How to debug high system load?

[Bryn M. Reeves]

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On 07/12/2012 02:36 PM, Steven Stern wrote:
> On 07/12/2012 06:00 AM, Suvayu Ali wrote: rottled down and are at
> about 1%.
>> 
>> So my question is since CPU usage is already so low, what could
>> be driving up the system load? This is bugging me since I also
>> see a general sluggishness compared to a much faster system early
>> this year.
>> 
>> Thanks for any ideas.
>> 
> 
> Are you listening to music?  Doing background uploads or
> downloads?
> 

That would normally also be reflected to some extent in CPU usage and
process activity as shown in top (unless the problem was buffering
them from a really slow network file system or something).

I would think Andrew is correct - the load is most likely being driven
up by processes that are waiting on I/O (D-state, aka uninterruptible
sleep).

On Linux the load average represents the 1m, 5m and 15m decaying
average of the number of processes that are either runnable (waiting
on a runqueue or actually executing on the CPU) /and/ the number of
processes blocked on I/O.

There's a fair bit of inaccurate information about the definition of
the load average on Linux floating around - this comes from the fact
that it's calculated slightly differently on the Sys V UNIXes and BSD
vs. Linux. Traditionally only the runqueue length (tasks running or
waiting to be scheduled) are counted as "active".

On Linux the "number of active processes" includes both tasks on the
runqueue and tasks blocked on I/O; this means it's hard to compare the
numbers meaningfully between BSD/Solaris and Linux even when running
on the same hardware.

Even well-known Linux magazines have managed to print entire articles
that got this completely wrong in the past..

Regards,
Bryn.
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