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

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Date:	Sat, 28 Apr 2007 11:21:17 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Chinner <dgc@....com>, Christoph Lameter <clameter@....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>,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [00/17] Large Blocksize Support V3

> > Also remember that even if you do larger pages by using virtual pairs or
> > quads of real pages because it helps on some systems you end up needing
> > the same sized sglist as before so you don't make anything worse for
> > half-assed controllers as you get the same I/O size providing they have
> > the minimal 2 or 4 sg list entries (and those that don't are genuinely
> > beyond saving and nowdays very rare)
> > 
> 
> Could you expand on that a bit please?  I don't get it.

Put a 16K "page" into the page cache physically and you need to allocate
1 sg entry and you get a clear benefit, IFF you can allocate the pages. 

Put a 16K "page" into the page cache made up of 4 x real 4K pages which
are not physically contiguous and you need 4 sg list entries - which is
no worse than if you were using 4K pages

	4 per 16K page cache "logcial page"		-> 4 per 16K

	1 per 4K physical page for 4K page cache	-> 4 per 16K

The only ugly case for the latter is if you are reading something like a
16K page ext3fs from an old IA64 box onto a real computer and you have a
controller with insufficient sg list entries to read a 16K logical page.
At that point the block layer is going to have kittens.

Alan
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