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

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Message-Id: <20080715113238U.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 15 Jul 2008 11:40:45 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	mpatocka@...hat.com
Cc:	davem@...emloft.net, andi@...stfloor.org,
	sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
	jens.axboe@...cle.com
Subject: Re: [SUGGESTION]: drop virtual merge accounting in I/O requests

On Mon, 14 Jul 2008 10:03:29 -0400 (EDT)
Mikulas Patocka <mpatocka@...hat.com> wrote:

> On Mon, 14 Jul 2008, David Miller wrote:
> 
> > From: Mikulas Patocka <mpatocka@...hat.com>
> > Date: Mon, 14 Jul 2008 08:16:13 -0400 (EDT)
> >
> >> As you mentioned ESP driver, it declares .sg_tablesize = SG_ALL, so
> >> BIO_VMERGE_BOUNDARY has no effect on the operation of this driver. Any
> >> other driver where BIO_VMERGE_BOUNDARY does matter?
> >
> > When BIO_VMERGE_BOUNDARY is turned on, requests that would not
> > otherwise fit into the device's limits, can.
> 
> Why would someone want to overshoot SG_ALL? ... and, shouldn't the 
> constant be increased then --- instead of making buggy BIO_VMERGE_BOUNDARY 
> expectations?

'buggy' is a wrong word. VMERGE works for some IOMMUs (though I think
that we need to fix it about max_segment_size).
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