On 2008-07-23, David L. Johnson <david.johnson@lehigh.edu> wrote:
> Davide Bianchi wrote:
>> On 2008-07-22, howa <howachen@gmail.com> wrote:
>>> 1. I have 4GB memory, people used to suggest I should set around 2GB
>>> to 4GB for swap partition.
>> 
>> If your system uses swap, it means that it lack real ram. Theoretically,
>> swap should not be used at all.
>
> Not really true.  It can safely swap out processes that are sitting 
> idle, taking up ram but not doing something at the moment, like that 
> browser window you have open while you are reading your news.  That way, 
> it can sometimes survive an overloaded situation.

At today's RAM prices, it's generally a good idea to buy
more RAM if you find your system paging to any significant
extent.  However, the above is true that idle processes can
be paged out without much harm.  Also, in many situations
some parts of active processes can be paged out without
causing serious harm.

For example, during the 1990s, I was working on schematic
formal verification of the Pentium Pro and Pentium 4
processors, using software based on binary decision diagrams
on IBM AIX workstations.  In those days, 192MB of RAM was
considered pretty big.  My experience was that the knee of
the curve was at a point where just about half of the active
process could be paged out.  If a little less than about
half of the active process was paged out, the job would
still make good progress at a pretty good fraction of full
speed.  If a little more than about half of the active
process was paged out, actual progress would slow to a
crawl.  In earlier days, that condition was known as
thrashing.

-- 
Robert Riches
spamtrap42@verizon.net
(Yes, that is one of my email addresses.)