[Beowulf] multi-threading vs. MPI

Douglas Eadline deadline at eadline.org
Sun Dec 9 10:19:23 PST 2007

I like answering these types of questions with numbers,
so in my Sept 2007 Linux magazine column (which should
be showing up on the website soon) I did the following.

Downloaded the latest NAS benchmarks written in both
OpenMP and MPI. Ran them both on an 8 core Clovertown
(dual socket) system (multiple times) and reported
the following results:

Test      OpenMP              MPI
       gcc/gfortran 4.2    LAM 7.1.2
CG         790.6             739.1
EP         166.5             162.8
FT        3535.9            2090.8
IS          51.1             122.5
LU        5620.5            5168.8
MG        1616.0            2046.2

My conclusion, it was a draw of sorts.
The article was basically looking at the
lazy assumption that threads (OpenMP) are
always better than MPI on a SMP  machine.

I'm going to re-run the tests using Harpertowns
real soon, maybe try other compilers and MPI
versions. It is easy to do. You can get the code here:



> On this list there is almost unanimous agreement that MPI is the way to go
> for parallelism and that combining multi-threading (MT) and
> message-passing
> (MP) is not even worth it, just sticking to MP is all that is necessary.
> However, in real-life most are talking and investing in MT while very few
> are interested in MP. I also just read on the blog of Arch Robison " TBB
> perhaps gives up a little performance short of optimal so you don't have
> to
> write message-passing " (here:
> http://softwareblogs.intel.com/2007/11/17/supercomputing-07-computer-environment-and-evolution/
>  )
> How come there is almost unanimous agreement in the beowulf-community
> while
> the rest is almost unanimous convinced of the opposite ? Are we just
> tapping
> ourselves on the back or is MP not sufficiently dissiminated or ... ?
> toon
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