What is easier to learn and debug OpenMP or MPI?

Question

Hi,

I have a number crunching C/C++ application. It is basically a main loop for different data sets. We got access to a 100 node cluster with openmp and mpi available. I would like to speedup the application but I am an absolut newbie for both mpi and openmp. I just wonder what is the easiest one to learn and to debug even if the performance is not the best.

I also wonder what is the most adequate for my main loop application.

Thanks

Answer 1

If your program is just one big loop using OpenMP can be as simple as writing:

#pragma omp parallel for

OpenMP is only useful for shared memory programming, which unless your cluster is running something like kerrighed means that the parallel version using OpenMP will only run on at most one node at a time.

MPI is based around message passing and is slightly more complicated to get started. The advantage is though that your program could run on several nodes at one time, passing messages between them as and when needed.

Given that you said "for different data sets" it sounds like your problem might actually fall into the "embarrassingly parallel" category, where provided you've got more than 100 data sets you could just setup the scheduler to run one data set per node until they are all completed, with no need to modify your code and almost a 100x speed up over just using a single node.

For example if your cluster is using condor as the scheduler then you could submit 1 job per data item to the "vanilla" universe, varying only the "Arguments =" line of the job description. (There are other ways to do this for Condor which may be more sensible and there are also similar things for torque, sge etc.)

Answer 2

OpenMP is essentially for SMP machines, so if you want to scale to hundreds of nodes you will need MPI anyhow. You can however use both. MPI to distribute work across nodes and OpenMP to handle parallelism across cores or multiple CPUs per node. I would say OpenMP is a lot easier than messing with pthreads. But it being coarser grained, the speed up you will get from OpenMP will usually be lower than a hand optimized pthreads implementation.