Path: utzoo!yunexus!geac!syntron!jtsv16!uunet!auspex!guy
From: guy@auspex.UUCP (Guy Harris)
Newsgroups: comp.arch
Subject: Re: RISC v. CISC (was The NeXT problem)
Message-ID: <306@auspex.UUCP>
Date: 26 Oct 88 16:30:39 GMT
Article-I.D.: auspex.306
References: <156@gloom.UUCP> <310@lynx.zyx.SE> <332@pvab.UUCP> <10447@cup.portal.com>
Reply-To: guy@auspex.UUCP (Guy Harris)
Organization: Auspex Systems, Santa Clara
Lines: 16

>>The more registers, the more to save at every context switch in a typical
>>OS (such as UNIX). Which will slow things down if you have many processes
>>running.

>What data do you have to substantiate this claim?  This is another popular
>misconception, I think.

There appears to be a belief out there that register windows slow
context switches down on Sun-4s; this may be the source of the claim. 
It isn't true; what slows them down is the expense of flushing the
entries for the U area from the virtual address cache, since the U area
is at the same address in kernel virtual space in all processes, and the
context number is the same for all those processes, so you can't rely on
the context number to distinguish between the virtual address of the U
areas in different processes.  The fix is to put them at different
virtual addresses....