On Sat, Feb 28, 2015 at 1:02 AM, Dave Angel <da...@davea.name> wrote:
> The term "virtual memory" is used for many aspects of the modern memory
> architecture. But I presume you're using it in the sense of "running in a
> swapfile" as opposed to running in physical RAM.
Given that this started with a quote about "you can't fake what you
ain't got", I would say that, yes, this refers to using hard disk to
provide more RAM.
If you're trying to use the pagefile/swapfile as if it's more memory
("I have 256MB of memory, but 10GB of swap space, so that's 10GB of
memory!"), then yes, these performance considerations are huge. But
suppose you need to run a program that's larger than your available
RAM. On MS-DOS, sometimes you'd need to work with program overlays (a
concept borrowed from older systems, but ones that I never worked on,
so I'm going back no further than DOS here). You get a *massive*
complexity hit the instant you start using them, whether your program
would have been able to fit into memory on some systems or not. Just
making it possible to have only part of your code in memory places
demands on your code that you, the programmer, have to think about.
With virtual memory, though, you just write your code as if it's all
in memory, and some of it may, at some times, be on disk. Less code to
debug = less time spent debugging. The performance question is largely
immaterial (you'll be using the disk either way), but the savings on
complexity are tremendous. And then when you do find yourself running
on a system with enough RAM? No code changes needed, and full
performance. That's where virtual memory shines.
It's funny how the world changes, though. Back in the 90s, virtual
memory was the key. No home computer ever had enough RAM. Today? A
home-grade PC could easily have 16GB... and chances are you don't need
all of that. So we go for the opposite optimization: disk caching.
Apart from when I rebuild my "Audio-Only Frozen" project [1] and the
caches get completely blasted through, heaps and heaps of my work can
be done inside the disk cache. Hey, Sikorsky, got any files anywhere
on the hard disk matching *Pastel*.iso case insensitively? *chug chug
chug* Nope. Okay. Sikorsky, got any files matching *Pas5*.iso case
insensitively? *zip* Yeah, here it is. I didn't tell the first search
to hold all that file system data in memory; the hard drive controller
managed it all for me, and I got the performance benefit. Same as the
above: the main benefit is that this sort of thing requires zero
application code complexity. It's all done in a perfectly generic way
at a lower level.
ChrisA
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