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From: dds@spsd.SPSD.DG.COM (Dennis D. Sherod)
Newsgroups: comp.os.misc
Subject: Re: DG/UX -- Seybold's Review ( long )
Message-ID: <943@spsd.SPSD.DG.COM>
Date: 21 Sep 88 06:35:44 GMT
References: <602@spsd.SPSD.DG.COM> <11590002@hpisoa1.HP.COM>
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Organization: Data General SPSD Santa Ana, CA
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In article <11590002@hpisoa1.HP.COM> vandys@hpisoa1.HP.COM (Andrew Valencia) writes:
>/ hpisoa1:comp.os.misc / dds@spsd.SPSD.DG.COM (Dennis D. Sherod) /  7:02 pm  Sep 11, 1988 /
>
>>DG needs to distinguish its approach to Virtual Memory Management from
>>those used by both Hewlett-Packard and IBM.  Both HP/UX and AIX allow
>>for users to have transparent access to programs that do not fit into
>>physical memory by swapping segments in and out of main memory.  DG's
>>approach allows users to have transparent access to programs that do
>>not fit into physical memory by demand paging on a per page basis.  DG
>>needs to be clear on how its approach is superior to this alternative.
>
>    Could you explain this statement?  Our VM system is essentially the
>BSD 4.2 VM system, with some modifications for performance and real-time
>latency.  Like 4.2, our system can swap out user processes.  But it is also
>perfectly capable of doing virtual paging.  Doing only demand-paging
>might be a win, but I'd be interested to hear in exactly what ways it
>manifests itself for DG over a system like 4.2/4.3.

I was not the author of the article.  Recall that this was a reprint of
a review made by a Patricia (think that's the name) Seybold publication.

SIDEBAR: They do a number of industry product comparisons often at a
vendor's request.

I don't wish to second guess the author's intent here, and I am not that
familiar with BSD's or HP's VM strategies, but as some general
statements:

Segment swapping is generally considered a process strategy while paging
is a memory management strategy.  There are also some connotations as to
the size of the VM objects, segments being higher in the hierarchy and
larger.  By process strategy I mean, how the operating system views a
process' intent on using CPU resources and decides to move parts or all
of the process in and out of memory.  While memory management strategy
is often largely handled by hardware architecture and works with much
smaller fixed size pieces 'pages'.

I know I didn't answer your question, but since I'm not familiar with
your VM architecture, I'm not even sure what the question is.  If there
is sufficient interest, I will prepare an article discussing in more
detail DG/UX's VM architecture, but I can not carry on an intelligent
discussion of alternate architectures and their merits.
-- 
Dennis Sherod, Data General Corporation UUCP:     ..!ucsd!spsd!dds
Software Products & Services Division   TELCON:   +1 714 835 3583
1224-B Village Way,Santa Ana,CA 92705   Internet: dds@spsd.SPSD.DG.COM