How much swap?

[Lennart Sorensen]

On Wed, Oct 25, 2006 at 03:09:57PM -0400, Robert Brockway wrote:
> 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].

I believe 2.4.0-2.4.10 may in fact have had such a requirement.  Or at
least swap > ram had to be the case if you had any swap, since I think
its VM did in fact back all ram with swap at all times.  I may have
misunderstood it at the time though.  I do remember the VM in those
versions really didn't work very well and a complete replacement was put
in for later 2.4 kernels which was a major improvement.

> 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!!!!

Yes, randomly killing a process does not seem like the best move.

--
Len Sorensen
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