[Gllug] Linux - big not small

[Ian Northeast]

John Winters wrote:

> I find that no matter how much RAM there is, if the system is up for a
> decent length of time than a little bit of swap will get used.  

Well this is probably a bit of an extreme example. It's an IBM blade 
which was purchased for a project which then decided it didn't want it, 
so when I asked for some machines to replace my 6 year old RS/6000 AIX 
based DNS servers I got a couple of these, as they were there:

sctd09 at bb02lp001:/home/sctd09> free
              total       used       free     shared    buffers     cached
Mem:       8312360     749292    7563068          0     291356     366564
-/+ buffers/cache:      91372    8220988
Swap:      1048568          0    1048568
sctd09 at bb02lp001:/home/sctd09> uptime
   8:43pm  up 51 days  9:19,  1 user,  load average: 0.00, 0.00, 0.00

So it's not swapped in nearly 2 months, but it hasn't even touched the 
sides of the available memory (or CPU come to that; during the working 
day the run queue can hit whole percents:). This is serving around 6000 
clients BTW.

Kernel is 2.6.5 but it's SuSE Enterprise so it's been "lightly modified".

And here's a web server for a largish company:

sctd09 at ihqx04a1:~> free
              total       used       free     shared    buffers     cached
Mem:       2586484    1990384     596100          0     342556    1448884
-/+ buffers/cache:     198944    2387540
Swap:      1048568          0    1048568
sctd09 at ihqx04a1:~> uptime
  21:22:43 up 329 days, 10:23,  2 users,  load average: 0.00, 0.00, 0.00

Again, out of hours, the US are just finishing and Australia havn't 
quite started yet. A bad time to look at resource utilisation. Also it's 
got an Akamai edge cache over it which significantly reduces its CPU 
consumption.


> This is
> a Good Thing(TM).  Without the swap, chunks of RAM would be used to
> store code and data which is never ever used.  By having the swap, this
> stuff can be moved out and the RAM can be used for more constructive
> purposes - even if only as a disk cache.  This is true no matter how
> much RAM you have.

I've only seen this happen to a significant degree on servers running
Orrible. Nothing else I use behaves so inefficently as to allocate
large amounts of memory then not use it.

Regards, Ian




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