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Date:      Mon, 27 Nov 1995 07:55:25 -0500 (EST)
From:      Peter Dufault <dufault@hda.com>
To:        sysseh@devetir.qld.gov.au (Stephen Hocking)
Cc:        current@FreeBSD.ORG
Subject:   Re: Prioritised disk requests
Message-ID:  <199511271255.HAA04725@hda.com>
In-Reply-To: <199511270628.GAA00550@netfl15a.devetir.qld.gov.au> from "Stephen Hocking" at Nov 27, 95 04:28:50 pm

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> Has anyone thought of a mechanism (similar to stuff I've seen on Masscomp 
> machines) where one could indicate that a particular stream of disk I/O had a 
> higher priority than normal? Masscomp used to have a B_URGENT flag for these 
> blocks in the buffer cache (for semi-realtime I/O when acquiring or 
> transmitting data). CD burners could do with this, or people trying to keep 
> tapes streaming on backups.

I actually had a tiny yet useful subset of this working a few months
back when I was talking about SCSI I/O queueing with Justin.  I
would jam rt_prio processes to the front of the queue to get a SCSI
slot on a host adapter; it made xcdplayer real snappy.  I was
thinking about the problems of scheduling I/O for tagged queueing
and doing multimedia and CDROM-burning.

The idea was given a certain number of possible transactions on a
resource (e.g.,, queue slots on a host adapter) you'd queue up to
get the resource, and the queue would be a priority queue based on
process priority for process context I/O.  It only addresses raw
transfers.

I have to formalize and generalize this and bounce it off the
architects; obviously it can't be a SCSI only solution.

I plan to have some time in the middle of December, around 12/13.

--
Peter Dufault               Real Time Machine Control and Simulation
HD Associates, Inc.         Voice: 508 433 6936
dufault@hda.com             Fax:   508 433 5267



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