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

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Date:	Fri, 27 Apr 2007 14:39:47 +0100
From:	Christoph Hellwig <hch@...radead.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Christoph Lameter <clameter@....com>,
	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

On Fri, Apr 27, 2007 at 10:25:44PM +1000, Nick Piggin wrote:
> Linus's favourite jokes about powerpc mmu being crippled forever, aside ;)

Different mmu.  The desktop 32bit mmu Linus refered to has almost nothing
in common with the mmu on 64bit systems.

> >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."

Real highend HBAs don't have that problem.  But for example aacraid
which is very common on mid-end servers is a _lot_ faster when it
gets continous memory.  Some benchmark was 10 or more percent faster
on windows due to this.
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