Subject: Re: SCSI TMF processing; tag allocation
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Matthew Wilcox <matthew@wil.cx>
Cc: Jens Axboe <jaxboe@fusionio.com>, Luben Tuikov <ltuikov@yahoo.com>,
        Greg KH <greg@kroah.com>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "tj@kernel.org" <tj@kernel.org>
In-Reply-To: <20101115144612.GA6178@parisc-linux.org>
References: <380694.87733.qm@web31812.mail.mud.yahoo.com>
	 <20101113123750.GF18258@parisc-linux.org> <4CE0FD24.4030000@fusionio.com>
	 <1289831595.2233.5.camel@mulgrave.site>
	 <20101115144612.GA6178@parisc-linux.org>
Content-Type: text/plain; charset="UTF-8"
Date: Mon, 15 Nov 2010 08:52:10 -0600
Message-ID: <1289832730.2233.7.camel@mulgrave.site>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1680
Lines: 35

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


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/