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This is the CPU usage graph of a server:

enter image description here

It seems that the CPU usage reached a hard limit at 24. This is a 32-core server so it should be able to go beyond 24 with ease. But that's not the case.

Is it possible that there's a cap on CPU usage on this server. How do I find out?

By the way, this is a database server running Percona (MySQL) Server 5.1 on CentOS 5.5

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Does top actually show eight CPU's "at rest" or just 75% load? –  Mark Mar 5 '11 at 20:13

2 Answers 2

You might be I/O or memory limited, rather than cpu limited. Your cpu won't burn cycles just to burn cycles if it's stuck waiting on disk, network, or memory. It could also be a software limitation, if you're using something that's not written to be aware of more than 24 cores.

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You have to see the CPU utilization for each CPU. I am using dstat -ta 10 to see that. Because the graph is a straight line it rules out that the load could be I/O bound. Now you have to see why MySQL is not using all CPUs. See also: http://dev.mysql.com/doc/refman/5.5/en/connection-threads.html

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