Subj : Re: virtual addresses
To   : comp.programming
From : Randy Howard
Date : Wed Sep 28 2005 06:24 pm

Brian wrote
(in article <dheh0s$ea2$1@gondor.sdsu.edu>):

>>> So your saying that it's always better to map virtual memory
>>> to magnetic media versus system memory?  I can't see how that
>>> would ever be true.
> 
>> I can't see how you came to that conclusion.  It's in fact the 
>> last thing I would have expected you to take away from the 
>> discussion.
> 
> Are you not arguing that disk based virtual memory is required?

No.  I'm arguing that it is a good thing, because you can not 
always predict/guarantee that it won't be needed.  I've said it 
several different ways previously, I won't try again.

> I thougt you were saying physical RAM will always be too little.

It can't be guaranteed to be sufficient for arbitrary usage. 

> If not, I'll have to admit your argument is elusive.

I'm sorry you're having difficulty with it, but it seems pretty 
straightforward from here.

> I contend, 
> as an example, that an XP system with 512MB of memory is useful.

As useful as most other XP systems, for whatever that is worth. 
:-)

> With 2048MB of memory, it should be just as useful 

"Useful" and not running out of memory aren't equivalent.  If 
you need more memory (i.e. you are hitting the page file) with 
512MB physical, than it is more "useful" to have 2GB.  It all 
depends upon what you are doing at a given point in time.

> I can't see the controversy in this at all.  

Neither can I, which is why I don't understand your "problem" 
with virtual memory.  Beating the hell out of the disk due to 
swapping is a bad thing, having it available when needed is not. 
 If you disagree, please indicate why you feel that way.

>>> The entire question boils down to this.  What type of memory
>>> is faster?  Is there enough of it?
> 
>> We definitely are concerned about two different questions, 
>> because I don't think that's even a question worth asking.  The 
>> answer has been back on that one for decades.
> 
> Exactly.  So why are we using disks for system memory?

For boundary cases, where the user needs more memory, even if 
only for a few minutes, than available on the system.  Without 
it, you either get "Sorry, you're out of memory, go away" 
messages, or you crash, or perhaps both if you're running a 
crappy OS.  If you personally want to live on the edge, turn off 
paging and see how it goes for you.

-- 
Randy Howard (2reply remove FOOBAR)

.