topic /Qparallel uses only 2 Cores on an 8 Core machine in Intel® Fortran Compiler

/Qparallel uses only 2 Cores on an 8 Core machine

onkelhotte — Mon, 22 Feb 2010 08:48:28 GMT

Hi there,

my colleague is on a construction site installing one of his programs on the clients machine.

The clients machine has8 Cores. His program is being compiled with the /Qparallel option, but the program uses only 2 Coreswith 100%.

So my question is, does the /Qparallel option "only" optimizes for 2 Cores? Would an OMP approach do better?

I dont know what the clients machine really is (Intel or AMD), but I asked my colleague. Because he cannot go online very often I havent got an answer yet. He usesVisual Studio 2008 9.0.21022.8 RTM andIntel Fortan Compiler 10.1.4159.2008 (seems likethe integration version).

Thanks in advance,
Markus

/Qparallel uses only 2 Cores on an 8 Core machine

bendel_boy1 — Mon, 22 Feb 2010 11:27:24 GMT

What version of Windows?

I think thatt he NT -> XP family allowed up to two processors as standard, and you had to move to server editions or pay extrat to access more core. Vista I believe supported four cores as standard, with 'higher' levels of Vista being required to access more cores.

/Qparallel uses only 2 Cores on an 8 Core machine

onkelhotte — Mon, 22 Feb 2010 11:37:44 GMT

Quoting bendel boy

What version of Windows?

I think thatt he NT -> XP family allowed up to two processors as standard, and you had to move to server editions or pay extrat to access more core. Vista I believe supported four cores as standard, with 'higher' levels of Vista being required to access more cores.

Thanks for that input, I sent my colleague an e-mail, what OS is used there.

I keep you informed.

Markus

/Qparallel uses only 2 Cores on an 8 Core machine

onkelhotte — Mon, 22 Feb 2010 12:57:42 GMT

The machine uses 2 Xeon E7340 CPUswith 32 GB RAM and Windows 2003 Server SP2 as OS.

Markus

/Qparallel uses only 2 Cores on an 8 Core machine

TimP — Mon, 22 Feb 2010 13:04:42 GMT

/Qparallel is simply automatic application of OpenMP. Most likely reasons for running 2 threads would be that the controlling environment variable is set to 2 threads, or that the loop chosen for parallelization has count 2. In the latter case, if OpenMP collapse were suitable, it could help to increase the number of threads.

/Qparallel uses only 2 Cores on an 8 Core machine

onkelhotte — Mon, 22 Feb 2010 13:08:06 GMT

Quoting tim18

Hi Tim, thanks for the answer.

What do you mean with "controlling environment variable"? Is there some Windows Variable / Definition / Registry entry to set this?

Markus

/Qparallel uses only 2 Cores on an 8 Core machine

TimP — Mon, 22 Feb 2010 14:18:23 GMT

When OMP_NUM_THREADS is set, that should take priority. I believe the ifort online doc attempts to describe how the choice is made when OMP_NUM_THREADS is not set.

/Qparallel uses only 2 Cores on an 8 Core machine

Steven_L_Intel1 — Mon, 22 Feb 2010 14:29:52 GMT

By default, the maximum number of threads is taken from the number the processor is capable of (including Hyperthreading). It's possible that the problem size is not large enough to split across 8 cores.