[Pw_forum] openmp vs mpich performance with MKL 10.x

[Eduardo Ariel Menendez Proupin]

Hi,
I have noted recently that I am able to obtain faster binaries of pw.x using
the the OpenMP paralellism implemented in the Intel MKL libraries of version
10.xxx, than using MPICH, in the Intel cpus. Previously I had always gotten
better performance using MPI. I would like to know of other experience on
how to make the machines faster. Let me explain in more details.

Compiling using MPI means using mpif90 as linker and compiler, linking
against mkl_ia32 or mkl_em64t, and using link flags -i-static -openmp. This
is just the  what appears in the make.sys after running configure in version
4cvs,

At runtime, I set
export OMP_NUM_THREADS=1
export MKL_NUM_THREADS=1
and run using
mpiexec -n $NCPUs pw.x <input >output
where NCPUs  is the number of cores available in the system.

The second choice is
./configure --disable-parallel

and at runtime
export OMP_NUM_THREADS=$NCPU
export MKL_NUM_THREADS=$NCPU
and run using
pw.x <input >output

I have tested it in Quadcores (NCPU=4) and with an old Dual Xeon B.C.
(before cores) (NCPU=2).

Before April 2007, the first choice had always workes faster. After that,
when I came to use the MKL 10.xxx, the second choice is working faster. I
have found no significant difference between version 3.2.3 and 4cvs.

A special comment is for the FFT library. The MKL has a wrapper to the FFTW,
that must be compiled after instalation (it is very easy). This creates
additional libraries named like libfftw3xf_intel.a and libfftw2xf_intel.a
This allows improves the performance in the second choice, specially with
libfftw3xf_intel.a.

Using MPI, libfftw2xf_intel.a is as fast as using the FFTW source
distributed with espresso, i.e., there is no gain in using
libfftw2xf_intel.a. With  libfftw3xf_intel.a and MPI, I have never been able
to run pw.x succesfully, it just aborts.

I would like to hear of your experiences.

Best regards
Eduardo Menendez
University of Chile
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