How much swap?

[Gordon Messmer]

Uno Engborg wrote:
> 
> Would I need a swap anyway.  There is some old rule of thumb to have
>  twice as much virtual memory as you have physical RAM, but that
> sounds a bit ridiculous  as it would take a lot of time to swap
> in/out this much memory from disk.

I used to think it was kind of ridiculous, myself, until I discovered a 
quirk of Posix's behavior.  If I get any of these details wrong, I 
invite corrections:

When a process called fork() on an old unix system, the OS required an 
amount of free memory equal to the size of the process, plus the size of 
a process table entry.  If this memory wasn't free, fork() would fail. 
After fork(), the OS would copy the full set of memory from the parent 
process to the new process.  Since fork() is so often followed by 
exec(), which throws away all of that memory, modern unix systems don't 
copy the whole set of memory when a process forks.  However, they still 
require that there is enough memory to do so (at least normally; Linux 
has an "overcommit" feature that you can enable).

With 8GB of RAM, that may not affect you directly, but it may, too. 
Lets say that you have no swap, and you were using a 3d modeler, or 
high-end graphics package.  If that application was using 5GB of your 8, 
and needed to launch a helper application, like an out-of-process perl 
or python script, it wouldn't be able to do so.  Even though that script 
only needs a few MB of the GB you have free, the parent is too big to 
fork(), so it can't spawn new processes.

So, when you decide whether or not to follow the traditional advice 
offered about the amount of swap to allocate, you should first 
understand how the Linux VM works:
http://www.redhat.com/magazine/001nov04/features/vm/

...if you don't want to spend the time learning the details, I'd just 
stick with the advice offered.