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From: rpw3@fortune.UUCP
Newsgroups: net.unix
Subject: Re: Semi-conductor Disk for VAX - (nf)
Message-ID: <2238@fortune.UUCP>
Date: Wed, 11-Jan-84 07:04:49 EST
Article-I.D.: fortune.2238
Posted: Wed Jan 11 07:04:49 1984
Date-Received: Thu, 12-Jan-84 00:47:23 EST
Sender: notes@fortune.UUCP
Organization: Fortune Systems, Redwood City, CA
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#R:sri-arpa:-1511100:fortune:26900017:000:1588
fortune!rpw3    Jan 11 04:01:00 1984

Tom Teixeira notes that disks are faster than memory becasue the
processor can do something else...

Well that is certainly true of PDP-11's and VAXen, but it just ain't
so for many of the modern micro-based systems. The Motorola 68000
in particular can use up as many memory cycles as you can give it
(use the fastest 64k RAMs on the market with the fastest 68k and the
68k will be waiting on the RAMs). Under these conditions, each DMA
memory cycle costs you one CPU memory cycle. In fact, if your bus
happens to be a little slow switching between bus masters (such as a
Multibus), each DMA cycle can cost you several CPU memory cycles.

I have seen systems (not ours) where the CPU would be better off
block moving data in/out to a passive (non-DMA) dual-ported RAM
on the controller card than it would be letting DMA steal (sic)
cycles.

Likewise, v.7 UNIX swaps instead of shuffling memory when it gets
internal fragmentation, under the assumptions that (1) DMA-ing out
to disk and back is a net savings in CPU cycles over block move
(true! for PDP-11), and (2) that there is something else going on
to use those cycles (the other 15 users). These assumptions should
be re-examined on 1-4 user micro-based systems, especially those with
sloooowwww access winchester disks.

Not criticizing any of the previous commentators; just noting again
the Murphy/tanstaafl correlary, "Things aren't always what they seem".

Rob Warnock

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