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cpu load and utilization correlation

Kenan Erdey
Honored Contributor

cpu load and utilization correlation

Hi all,

i am trying to understand what cpu utilization and load means and correlation between them. i've read many articles about them. but cannot got exactly anything.

firstly i thought, up to %100 cpu utilization and load avarage 1, they are same. after load avarage 1 they differs because utilization can't get over %100. but i investigate some servers and realised that over %65-70 utilization, cpu load goes over 1.

i also read this artice that explains that %100 cpu utilization is not perfect situation ( we get all from cpu as much as we pay)

http://hoopercharles.wordpress.com/2010/02/05/faulty-quotes-6-cpu-utilization/

what do you think ? is there any correlation according to calculation methods, anything consider ?

thanks. any nice answer will be appreciated with points.




Computers have lots of memory but no imagination
9 REPLIES
Dennis Handly
Acclaimed Contributor

Re: cpu load and utilization correlation

CPU usage is better than load average.
The latter just measures how processes are being started and stopped and short lived processes can really make it large.

Of course, you need to really measure more than CPU, glance can measure, CPU, disk I/O, networking and memory usage, and much more.
Hakki Aydin Ucar
Honored Contributor

Re: cpu load and utilization correlation

>Kenan: http://hoopercharles.wordpress.com/2010/02/05/faulty-quotes-6-cpu-utilization/

This page appears broken I am afraid.

I am agree with Dennis.As addition:

When it comes to CPU Load; CPU queue metrics, run queue and the load average together make sense of the CPU load;
-The run queue is the current instantaneous number of processes waiting to use the CPU
-The load average, shows 1-minute, 5-minute, and 15-minute averages of CPU use.
Kenan Erdey
Honored Contributor

Re: cpu load and utilization correlation

page is working for me. could you paste it to browser ?

Computers have lots of memory but no imagination
Hakki Aydin Ucar
Honored Contributor

Re: cpu load and utilization correlation

>Kenan: page is working for me. could you paste it to browser ?

Kenan, it is working now. But it did not work by the time of my post.

By the way, I would like to add about one word glance quoted from HP-UX Performance & Tuning book; Glance is the best tool for obtaining CPU usage information because it uses more accurate data collected by the MI.(measurement process, midaemon)
Kenan Erdey
Honored Contributor

Re: cpu load and utilization correlation

Hi,

@Dennis, can you explain a bit more detail ?

> The latter just measures how processes are being started and stopped and short lived processes can really make it large.

in what circumstances load and utilization over a time can be different ?
Computers have lots of memory but no imagination
Dennis Handly
Acclaimed Contributor

Re: cpu load and utilization correlation

>in what circumstances load and utilization over a time can be different?

Suppose you have a lot of short term processes or processes that are continually waiting for I/O. This will cause the load average to go up even if the CPU utilization isn't high.
Kenan Erdey
Honored Contributor

Re: cpu load and utilization correlation

to be in the run queue, must a process be in runnable state ? if a process is waiting for io, it should be in sleep state. am i wrong ? so it shouldn't increase load average.
Computers have lots of memory but no imagination
Hakki Aydin Ucar
Honored Contributor

Re: cpu load and utilization correlation

I believe they are different;

Run Queue:A process must be on a queue of runnable processes before the scheduler can choose it to run.
http://docs.hp.com/en/5965-4642/ch01s14.html

Sleeping a process:A process may have to wait for a resource or event that is currently not available. It makes a sleep system call that puts the process on a queue of sleeping processes.

So run queue , process did not go to scheduler to run yet, mean you cannot see it in the list (with command top for example)
Dennis Handly
Acclaimed Contributor

Re: cpu load and utilization correlation

>it should be in sleep state. am I wrong? so it shouldn't increase load average.

I was thinking it quickly finished the I/O and then went back into the run queue.