On 05/04/2011 17:11, Stephen Burke wrote:
> Testbed Support for GridPP member institutes
>> [mailto:[log in to unmask]] On Behalf Of Andrew McNab said:
>> The most you could ever deliver in total is one second per second for
>> each core, irrespective of hyperthreading, but two cores can
>> potentially
>> deliver two seconds per second in total for some types of task.
>
> Only if the code is running multiple threads, and the underlying
> assumption is that HEP code is single-threaded.
When I say "the most you ever deliver in total", the total I'm referring
to is across all jobs on that machine.
The most CPU time you can ever deliver in total to all jobs on a machine
"is one second per second for each core, irrespective of hyperthreading".
But for some extreme types of task (eg generating random numbers), two
cores can deliver two seconds per second in total to the jobs.
The point is that although sites can use hyperthreading to increase
their efficiency (by using the execution units of cores to deliver CPU
time to other jobs while one is waiting for I/O) it's not the same as
having more cores. So when we deliver a second of CPU time to a job,
it's had the exclusive use of one core's execution resources during all
the time slices that make up that second. So it's really the cores that
count.
Cheers,
Andrew
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Dr Andrew McNab, High Energy Physics, University of Manchester
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