Subject: Re: siop(4) and tagged queuing
To: Manuel Bouyer <>
From: Thor Lancelot Simon <>
List: current-users
Date: 10/23/2000 23:09:56
On Mon, Oct 23, 2000 at 09:40:57PM +0200, Manuel Bouyer wrote:
> On Mon, Oct 23, 2000 at 06:46:28PM +0200, Manuel Bouyer wrote:
> > Hi,
> > I just added tagged queuing support to siop(4), and also reworked a bit the
> > driver so that it should create less interrupt and PCI load.
> > It should also support the QUEUE FULL status (which ncr(4) failed to handle),
> > but I didn't find a drive which would generate such a condition. If someone
> > has the hardware to test please tell me how it goes.
> Don't bother to try it. I've been able to reproduce it at home, by bumping
> the number of tags to 32. It's brocken :) I'm working on it.

I think the "leaky bucket" algorithm Matt proposed is the correct one.  It
is *not* the case that drives always report QUEUE FULL at the same number of
tags, and some drives that occasionally report QF at a small number of tags
really want a large number to perform well.

Of course, the "drives" that *really* want a lot of tags are RAID 
controllers.  For some I/O loads, the performance of the CMD CRD-5440 (just
to pick an old example, but one with which I'm very familiar) scales almost
linearly with the number of tags in use.

Thor Lancelot Simon	                            
	the effort to perceive simply the cruel radiance of what is