Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site sftig.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxn!mhuxm!sftig!rbt
From: rbt@sftig.UUCP (R.Thomas)
Newsgroups: net.arch
Subject: Re: Cache revisited
Message-ID: <563@sftig.UUCP>
Date: Fri, 2-Aug-85 14:59:51 EDT
Article-I.D.: sftig.563
Posted: Fri Aug  2 14:59:51 1985
Date-Received: Sat, 3-Aug-85 10:23:43 EDT
References: <5374@fortune.UUCP> <268@gcc-bill.ARPA>
Distribution: net
Organization: AT&T Bell Laboratories, Summit, NJ
Lines: 30

> 
> I'd like to compare and contrast the difference in performance between a
> simple single level paged memory manager using a ram (a la Sage 68000) and
> a system like the IBM DAT box, where the page tables are stored in main memory
> and cached in hardware. The point being that switching context is MUCH
> faster if you only need to change the pointer to the page tables, rather than
> copy 8K of paging information into the page table ram. It is assummed that
> the cache used to speed up the main memory page table accesses is sufficiently
> large to get a good hit rate (what ever that may be).
>

You pay to load the page table when you context switch with either system. 
In one system, you load the page table registers explicitly all at once during
the actual context switch, in the other you pay to load it in a more lesurely
fashion as the DAT box cache faults it in.  The reference to main memory to
load the DAT box cache line cost just as much as the reference to main
memory to load the page table entry.

Mitigating circumstances -- with the DAT box, you only pay to load the ones
you actually use, with the page table in its own register file, you have to
load each register in the file whether you intend to use that page or not.
But if the cache is too small (and it *always* is too small -- There is no
economic incentive to make it too big!), you may have to load each cache
line several times.

If you only care about interrupt response time, then the DAT-box/cache is a
win.  But you take the same thruput hit either way.


Rick Thomas