linux-kernel - Re: SCSI TMF processing; tag allocation

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Date:	Mon, 15 Nov 2010 08:52:10 -0600
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Jens Axboe <jaxboe@...ionio.com>, Luben Tuikov <ltuikov@...oo.com>,
	Greg KH <greg@...ah.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tj@...nel.org" <tj@...nel.org>
Subject: Re: SCSI TMF processing; tag allocation

On Mon, 2010-11-15 at 07:46 -0700, Matthew Wilcox wrote:
> On Mon, Nov 15, 2010 at 08:33:15AM -0600, James Bottomley wrote:
> > Right, it's the clock algorithm to prevent tag starvation.  If you have
> > hands representing the first and last tag and they're never allowed to
> > cross, the device can't starve any tag for too long because eventually
> > it will be the only outstanding command.
> > 
> > It's not the only algorithm however.  Banging down an ordered tag every
> > 200 or so commands has exactly the same effect.  In fact the clock
> > algorithm was what the 53c700 driver used (before it was converted to
> > generic tags) and the ordered tag what aic7xxx uses.
> > 
> > Realistically, tag starvation isn't really a problem.  It was a known
> > issue for 80s era hardware.  I've got some of the oldest drives on the
> > planet and I didn't see a problem when the clock algorithm was removed
> > from 53c700.
> 
> The problem is that each driver is solving the problem in its own way
> right now, which is clearly daft.  And no drive manufactured in the past
> fifteen years supports ordered tags anyway, so they're only a placebo
> at this point.

Actually, most drivers ignore the issue ... which is why I don't think
there's much of a problem.  The aic7xxx is the only one I know doing
something about this.  Like I said, I removed the clock algorithm from
53c700 and haven't had a problem (yet, I suppose).

James


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