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

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Message-ID: <Pine.LNX.4.64.0807141903001.2825@devserv.devel.redhat.com>
Date:	Mon, 14 Jul 2008 19:16:17 -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

>>>> 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?
>
> I'm talking about other devices, not about the ESP, here.

And which ones are those important drivers that need merge accounting?

A100U2W is an old card, I got it for $8.5 in bazaar, it does 38MB/s. This 
virtual merge accounting helps to stuff on average 4 more segments into 
the 32-entry table.

So the question is: to reduce number of requests by 12% on an outdated 
SCSI card, it is sensible to maintain complicated merge accounting logic 
in the core block layer? To me, it doesn't seem sensible.

Mikulas
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