How is CPU usage calculated?

[Martin Turgeon]

  Le 2010-10-18 15:12, Adam Vande More a écrit :
> On Mon, Oct 18, 2010 at 1:54 PM, Martin Turgeon
> <freebsd at optiksecurite.com>wrote:
>
>> I didn't knew about -H to show individual threads, but my problem isn't
>> there. My problem is that the summary printed in the first lines doesn't
>> match the total of the process detailled under the summary. For example, I
>> have a user CPU usage est 83.1% in the summary, but the only process worth
>> mentionning in the list is mysqld that is taking 255.52% WCPU. That's far
>> less than half the "CPU power" but 83% is far more. It's that difference
>> that I don't understand.
>>
>> How is this possible?
>>
> man 1 top
>
> 83.1% doesn't mean what you think it does.
>
I just reread it and it isn't clearer what is using the CPU so much. Can 
you please give me a little more explanation?