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From: gnu@l5.uucp (John Gilmore)
Newsgroups: net.arch
Subject: Re: Why Virtual Memory
Message-ID: <239@l5.uucp>
Date: Wed, 30-Oct-85 17:43:43 EST
Article-I.D.: l5.239
Posted: Wed Oct 30 17:43:43 1985
Date-Received: Sun, 3-Nov-85 07:51:08 EST
References: <480@seismo.CSS.GOV>, <384@unc.unc.UUCP> <6086@utzoo.UUCP>
Organization: Nebula Consultants in San Francisco
Lines: 20
Summary: I. P. Sharp no longer runs their Amdahl without paging

In article <6086@utzoo.UUCP>, henry@utzoo.UUCP (Henry Spencer) writes:
> Virtual memory has always meant some speed penalty, although clever design
> can minimize it.  Even 10-year-old big machines run more quickly with
> address translation switched off, as witness IP Sharp [big APL timesharing
> firm] which runs its monster Amdahl unmapped and sees about a 15% speed
> improvement as a result.  (They can get away with this because they run no
> directly-executable user code.)

Any machine has some overhead due to updating page table entries -- in
hardware, or in software, or both.  You get what you pay for.

Sharp used to run DOS/360 (heavily hacked) on their Amdahl, since their
APL time sharing system was written to timeshare efficiently on the 360
which did not have address translation or paging (remember those days??).
They ported their APL to MVS a few years ago because a lot of customers wanted
to run it that way, and eventually converted their data centre to MVS too.
I think it cost them about 40% in overhead, but they could stop maintaining
DOS/360 (which IBM dropped 10 years ago and Amdahl had no interest in).
It got to be a pain writing new disk drivers and machine check handlers
to keep up with the latest in IBM and 3rd party mainframe fashions.