linux-kernel - Re: [patch] scsi: use lock per host instead of per device for shared queue tag host

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Date:	Thu, 25 Jan 2007 20:21:01 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Ed Lin <ed.lin@...mise.com>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	David Somayajulu <david.somayajulu@...gic.com>,
	Michael Reed <mdr@....com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"james.Bottomley" <james.Bottomley@...elEye.com>,
	Promise_Linux <Promise_Linux@...mise.com>
Subject: Re: [patch] scsi: use lock per host instead of per device for shared
 queue tag host

Ed Lin wrote:
> There may possibly be some other errors. So we need a lock here.
> I think the simple but reliable way to do it is just to replace
> queue lock with a host lock. James pointed out that there may be
> performance slow down when many devices are accessed at the
> same time. But I think the major part is still on the hardware,
> and a host lock is the price these kind of controllers must pay.


I agree.

Further, a host lock is (a) common across many controllers, to protect 
host-wide resources and (b) only limits us when the controller is 
CPU-limited, a very rare scenario.

	Jeff


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