linux-kernel - Re: [SUGGESTION]: drop virtual merge accounting in I/O requests

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Message-Id: <20080713.124610.193703496.davem@davemloft.net>
Date:	Sun, 13 Jul 2008 12:46:10 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	andi@...stfloor.org
Cc:	mpatocka@...hat.com, sparclinux@...r.kernel.org,
	linux-kernel@...r.kernel.org, jens.axboe@...cle.com
Subject: Re: [SUGGESTION]: drop virtual merge accounting in I/O requests

From: Andi Kleen <andi@...stfloor.org>
Date: Sun, 13 Jul 2008 15:50:55 +0200

> Still I would expect that modern IO controllers are typically fast
> enough at processing SG lists that it shouldn't matter much.

I know it matters a lot on sparc64 ESP scsi controllers.

You can only have one address/len pair DMA'ing at a time and you have
to service an interrupt to load in the the next DMA sg elements into
the chips registers.

Merging is essentially a must for performance on those cards.
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