Load

Michael Torrie torriem at gmail.com
Tue Sep 30 17:09:21 MDT 2008


Andrew Jorgensen wrote:
> On Tue, Sep 30, 2008 at 4:54 PM, Michael Torrie <torriem at gmail.com> wrote:
>> When the load is high, the first thing to check is the amount of swap
>> being used.  If the swap is exceeding 50%, I've found that Linux tends
>> to thrash a lot.  In this case adding RAM will help, although if this is
>> not a normal server condition, maybe you have a process with a memory leak.
> 
> We suspected this, of course, but it appears that swap is not in use.
> SwapTotal == SwapFree means what I think it does, doesn't it?

Well on my machine, /proc/meminfo says:

SwapTotal:     2096472 kB
SwapFree:      1607952 kB

So I'm only using about 25% of my swap space (I have 2 GB of ram).  If
the SwapFree got below 1GB, I'd expect to see some major thrashing.

> 
>> If not, then check your disk i/o stats.  If the disk is maxed out, or
>> even in action the majority of the time, then your load average is due
>> to disk wait, which is a hard one to solve.  Sometimes a better i/o
>> scheduler can improve things.  Sometimes faster disks.
> 
> How do we check these stats?

A utility called iotop may help here:

http://guichaz.free.fr/iotop/

> What tools can I use to see if network is bottlenecking here?  I doubt
> that's it but if it's not disk IO then I do need to check there.

I'm not really sure.  I think there was a program called ntop, which
gave bandwidth for each process.  But that's not the entire story.  A
network can be nearly idle but still waiting on NFS, which causes load
average because of the way the kernel treats NFS operations.  On the
other hand tons and tons of connections to a service, say httpd, can
drive up the load in a hurry.  This would likely be represented by
saturated bandwidth, though, which ntop would show.

> Can it also be caused, for instance, by an apache thread waiting for
> mysql to respond?  If so then how do I tell if that's the case?

I believe that can certainly influence the load, yes.  Tuning apache and
mysql are not things I am familiar with.  I deal mainly in file servers.



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