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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