Speed up issues with 4 threads on quadcore system

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Speed up issues with 4 threads on quadcore system

Postby parmeet » Fri Sep 14, 2012 4:37 am

Hi all,

I have a speed up problem using 4 threads on quadcore system. With 2 threads the efficiency is close to 1 but with 4 threads it reduces to half that is the running time is more or less same as when running the code using 2 threads. I searched this forum and i find similar issue before which is because of Inter turbo boost technology. Please refer to this post viewtopic.php?f=3&t=1289&start=0&hilit=intel+turbo+boost

So i have tried to disable turbo boost on all the 4 processors of my machine but couldn't get rid of the problem.

I took the benchmark code from above link only.

I have a DELL laptop and my harware/OS information summary is as follows:

OS : Linux3.0.0.12-generic , Ubuntu
KDE SC Version : 4.7.1

Processor: Intel(R) Core(TM) i7-2620M CPU @ 2.70GHz

Please let me know what could be other possible problems that is not allowing me to speed up using 4 threads/cores. As an additional info. i have checked that all the 4 threads are running on different cores.

Looking forward to your answers.

Parmeet
parmeet
 
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Re: Speed up issues with 4 threads on quadcore system

Postby ftinetti » Fri Sep 14, 2012 5:22 am

Hi Parmeet,

According to

http://ark.intel.com/products/52231/Int ... e-2_70-GHz)

the processor you are using has two cores + HT so I would not expect a lot of performance improvement beyond 2 threads (even when the OS reports 4 processors).

HTH,

Fernando.
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Re: Speed up issues with 4 threads on quadcore system

Postby parmeet » Fri Sep 14, 2012 6:06 am

Hi Fernando,

Many thanks for pointing out.. Yes indeed its duo-core with HT..

Just a related question: Why wouldn't openMP be able to report above information. Infact when i call omp_get_num_procs() , it reports 4. Is it because somehow OpenMP cannot distinguish between physical and logical cores or is it something else?

Thanks,
Parmeet
parmeet
 
Posts: 2
Joined: Thu Sep 13, 2012 2:56 am

Re: Speed up issues with 4 threads on quadcore system

Postby ftinetti » Fri Sep 14, 2012 6:12 am

Hi Parmeet,

Just a related question: Why wouldn't openMP be able to report above information. Infact when i call omp_get_num_procs() , it reports 4. Is it because somehow OpenMP cannot distinguish between physical and logical cores or is it something else?

I think it would be possible... but it is not included in the current (or any previous) Spec. so I think it is not currently possible. Even the OS does not distinguish between physical and logical cores...

Fernando.
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