[CentOS] Swap Considerations

[Phil Schaffner]

On Thu, 2007-03-01 at 23:17 +0800, Feizhou wrote:
> It comes down to whether the use of swap has a significant effect on 
> your operations. You can tell the kernel to not ever use swap unless 
> absolutely necessary: swappiness = 0. Normally it is set at 60 (range
> is 
> from 0 - 100). Let me explain how I would handle swap.
> 
> Take a case of mail servers with hundreds of processes that only have 
> 1GB of RAM and a pair of disks. I would use vmstat to check the 'si
> so' 
> columns to see whether the box is swapping. Positive figures in both 
> columns indicates use of swap and if I see constant reports of
> positive 
> figures for these two columns, I start reducing the number of
> processes 
> allowed to run to increase performance. This is how I identify active 
> swap usage as opposed to swap in use. Swap in use is not the same as 
> active swap usage. I could get maybe tens or even hundreds of
> megabytes 
> being reported in swap but if vmstat 'si so' columns report zeros, I 
> don't worry about its effect on disk i/o because there is none.
> However, 
> if the amount of swap in use continues to grow over time, I start 
> looking for memory leaks. 

This has been a very interesting thread, and given the length
(discounting a few digressions), and the general number of occurrences
of the subject of swap in the list archives, there is a high level of
interest in the topic.  My search of the Wiki turns up nothing helpful.
Sounds like a good candidate for an entry on the Wiki FAQ page:

http://wiki.centos.org/HowTos

Anyone (like Feizhou :-) who has a through grasp of the topic willing to
step up?

Cheers,
Phil