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

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Message-ID: <Pine.LNX.4.64.0807141000340.24718@devserv.devel.redhat.com>
Date:	Mon, 14 Jul 2008 10:03:29 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	David Miller <davem@...emloft.net>
cc:	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, 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?

Mikulas

> It's the only way to know ahead of time that the IOMMU is going
> to merge things for us.

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