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From: stubbs@ncr-sd.UUCP (Jan Stubbs)
Newsgroups: net.arch
Subject: Re: Very large memories
Message-ID: <1172@ncr-sd.UUCP>
Date: Tue, 16-Sep-86 16:53:53 EDT
Article-I.D.: ncr-sd.1172
Posted: Tue Sep 16 16:53:53 1986
Date-Received: Tue, 16-Sep-86 22:04:38 EDT
Reply-To: stubbs@ncr-sd.UUCP (0000-Jan Stubbs)
Organization: NCR Corporation, San Diego
Lines: 45

In article <2383@peora.UUCP> joel@peora.UUCP (Joel Upchurch) writes:
>
>>Jan Stubbs    ....sdcsvax!ncr-sd!stubbs
>>3) The real reason for virtual memory (and the one which won't
>>away when memories get big) is that I can quickly load the
>>working set of a large program, execute it, and go on to the next
>>program, while a real memory system might be still loading its program.
>>
>>In fact, I can start executing when the first page gets in, on
>>a properly implemented system where everything is already
>>in paging format.
>>
>>This assumes that the working set is smaller than the program,
>>and that the programmer didn't do some work to only load those
>>pieces of program and data which were needed this time.
>
>	I may be missing something important, but this doesn't
>	sound right to me. I followed what Jan said about starting
>	up faster (more about that later), but it seems to me that
>	you'll use up that and a lot more in page faults. Since you
>	are reading the program piecemeal into virtual memory you
>	are going to be a lot slower because of the extra seek and
>	rotational delays. It is sort of like a guy driving on a
>	surface street, instead of going over and getting on the
>	freeway. You get started faster, but you hit all those
>	traffic lights. The only case that is valid is if the
>	overwhelming majority of the pages in the program are
>	never referenced.

Exactly, and I contend that this is often the case. Take an ADA
compiler for example, maybe most programs won't use but a small
subset of the constructs available, then the code to handle
those constructs won't need to be paged in. Your freeway analogy
is perfect - most trips are short, so take the direct route
by surface street.
>
>	Another consideration, is that on at least some virtual
>	memory systems I've worked with, the the system transfers
>	the program over to some sort of paging file, before it
>	starts, which means that you end up reading the whole
>	program in anyway, and writing it out to boot.

Yes, but maybe you only load the page file once (kind of like
setting the sticky bit in a UNIX swap file). 
>