Optimal # of threads for multi multi-core processors?

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Optimal # of threads for multi multi-core processors?

Postby ntt2011 » Mon Dec 31, 2012 2:58 am

Dear all,

I have a beginner question regarding openmp on a multi multi-core processors machine.
I'm buying a 4-processor mainboard with AMD SR5690/SR5670+SP5100 Chipset and
plan to put the Opteron 6200 12-cores on each CPU sockets (totally 48 cores).
I noticed on the manufacturer's website that there are 8 RAM banks near each CPU socket.
I don't know the RAM belong to only that processor, or all 32 RAM banks are shared.

I used openmp on a 1 multi-core processor before, but don't know how it is with multi multi-core processors.
My question is, for optimal implementaion, should I use openmp with 12 threads maximum, or I can use
all 48 threads for openmp? If I set omp_set_num_threads(12), is it guaranteed that all 12 threads
will run on a single CPU (to have better memory access?).

My simulation program is small scale, I could run 4 simulations, each uses 12 threads to get 4 data points, or
I could run 1 simulation with 48 threads to get 1 data point in 1/4 of time. Do I need to adjust my openmp
code for later case?

Thanks
Toan
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Re: Optimal # of threads for multi multi-core processors?

Postby Oldboy » Wed Jan 02, 2013 8:25 am

You have an example on SPEC here:

http://www.spec.org/omp2012/results/res ... 00007.html

The only new Opteron I have found. You will find more Opterons in SPEC2006fp rate but I guess you are not interested in servers typical for Internet.
One thread/core looks to be the best for Opteron. One test I have done is 7-zip benchmark (free to download) with compress and decompress of files.
One thread/core will result in the best total perf. but in fact 2 threads/core gives higher compress rate.
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Re: Optimal # of threads for multi multi-core processors?

Postby ntt2011 » Sat Jan 05, 2013 9:26 pm

My question is actually whether or not, this system has distribution memory or shared memory :D. I think OpenMP can be used for share memory only.
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Re: Optimal # of threads for multi multi-core processors?

Postby ftinetti » Mon Jan 07, 2013 4:58 am

Hi,

My question is actually whether or not, this system has distribution memory or shared memory

As far as I know, this is a shared memory system with NUMA. It is easily verified if you actually have the computer: if you see all the installed RAM reported by free or a similar command, then you have a shared memory computer.

I think OpenMP can be used for share memory only.

You are right.

HTH,

Fernando.
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