Help with CPU Check Thresholds

[Frost, Mark {BIS}]

From: nap [mailto:naparuba at gmail.com]
Sent: Tuesday, April 02, 2013 2:48 AM
To: Nagios Users List
Subject: Re: [Nagios-users] Help with CPU Check Thresholds


On Mon, Apr 1, 2013 at 2:05 PM, Scott Wilkerson <swilkerson at nagios.com<mailto:swilkerson at nagios.com>> wrote:
Alex,

It would be very common for a machine with multiple CPU's to have a
higher load.  On a 16 CPU machine, no processes would be waiting at all
with a load under 16.

Hi,
Yes if all the load is consumed by CPUs, but it's quite rare. For example with a database the most time is spend on disks I/O, and a 16cpu server won't help here, you can be overload with a load average > 1 in this case.
The "good" load average is very specific for each server/application. CPU number is a just a part of the "load" equation.

Jean

I think that's rather a misunderstanding of what load average is.   It is not based on the CPU (otherwise it would be called something stupid like CPU % on Windows).  There's lots of resources on the Googles that explain what goes into the load average calculation so I won't go into it here, but it is intended to be a general number indicating the load of the system and i/o wait times are definitely a factor in that calculation.   It is intended to be a representation of how busy a machine is overall.   The general rule is as Scott indicates that for a machine with 'n' CPUs (cores, kind of as well) if the load average is also 'n' then that machine is 100% utilized.  That is, it is completely able to keep up with what it's being asked to do.   If the load average is '2n' then it's 200% utilized and the machine is effectively getting behind.

But anyway, there are some really good write-ups as to how the load average calculation is made, but it's definitely not just CPU.    CPU utilization by itself is pretty useless nowadays in my opinion.

Mark
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