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Memory size equivalences
Date: Tue, 11 Dec 1990 20:37 EST
From: firstname.lastname@example.org (John C. Mallery)
Date: Tue, 11 Dec 90 13:30:07 -0800
From: email@example.com (Jeff Barnett)
We are thinking of replacing several 36xx machines with the new 1200's.
The 3600 machines each have either 4 or 6 mega-word memories. From your
experiences with similar conversions, can anyone estimate the amount of
memory necessary for the 1200's to get similar performance. By similar
performance, I mean how much memory do I need so that I page thrash about
the same fraction of the time that I do now. I understand that this
question is not particularly well-formed but I would appreciate any
heuristic advise that is available.
Assuming you're running roughly the same software on each machine
(Genera 8.0.x and comparable versions of your application), there
shouldn't be much difference in memory requirements for 3600s versus
XL1200s. Replace your 4M 3600s with 4M XL1200s, and your 6M 3600s with
8M XL1200s, and you'll get a very significant performance improvement.
Some of the factors involved include:
- I-machines use some main memory (on the order of a hundred kilowords)
for the IFEP operating system; the 3600 FEP had dedicated RAM for this
- I-machines use slightly more main memory to describe their potentially
much larger virtual memory
- I-machines have better code density and so tend to use main memory
- Embedded I-machines (MacIvories, UX1200s) have a somewhat larger working
set due to their larger user interface support (integration with MacOS
or X). This doesn't affect XL1200s.
- Color systems generally need more memory than monochrome, to store
deep bitmaps being pushed around by the user interface
From my experience, an XL1200 with 4 megawords of memory runs the same natural
language application 6 times faster with 10% of the page faults (if you belief
the time macro) as a 3640 with the same amount of memory. This application is
memory-intensive because it builds a knowledge base from incoming sentences
and sentence may refer to prior knowledge. Some of this performance doubtless
comes from using areas localize memory. I suspect somebody fixed up paging
performance for the I-machine. Perhaps someone on this list can tell us about
it. (Note that paging performance is currently one of the major advantages of
the lisp machine over unix lisps).
The I-machine virtual memory architecture is indeed better than the
L-machine's, which takes more "soft" page faults and generally incurs
more overhead to manage a given set of pages. Also, in Genera 8.0 the
virtual memory system for both machines is substantially improved over
Genera 7.2, particularly for systems with larger physical or virtual
memories; the major difference is the implementation of a cheaper and
better page replacement policy.