Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!pasteur!ucbvax!decwrl!nsc!stevew
From: stevew@nsc.nsc.com (Steve Wilson)
Newsgroups: comp.arch
Subject: Re: VM needed for rapid startup
Keywords: paging virtual-memory speed
Message-ID: <5129@nsc.nsc.com>
Date: 26 May 88 16:06:02 GMT
References: <463@cvaxa.sussex.ac.uk> <19322@beta.UUCP>
Reply-To: stevew@nsc.UUCP (Steve Wilson)
Organization: National Semiconductor, Sunnyvale
Lines: 35

In article <19322@beta.UUCP> jlg@beta.UUCP (Jim Giles) writes:
>In article <463@cvaxa.sussex.ac.uk>, aarons@cvaxa.sussex.ac.uk (Aaron Sloman) writes:
>> [...]
>> Anyhow, while disk access remains significantly slower than main memory,
>> something like paged virtual memory will remain highly desirable for
>> those of us who want to use large sophisticated software packages like
>> AI development environments, powerful editors, etc. EVEN if there is
>> enough physical memory for the whole package.
>
>One of the things that makes virtual memory LESS useful than it used to be
>is the fact that disk is so slow compared to main memory (and CPU speed).
>If it takes less than the disk access time for your program to execute
>through a whole page of code, a busy multiprocessing environment may spend
>nearly all its time waiting on page faults. What's really needed is some
>form of mass storage that is faster than disk - implement a paging scheme
>off the intermediate storage (which in turn can page off disk in larger
>chunks?).

If a system is running below its "thrash" point, and assuming there are
other useful things to do beside hang around in the idle loop, a 
multi-tasking system will suspend a job that ran into a page fault
or blocked for I/O.  At this point they'll move on to some other
job.  Also consider that USUALLY the execute-a-page then fault situation
only occurs at process startup.  This is where the concept of a "working
set" comes from.  Once that working set of pages is resident in main memory
the occurance of page faults drop off.  

It makes more since to place more storage in the primary memory instead
of an intermediate store.  After all, that is where you want the code/data
to wind up in the first place.

Steve Wilson
National Semiconductor

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