What is eating swap?

[Jamon Camisso]

Fraser Campbell wrote:
> Funny that I haven't really run into this one before, hoping someone here has 
> ideas ...
> 
> I have a box with lots of free memory where swap usage is growing (slowly) 
> despite lots of free memory and swap is actively being used (according to 
> vmstat).  Here's what free has to say:
> 
>                  total       used       free     shared    buffers     cached
>     Mem:       1034640     439260     595380          0      34404      84544
>     -/+ buffers/cache:     320312     714328
>     Swap:      2097144    1793680     303464
> 
> It is RHEL4 (kernel 2.6.9-22.ELsmp 32-bit).
> 
> There is no single process whose virtual size wouldn't fit within the 700MB of 
> supposedly free memory.  Interestingly although swap use is growing, "used" 
> memory (the 320312 figure) is supposedly dropping.
> 
> Normally when a box has free memory I can coax it into increasing the 
> filesystem cache easily (find /usr -type f -exec cat '{}' \; > /dev/null), in 
> this case filesystem cache does not grow at all no matter how much disk I 
> read ... this suggests to me that there really is no free memory, or perhaps 
> just that the kernel is in a really bad state.
> 
> Is there a tool which can dig through memory to say which processes are 
> pointing at the swapspace?
> 
> Red Hat has an FAQ on this topic and they suggest looking at the swapped field 
> in top (http://kbase.redhat.com/faq/FAQ_85_9807.shtm).  As far as I can tell 
> they are completely wrong, all of the numbers from top look quite bogus (on 
> this and other machines).
> 
> My other thought was that the box was hacked and some processes hidden from 
> me ... at least so far I don't see any such indication.
> 

Try altering /proc/sys/vm/swappiness to a lower value (can only be 
between 0-100) and see if that decreases your swap usage. Won't find the 
problem, but will help troubleshoot I'd imagine.

Jamon
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