Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!lamaster
From: lamaster@ames.arc.nasa.gov (Hugh LaMaster)
Newsgroups: comp.arch
Subject: Re: RISC v. CISC (was The NeXT problem)
Message-ID: <17257@ames.arc.nasa.gov>
Date: 28 Oct 88 16:01:51 GMT
References: <156@gloom.UUCP> <310@lynx.zyx.SE> <332@pvab.UUCP> <15964@agate.BERKELEY.EDU> <7681@boring.cwi.nl> <7180@winchester.mips.COM>
Reply-To: lamaster@ames.arc.nasa.gov.UUCP (Hugh LaMaster)
Organization: NASA Ames Research Center, Moffett Field, Calif.
Lines: 34

In article <7180@winchester.mips.COM> mash@winchester.UUCP (John Mashey) writes:
>In article <7681@boring.cwi.nl> jack@cwi.nl (Jack Jansen) writes:
>>Well, 100 usec might be fine for standard unix, it is definitely not
:
>BTW: it is not instantly obvious that one would add a bit in for just this
>purpose.  On a 16.7MHz M/120, it takes something like 4-30 microseconds
>to save 32 registers and restore 32 registers [the 4 is all cache hit,
>the 30 is all cache miss].  On a 25MHz M/2000, it takes 3-10 microseconds,

On a 50MHz Cyber 205, it takes approximately 200 minor cycles = 4 microseconds
to swap the entire processor context.  128 of those cycles are used to
swap the entire 256 general purpose registers.

Sometimes people confuse procedure call time (which requires saving only
those registers which will be used - small for small procedures) with
the context switching time (time to switch to another user/process).

There is no need for context switching time to be tiny in an ordinary
(no fine-grained parallelism) system, since even on a very large machine
context switch shouldn't occur more frequently than several thousand/second.

Procedure call time must be very fast, obviously, since it may occur
several thousand times more often than context switching does. 

The Cray-2 is the only machine that I know of that has a real problem with
context switching.  The reason is that it has an extraordinarily large
user context in the form of "local memory".


-- 
  Hugh LaMaster, m/s 233-9,  UUCP ames!lamaster
  NASA Ames Research Center  ARPA lamaster@ames.arc.nasa.gov
  Moffett Field, CA 94035     
  Phone:  (415)694-6117