Path: utzoo!utgpu!water!watmath!clyde!att!mtunx!rutgers!gatech!bloom-beacon!bu-cs!purdue!decwrl!sun!aeras!elxsi!beatnix!rw
From: rw@beatnix.UUCP (Russell Williams)
Newsgroups: comp.arch
Subject: Re: VM needed for rapid startup
Message-ID: <802@elxsi.UUCP>
Date: 7 Jun 88 14:43:56 GMT
References: <19730@beta.UUCP> <4332@killer.UUCP>
Sender: news@elxsi.UUCP
Reply-To: rw@beatnix.UUCP (Russell Williams)
Organization: ELXSI Super Computers, San Jose
Lines: 30

In article <4332@killer.UUCP> elg@killer.UUCP (Eric Green) writes:
>
>The stereotypical example of a program that does not run well under VM is the
>large scientific array processing program. Such a program marches from one end
>of a huge array to the other, and by the time the last pages are accessed, the
>first pages have already been paged out of RAM (meaning that the next pass
>results in the whole array being paged back into RAM).
>
>Without VM, the program must explicitly handle the segmentation of data, and
>can make sure that as much is kept loaded as will fit in memory. With VM, the
>problem could possibly be handled -- but hasn't been, in the past. 

   Presumeably you mean to imply that the entire array won't fit in physical
memory, but with explicit assignment of what goes and comes when,
reasonable efficiency could be achieved.  There's absolutely nothing incom-
patible between VM and explicit control over page residence.  Many
memory managers support asynchronous (without-wait) requests to touch pages
so they will be in memory when referenced later, or release pages which are
no longer needed so that they are the prime candidates for replacement by
incoming pages.  It's far more convenient (and thus efficient) to have
VM to manage large data spaces for you and provide explicit control only
when needed, than to require explicit management of data residence.  
If explicit loading and unloading of data is more than hints to an MM, 
it is another serious source of bugs.  Errors in overlay schemes result in
wrong answers, errors in MM hints only in performance degradation.
scheme

Russell Williams
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