check_load --- divide by number of cpus?

[Mike Emigh]

On Fri, May 9, 2008 at 4:48 AM, Thomas Guyot-Sionnest <dermoth at aei.ca> wrote:
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> On 08/05/08 10:31 AM, Mike Emigh wrote:
>> On Thu, May 8, 2008 at 9:49 AM, Terry <td3201 at gmail.com> wrote:
>>> I am sitting here racking my brain on this one.  Is dividing the load
>>> average by the number of CPUs a smart thing to do?  the 'uptime'
>>> command on a box does not do this.  So, if I have an 8 core box
>>> sitting with a load average of 8, its the same as a single core box
>>> sitting with a load average of 1?  I'll see the same type of server
>>> response?  Thoughts?
>>>
>>
>> It's not necessarily the same.  If you have a single-threaded process
>> pegging one of the cores, your load average can hit 8 even if the 7
>> other cores are sitting idle.
>
> Not possible. The definition of load average is simply "the number of
> processes in the run queue". If you have only one process running (no
> multi-threading) it can only run on a single CPU at any given time, so
> the run queue can only be 1. A multi-threaded application will, on the
> other hand, be able to run on multiple CPUs (obviously depending on its
> design) and cause higher loads.
>
> When comparing load averages between servers you should divide it by CPU
> because the more CPUs you have, the faster the run queue is processed.
> Think of it like a reservoir with pipes: if you have one with 8 pipes,
> and another with only one, the 8-pipe reservoir will be able to take 8
> time as much water and still be able to empty it as fast as the one-pipe
> one.
>

You're right, it wouldn't work with just a single-threaded process.
I'm not sure of the specifics of how this would happen, but with
Oracle we've seen it maxing two CPUs and raising the load to 56 while
the 6 other cores in the 8 core machine sat idle.  As these situations
are possible simply dividing by the number of cores wouldn't provide
precise insight into what's going on.

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