interpreting top

[Dan Ritter]

On Fri, May 14, 2010 at 03:47:27PM -0400, Doug wrote:
> Hello:
> 
> I work on a big machine at work.  Here is the command top:
> 
> 
> top - 15:36:46 up 80 days,  3:14,  4 users,  load average: 0.00, 0.00, 0.00
> Tasks:  89 total,   2 running,  87 sleeping,   0 stopped,   0 zombie
> Cpu(s):  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0
> Mem:  16440256k total, 16280920k used,   159336k free,    67484k buffers
> Swap:  7807580k total,    83564k used,  7724016k free, 15646216k cached
> 
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>     1 root      15   0  5076  152   68 S    0  0.0   0:02.49 init
>     2 root      RT   0     0    0    0 S    0  0.0   0:00.01 migration/0
>     3 root      34  19     0    0    0 S    0  0.0   0:00.66 ksoftirqd/0
>     4 root      RT   0     0    0    0 S    0  0.0   0:00.00 watchdog/0
>     5 root      RT   0     0    0    0 S    0  0.0   0:00.02 migration/1
> 
> Lots of sleeping is happening.  Nothing has been going on for a while
> (load average: 0.00, 0.00, 0.00)  One thing I found curious is the
> line started Mem: there still is a large value under used.  I can see
> that some swap was used (when, I do not know).  I guess the rational
> explanation is that nearly all of this is cache memory.  I guess linux
> as an OS is passive about tossing this stuff away.  It is hard to take
> the Mem: line as a warning marker if it remains so high when times are
> calm.  Can I/should I do anything?


15646216k cached

That's what most of your RAM is doing right now. It's disk
cache. And it's probably all in sync with the disk, since it's
been so long since anything interesting happened on the system.

I would diagnose this machine as underutilized.

There is absolutely nothing to worry about right now. The kernel will
discard clean disk cache space in favor of running applications as soon
as that's needed. It's normal to have a small amount of swap being used,
too -- that's stuff which the kernel thinks is quite likely to never be
called up at all.

If you're concerned about eventual performance, install systat/sar and
learn to read the reports that generates.

-dsr-

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