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From: blm@cxsea.UUCP (Brian Matthews)
Newsgroups: comp.sys.mac
Subject: Re: Virtual Memory INIT
Message-ID: <2598@cxsea.UUCP>
Date: 19 Jan 89 22:08:27 GMT
References: <8901151644.AA17944@Portia.stanford.edu> <870@mailrus.cc.umich.edu>
Reply-To: blm@cxsea.UUCP (Brian Matthews)
Organization: Computer X Inc.
Lines: 40

Shane Looker (shane@chablis.cc.umich.edu) writes:
|In article <8901151644.AA17944@Portia.stanford.edu> name@PORTIA.STANFORD.EDU (tony cooper) writes:
|> What would you do with all that
|>much memory? I guess you could use it as a RAM disk!?!
|
|I sure hope you are joking about this...

I don't know if Tony was joking or not, but I would certainly consider
using the extra memory as a RAM disk.  In fact, I have a paged RAM disk
on my Unix machine.

Why is this not as silly as it sounds?  Because an application usually
doesn't use all of your physical memory, you generally only use a small
part of the RAM disk (within a small amount of time, you may fill the
thing up eventually), and even if the application is using all of
physical memory (or more), it isn't touching all of those pages very
often.

Say that you have 2 Meg of physical memory, and the application you
happen to be running only uses 1.5 Meg.  Then you've got .5 Meg for the
RAM disk to be paged into, even if the application is touching all of its
memory.  Maybe 1 Meg of the applications memory can be paged out.  Unless
you're dealing with some large files, chances are your data will fit in
the 1.5 Meg, and run at RAM disk speed.  If the application snarfs some
more memory, the least recently used pages of the RAM disk will get paged
out.  In the worst case, when the application grows larger than physical
memory, the RAM disk will work like a normal drive.  But even in this
case, the chances are pretty high that some pages owned by the
application aren't being touched, and will get paged out for the RAM
disk.

Basically the RAM disk gives you a cache that uses whatever unused memory
is available, and in the worst case acts like a normal disk drive.  And
consider that this worst case only happens when the application owns more
memory than is available in physical memory, and is touching all of these
pages fairly often.  A paged RAM disk can be a smart way to go.

-- 
Brian L. Matthews  blm@cxsea.UUCP   ...{mnetor,uw-beaver!ssc-vax}!cxsea!blm
+1 206 251 6811    Computer X Inc. - a division of Motorola New Enterprises