Date: Mon, 20 Feb 1995 10:10:49 +1100 From: Bruce Evans <bde@zeta.org.au> To: estienne.cs.berkeley.edu!gibbs@implode.root.com, toor@jsdinc.root.com Cc: CVS-commiters@freefall.cdrom.com, bde@zeta.org.au, bde@freefall.cdrom.com, cvs-sys@freefall.cdrom.com, toor@Root.COM Subject: Re: cvs commit: src/sys/sys buf.h Message-ID: <199502192310.KAA05252@godzilla.zeta.org.au>
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>Well, it appears that on an IDE drive there is the >possibility< for it to >stream data for a pretty long while. It would cause the system to be I/O >(interrupt) bound (because of the slow ISA bus) for a long time. There is the >possibility for this I/O operation to last approx 64K * ??usecs/transfer :-(. >(Bruce knows more about ISA bus timing than I do, but I guess that it is about >.5usecs??? per word or more). That is a long, long time. The kernel cannot >do much about it once the (long) I/O operation is queued. I think one IDE PIO speed is 3.3M/sec so the time per word is 0.67usec. The new EIDE PIO speed of 11M/sec will reduce the problem in a while for a while. I don't see how reducing the I/O size makes any difference if the disk+controller can transfer faster than the bus. Won't the driver just loop over more buffers? There may be short pauses in I/O while the drive+controller prepares the next I/O, but only if drive+ controller didn't do enough read ahead. >Bus mastering SCSI worries me much less, but still is a concern (as drives >get faster and the old ISA-bus stays the same speed) :-). How fast can bus mastering DMA on ISA systems go? The problem (and the throughput!) may be reduced by the controller having to follow bus-on/ bus-off timing rules. Bruce
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