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

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Date:	Thu, 26 Apr 2007 19:42:54 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	David Chinner <dgc@....com>, Christoph Lameter <clameter@....com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>
Subject: Re: [00/17] Large Blocksize Support V3

On Fri, Apr 27 2007, Nick Piggin wrote:
> Why do we limit drivers to 128 sg entries?

No particular reason, except than to avoid 2^bigger order allocations.
2MiB requests would require 3 contig pages to setup the sg list, which
is (probably) a little troublesome especially since it's sometimes
atomically allocated.

Larger pages are by no means a prerequisite to getting larger requests,
assuming your hardware can handle the bigger sglist. There are other
ways of doing that, I've contemplated doing chained sglists and adding
sg_for_each_segment() macros for iterating these things. Drivers that
want larger sglists would then be required to update their sg mapping
loop to use the provided macros. It wouldn't be too hard.

-- 
Jens Axboe

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