How is CPU usage calculated?

[Bas Smeelen]

From: Martin Turgeon [mailto:freebsd at optiksecurite.com]
To: freebsd-questions at freebsd.org
Sent: Thu, 14 Oct 2010 21:46:13 +0200
Subject: How is CPU usage calculated?

  Hi list!
  
  I did a strange observation yesterday night. The CPU usage reported by 
  top doesn't match what is indicated under it. I was seeing around 80-90% 
  user, 5% system, 1% interrupt and 10% idle. But the process details 
  under it doesn't match. mysqld was taking around 250% (WCPU) with a few 
  httpd processes at 1-2%. The system is running GENERIC 8.0-REL on a Xeon 
  E5630 (quad core with hyperthreading so 8 CPUs). MySQL and Apache each 
  have their own jail (I don't think it will matter but just in case)
  
  I understand why the mysqld process take more than 100% but how can I 
  know what's taking the rest? Why doesn't the total user CPU usage match 
  the total of the CPU usage of each process? Is there a link with 
  hyperthreading?
  
  There sure is a logical answer and I would really like to know it :)
Yes if you have multiple cpu's, cores or hyperthreading than each unit can be used up to 100%
Mysql has multiple threads, you can check with top -P to see the multiple cores and when top is running give the command H (capital H) to see the threads of each process    


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