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Date: Thu, 26 Sep 2002 00:12:41 -0700
From: Andrew Morton <akpm@digeo.com>
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To: Jens Axboe <axboe@suse.de>, Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] deadline io scheduler
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Andrew Morton wrote:
> 
> I'll test scsi now.
> 

aic7xxx, Fujitsu "MAF3364L SUN36G" (36G SCA-2)


Maximum number of TCQ tags=253

	fifo_batch		time cat kernel/*.c (seconds)
	    64				58
	    32				54
	    16				20
	     8				58
	     4				1:15
	     2				53

Maximum number of TCQ tags=4

	fifo_batch		time cat kernel/*.c (seconds)
	    64				53
	    32				39
	    16				33
	     8				21
	     4				22
	     2				36
	     1				22


Maximum number of TCQ tags = 0:

	fifo_batch		time cat kernel/*.c (seconds)
	    64				22
	    32				10.3
	    16				10.5
	     8				5.5
	     4				3.2
	     2				1.9

I selected fifo_batch=16 and altered writes_starved and read_expires
again.  They made no appreciable difference.

>>From this I can only conclude that my poor little read was stuck
in the disk for ages while TCQ busily allowed new incoming writes
to bypass already-sent reads.

A dreadful misdesign.  Unless we can control this with barriers,
and if Fujutsu is typical, TCQ is just uncontrollable.  I, for
one, would not turn it on in a pink fit.