerpc mmu being crippled forever, aside ;)

This seems like just speculation. I would not be against something which,
without, would "cripple" some relevant hardware, but you are just handwaving
at this point. And you are still ignoring the alternatives.


>>Anyway, for io performance, there are alternatives, dispite what
>>some people seem to be saying. We can submit larger sglists to the
>>device for larger ios, which Jens is looking at (which could help
>>all types of workloads, not just those with sequential large file
>>IO).
> 
> 
> Right this could help but it is not addressing the basic requirement for
> devices that need large contiguuos chunks of memory for I/O.

Did you read the last paragraph? Or anything Andrew's been writing?

  "After that, I'd find it amusing if HBAs worth thousands of $ have
   trouble looking up sglists at the relatively glacial pace that IO
   requires, and/or can't spare a few more K for reasonable sglist
   sizes, but if that is really the case, then we could use iommus
   and/or just attempt to put physically contiguous pages in pagecache,
   rather than require it."

-- 
SUSE Labs, Novell Inc.
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