openmp with single processor

Ravindra Shende · July 12, 2013, 06:23

Hi everyone,

I found that my openmp parallelized fortran code runs slower when run on one processor as compared to the same but unparallelized code. (I have used ifort compiler)

Can someone tell me why is this so?

Is it because the compiler itself tries to parallelise a serial code and when the code is parallel it doesn't?

flotus1 · July 15, 2013, 10:50

How high is the slowdown you observe for the parallel code?
A few percent could be explained by the overhead introduced by the parallelization.

FMDenaro · July 15, 2013, 10:51

how do you set the OS parameters NCPUS and OMP_NUM_THREADS?

July 12, 2013, 06:23	openmp with single processor	#1
Ravindra Shende Member Ravindra Shende Join Date: Feb 2011 Location: Pune, India Posts: 45 Rep Power: 15	Hi everyone, I found that my openmp parallelized fortran code runs slower when run on one processor as compared to the same but unparallelized code. (I have used ifort compiler) Can someone tell me why is this so? Is it because the compiler itself tries to parallelise a serial code and when the code is parallel it doesn't?

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July 15, 2013, 10:50		#2
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,398 Rep Power: 46	How high is the slowdown you observe for the parallel code? A few percent could be explained by the overhead introduced by the parallelization.

July 15, 2013, 10:51		#3
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,763 Rep Power: 71	how do you set the OS parameters NCPUS and OMP_NUM_THREADS?