12 Core OpenMP

General OpenMP discussion

12 Core OpenMP

Postby surgjw » Sun Oct 16, 2011 7:20 pm

Dear all,
Have used OpenMP for several years, and run a program which is SIGNIFICANTLY faster with more threads.
Example;
Language = C#, Running on Windows 32 bit OS (Windows XP), Compiled with MS VS 2008:
1. Specify thread number = 1, execution-time = 3 min 22 sec
2. Dual Core, (my laptop) specify thread number = 2, execution time = 2 min 45 sec
3. QuadCore (desktop) specify thread number = 4, execution time = 1 min 50 sec

All is good. I'll just get a multicore Workstation to go even faster (12 core)!!!!!!!!!!!
Now: same Language and Compiler, but running Windows 7 (64 bit)
1. Specify thread number = 1, execution-time = 3 min 10 sec
2. Specify thread number = 2, execution time = 2 min 25 sec
3. Specify thread number = 4, execution time = 1 min 45 sec
4. Specify thread number = 6, execution-time = 1 min 22 sec
5. Specify thread number = 8, execution time = 2 min 10 sec
6. Specify thread number = 10, execution time = 2 min 55 sec
7. Specify thread number = 12, execution time = 3 min 05 sec

Same answer for all, so no data race (I Think!!!!!)
BUT my question(s) is/are;
1. Why is the program speeded up till number of threads = cores/2 and then as further threads are added, it appears to slow back down again.
2. Is this a 64 bit issue with my compiler only compiling 32 bit versions (Win32)
3. MSVS 2008 uses OpenMP v 2.0. Should I upgrade to MSVS 2010. and does 2010 compile a Win64 bit version of the program, and does it use a later version of OpenMP (with presumably lower overhead)?

Thanks in anticipation

Surgjw

PS edj has been HUGELY helpful to two of my queries in the past, are you still there?
surgjw
 
Posts: 4
Joined: Fri Apr 23, 2010 10:48 pm

Re: 12 Core OpenMP

Postby ctierney » Fri Nov 18, 2011 10:19 pm

Have you done anything specific do manage process affinity? You didn't say what hardware you are using
but if you are on an Intel six-core (Westmere) system, then you could be running into affinity problems.

Depending on your compiler you can manage the affinity through environment variables.
ctierney
 
Posts: 1
Joined: Fri Nov 18, 2011 10:14 pm


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