Re: [OM] Missing in action . . . and OT Mac question

Subject: Re: [OM] Missing in action . . . and OT Mac question
From: David Thatcher <davidt@xxxxxxxxxxxxxxxxxxx>
Date: Sat, 7 Apr 2012 19:22:41 +0930
On Fri, Apr 06, 2012 at 09:22:31PM +0100, Chris Barker wrote:
> I gather that the limitations initially placed on computers have been
> made void by improvements in the RAM.  But I might have got the wrong
> end of the stick there . . .
> Chris

To comment on this general non-OS-specific statement: Yes, more RAM is
better - up to a point. Modern operating systems use a 'protected paged
virtual' memory model. As applications start up, they request some
pages/blocks of memory from the operating system, can ask for more as
they need it, and -if the OS & app are properly written- can return it
to the free pool when they are finished with it (if not done properly it
is one form of 'memory leak' and why some systems need a 'maintenance
reboot' periodically (pronounced 'often' or 'daily' :) ).

If there is no free ram when an application asks for it, the OS will
make decisions about what applications are idle & will 'swap' those
applications' memory pages out into a hard disk 'swapfile'. When swapped
pages are required for use (the application wakes up for some reason),
they are swapped back in, and other pages are swapped out.  All of this
swapping in/out takes enormous amounts of time, as disk input/output is
thousands to million times slower than memory accesses -  and if swapping
is happening a lot, the machine seems to grind to a halt.

Basically by stuffing your machine full of RAM, you are minimising the
swapping, and thus the machine appears to run faster - until you run out
of free RAM again!


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