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Date: Mon, 20 Feb 1995 10:10:49 +1100
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199502192310.KAA05252@godzilla.zeta.org.au>
To: estienne.cs.berkeley.edu!gibbs@implode.root.com, toor@jsdinc.root.com
Subject: Re: cvs commit: src/sys/sys buf.h
Cc: CVS-commiters@freefall.cdrom.com, bde@zeta.org.au, bde@freefall.cdrom.com,
        cvs-sys@freefall.cdrom.com, toor@Root.COM
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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