How much swap?

[Robert Brockway]

On Wed, 25 Oct 2006, Ian Petersen wrote:

> Chances are, there are a bunch of processes on your system that spend
> most of their time idling.  If you have no swap, these idle processes
> _must_ reside in real RAM, which eats away at the amount of RAM that
> can be used for caching and buffering.  On the other hand, if you
> allow the kernel to swap out mostly-idle processes, it has more room
> for these speed-enhancing uses of physical memory.

Hi Ian.  You took the words right out of my mouth.  Ram being cheap is not 
sufficient reason not to put swap in the system for exactly the reason 
that you mention.

Allowing unused apps to swap and thus allowing the system to reclaim the 
memory and use it for cache and buffering is an overall win for 
performance, even after factoring in the need to swap apps back in from 
time to time.

As for the original poster's query - the old "swap must equal 2x ram" 
formula was derived from the swapping algorithms used on some versions of 
Unix in the past.  This is not and has never been necessary on Linux[1].

Basically the amount of ram+swap should be higher than you would 
reasonably expect the system to demand.  On systems with lots of ram (say 
4GB) I'll throw in a few GB of swap (disk is cheap too).  Watch out for 
runaway processes that gobble lots of memory - having swap can prevent the 
OOM killer[2] coming in to play and the OOM killer is nobody's friend.

On a side note I highly recommend setting per process limits.  See 
limits.conf and ulimit.

[1] Actually there was one kernel which had a bug which meant that if swap 
was not at least 2x ram the system would not swap at all.  This was 
quickly fixed and was entirely unintentional.

[2] Out Of Memory killer.  ROARRR!!!!

Rob

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