linux-kernel - RE: [patch] scsi: use lock per host instead of per device forshared queue tag host

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Message-ID: <B70A50D07063384EB9BCE3330D18414F02F5AF9D@nonameb.ptu.promise.com>
Date:	Wed, 24 Jan 2007 19:25:11 -0800
From:	"Ed Lin" <ed.lin@...mise.com>
To:	"James Bottomley" <James.Bottomley@...elEye.com>
Cc:	"linux-scsi" <linux-scsi@...r.kernel.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"jeff" <jeff@...zik.org>,
	"Promise_Linux" <Promise_Linux@...mise.com>
Subject: RE: [patch] scsi: use lock per host instead of per device forshared queue tag host



> -----Original Message-----
> From: James Bottomley [mailto:James.Bottomley@...elEye.com] 
> Sent: Wednesday, January 24, 2007 9:00 AM
> To: Ed Lin
> Cc: linux-scsi; linux-kernel; jeff; Promise_Linux
> Subject: Re: [patch] scsi: use lock per host instead of per 
> device forshared queue tag host
> 
...
> 
> This patch looks OK in principle.
> 
> However, are you sure you're not using a sledgehammer to crack a nut?
> If the only reason you're doing this is because of the shared tag map,
> then probably that should be the area you protect with a per-tag-map
> lock.   The net effect of what you've done will be to serialise all
> accesses to your storage devices.  For a small number of devices, this
> probably won't matter than much, but for large numbers of devices,
> you're probably going to introduce artificial performance 
> degredation in
> the I/O scheduler.
> 
> James
> 

Thanks.
Maybe this issue needs more discussion. I'll follow up.

Ed Lin
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