How is CPU usage calculated?

[Lowell Gilbert]

Martin Turgeon <freebsd at optiksecurite.com> writes:

>  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?

They are calculated over different time periods, averaged in different
ways (that's they "weighted" part), and processes only show up until
they exit.