Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/3/84; site soph.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!bellcore!decvax!genrad!panda!enmasse!enms2!soph!dave
From: dave@soph.UUCP (Dave Brownell)
Newsgroups: net.micro
Subject: Re: What Unix needs is ...
Message-ID: <154@soph.UUCP>
Date: Tue, 30-Apr-85 21:51:53 EDT
Article-I.D.: soph.154
Posted: Tue Apr 30 21:51:53 1985
Date-Received: Fri, 3-May-85 23:43:17 EDT
References: <381@wdl1.UUCP> <>
Reply-To: dave@soph.UUCP (Dave Brownell)
Organization: Enmasse Computer Corp., Acton, Mass.
Lines: 30
Summary: There IS a troublesome semantic for shared memory.

I agree, it is hard to say UNIX lacks shared memory when I can use
it on this very (UNIX) machine!!

But the person who said that there are no machine-dependant semantics
was not thinking of multiprocessor environments.  Consider the case of
two CPUs on a bus; each one has a cache of some size, and each cache
has what I will call a "cache chunk size":  the size of the minimum
cacheable bit of memory.

The problem is simple:  suppose two processes, communicating with
shared memory, need to access locations N and N+X?  If they are
running on the two different CPUs, there is no way to determine whether
mutual exclusion has been achieved on the chunk of memory with location
N unless you know the cache chunk size.

To bring this to a concrete point, say you have two 68020 processors
and the data space is cached on longword boundaries (4 bytes).  If
processor A reads location 1000, and processor B reads location 1002,
even a lock on every byte would not detect the collision.  (Both bytes
are in the same longword.)  The programs need to know that hardware
exclusion operates in longword units.

Naturally, this crops up only in multiprocessor cached environments.
(Which we hope will become much more common...)

-- 
	Dave Brownell
	EnMasse Computer Corporation
	enmasse!dave@Harvard.ARPA
	{genrad,harvard}!enmasse!dave