I'm developing a piece of code, and to my surprise I have found that when compiling, if I add -openmp option I get around a 36% reduction in execution time (not using any OMP directives in the code, and just in case setting OMP_NUM_THREADS to 1).
Regular compilation is with -fast
If I compile with -fast -openmp then I get this 36% time reduction.
This happens in a 64-bits Ubuntu 11.10 box, running ifort 12.1.3
How is this possible? I read the documentation and I only found that -openmp adds -automatic option, but removing -openmp and adding -automatic doesn't give any performance boost. Any clues?
Thanks a lot,
ngel de Vicente
angelv@carro:~/SPIA.WC/mancha_src$ ifort -V
Intel Fortran Intel 64 Compiler XE for applications running on Intel 64, Version 184.108.40.2063 Build 20120212
Copyright (C) 1985-2012 Intel Corporation. All rights reserved.
angelv@carro:~/SPIA.WC/mancha_src$ uname -a
Linux carro 3.0.0-19-generic #33-Ubuntu SMP Thu Apr 19 19:05:14 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux