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From: ed@mtxinu.UUCP (Ed Gould)
Newsgroups: net.arch
Subject: Re: Re: I don't believe your statements abou
Message-ID: <402@mtxinu.UUCP>
Date: Mon, 3-Jun-85 16:53:37 EDT
Article-I.D.: mtxinu.402
Posted: Mon Jun  3 16:53:37 1985
Date-Received: Thu, 6-Jun-85 05:12:14 EDT
References: <7202@Glacier.UUCP> <36900002@ima.UUCP> <579@lll-crg.ARPA> <5593@utzoo.UUCP>
Reply-To: ed@mtxinu.UUCP (Ed Gould)
Organization: mt Xinu, Berkeley, CA
Lines: 30

In article <5593@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
>Actually, in Computing Surveys some years ago there was mention of another
>case where a multiprocessor got super-linear performance.  "Multi" in that
>case was 2, so it's not a striking example, but even so it was interesting.
>The machine was a dual-processor IBM 360/67.  The slightly-better-than-2
>factor of performance was because some system overhead is not proportional
>to the user load, and that overhead has to be incurred only once for both
>processors.  The 67 had an unusual i/o architecture that tended to direct
>interrupts to the processor (if any) that wasn't busy at the time, which
>helped.

Probably true, but to some extent it's an apples-to-oranges comparison.
The real reason that the /67 ran faster with two processors is really
that the OS overhead was too large, and adding more user horsepower
got a net improvement in throughput.  I doubt that there were really
more than 2 times as many instructions executed, total.

IBM operating systems are noted for large overheads.  One of theirs
(OS/VS1 if memory serves correctly) ran faster under VM than it did
native, i.e. the throughput went up by adding an *additional* layer
of software between the OS and the hardware.  (There is an explanation -
we don't have a perpetual motion machine here:  VS1's paging algorithm
was *terrible*, so telling it that it had infinite amounts of real
memory and letting VM do the pageing did the trick.)  (This was true
about 8 years ago when I had my few dealings with IBM systems.  I
don't think it's so any more.)

-- 
Ed Gould		    mt Xinu, 2910 Seventh St., Berkeley, CA  94710  USA
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