Path: utzoo!attcan!uunet!actnyc!prh
From: prh@actnyc.UUCP (Paul R. Haas)
Newsgroups: comp.arch
Subject: Re: dedicated vs general-purpose CPUs
Summary: CDC stopped using general purpose PP's
Message-ID: <1018@actnyc.UUCP>
Date: 8 Aug 88 19:48:17 GMT
References: <5254@june.cs.washington.edu> <76700032@p.cs.uiuc.edu> <1988Aug3.180947.12070@utzoo.uucp> <1221@ficc.UUCP> <1173@garth.UUCP>
Reply-To: prh%actnyc@uunet.uu.net (Paul R. Haas)
Organization: InterACT Corporation
Lines: 28

In article <1173@garth.UUCP> smryan@garth.UUCP (Steven Ryan) writes:
>If you want an expensive example of a machine made of diverse processors,
>consider the CDC 6600->170s. A machine consists of 1 or 2 60-bit CPs and
>10 to 20 12-bit PPs. All the number crunching goes in the CP and the
>io in the PPs with all processors in parallel. (Well actually, PPs share
>a barrel.)
[a PP is a peripheral processor, a CP is a Central Processor.]
>
>Most of the operating system for NOS actually runs in the PPs so that the
>CP spends much of its time in user state.
Nice idea, unfortunately, it was frequently faster and easier to do things
in the CP.  The PPs are slow, compared to the CP, and have only 4096, 12 bit
words.  If you try to do all of the operating system system in the PPs what
really happens is the CP spends much of its time in the idle state waiting
on the PP.
>
>The 180 retains the same philosophy with 64-bit CPs and 16-bit PPs, though
>apparently NOS/VE uses the PPs as just drivers with operating system mostly
>in the CP.
CDC does learn from experience.

On a machine with slow context switches an IO processor which is just bright
enough to buffer transactions so as to avoid some context switches, is a net
win.  There doesn't seem to be any reason to divert any more resources from
the CP.
------
Paul Haas
uunet!actnyc!prh