check_load --- divide by number of cpus?

[Thomas Guyot-Sionnest]

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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.

Thomas
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