Path: utzoo!attcan!uunet!husc6!mailrus!ames!oliveb!sun!ember!dre
From: dre%ember@Sun.COM (David Emberson)
Newsgroups: comp.arch
Subject: Re: Costs of physical vs virtual caches
Summary: fphysical caches and VM
Message-ID: <61192@sun.uucp>
Date: 25 Jul 88 16:31:01 GMT
References: <60952@sun.uucp> <1331@basser.oz>
Sender: news@sun.uucp
Lines: 20


 >Even on some machines with physical caches, address translation can take place
 >in parallel with the cache tag access--thus no penalty.
 >			Dave Emberson (dre@sun.com)
> 
> This is the real point: physical caches avoid the problems above, and can be
> built to have NO (or acceptably small) speed penalty. So, they don't impact
> CPU intensive jobs, but win big for highly interactive or real time systems
> where OS overhead costs are significant.

This is only true if the cache set size is less than or equal to the page
size, or some restriction is placed on virtual to physical mapping to avoid
aliases.

There are easy and efficient ways to selectively flush virtual caches, so
I think in the future the argument over physical v.s. virtual caches will 
turn out to be a religious debate.  The statement that snoopy caches must
have some sort of physical tags if aliasing is allowed is certainly correct.

			Dave