linux-kernel - Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

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Date:	Thu, 13 Dec 2007 13:48:18 -0500
From:	Mark Lord <lkml@....ca>
To:	Jens Axboe <axboe@...nel.dk>,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

Mark Lord wrote:
> (resending with corrected email address for Jens)
> 
> Jens,
> 
> I'm experimenting here with trying to generate large I/O through libata,
> and not having much luck.
> 
> The limit seems to be the number of hardware PRD (SG) entries permitted
> by the driver (libata:ata_piix), which is 128 by default.
> 
> The problem is, the block layer *never* sends an SG entry larger than 
> 8192 bytes,
> and even that size is exceptionally rare.  Nearly all I/O segments are 
> 4096 bytes,
> so I never see a single I/O larger than 512KB (128 * 4096).
> 
> If I patch various parts of block and SCSI, this limit doesn't budge,
> but when I change the hardware PRD limit in libata, it scales by exactly
> whatever I set the new value to.  This tells me that adjacent I/O segments
> are not being combined.
> 
> I thought that QUEUE_FLAG_CLUSTER (aka. SCSI host .use_clustering=1) should
> result in adjacent single pages being combined into larger physical 
> segments?
> 
> This is x86-32 with latest 2.6.24-rc*.
> I'll re-test on older kernels next.
...

Problem confirmed.  2.6.23.8 regularly generates segments up to 64KB for libata,
but 2.6.24 uses only 4KB segments and a *few* 8KB segments.

???
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