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On 06/04/2011 12:28, John Gordon wrote: > > Andrew, if you look at the HEPiX Benchmark site http://w3.hepix.org/benchmarks/doku.php?id=bench:results_sl5_x86_64_gcc_412 you will see multiple examples of the same chip being benchmarked with one job/core HT off and 2 jobs/core HT on. The latter is typically 20-25% bigger in HS06/chip. Obviously you get less HS06/logical cpu but it looks like there will be a benefit in total throughput. > > You are obviously right about the theoretical seconds/core but HEP codes and the HS06 benchmark are not 100% efficient so doubling up helps. I guess there is also some parallel capability in even a single core. Yes to all of the above. I'm not arguing against hyperthreading - just saying it's not true that "hyperthreaded CPUs and cores are effectively the same thing". There's a further complication that the "CPU seconds used" reported by the operating system _includes_ the idle times when one hyperthread is waiting for access to the shared execution units etc (ie due to the internal queue within each core) even though that time doesn't actually correspond to instructions being carried out by that hyperthread. Andrew > -----Original Message----- > From: Testbed Support for GridPP member institutes [mailto:[log in to unmask]] On Behalf Of Andrew McNab > Sent: 06 April 2011 11:21 > To: [log in to unmask] > Subject: Re: HEPSPEC06 numbers for GridPP metrics > > 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 > > -------------------------------------------------------------- > Dr Andrew McNab, High Energy Physics, University of Manchester -- Cheers, Andrew -------------------------------------------------------------- Dr Andrew McNab, High Energy Physics, University of Manchester