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

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Date:	Sat, 28 Apr 2007 12:27:45 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Christoph Hellwig <hch@...radead.org>
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

Christoph Hellwig wrote:
> 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.


Well I wasn't trying to make a point there so it isn't a big deal... but
he has known to say the 64-bit hash table is insane or broken. If he's
since recanted, I'd be interested to read the post :)


>>>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.

And that wasn't due to the 128 sg limit?

I guess 10% isn't a small amount. Though it would be nice to have
before/after numbers for Linux. And, like Andrew was saying, we could
just _attempt_ to put contiguous pages in pagecache rather than
_require_ it. Which is still robust under fragmentation, and benefits
everyone, not just files with a large pagecache size.

-- 
SUSE Labs, Novell Inc.
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