Hybrid OpenMP + OpenMPI for mixed distributed & shared memory?

Question

I am developing a code to perform a few very large computations by my standards. Based on single-CPU estimates, expected run-time is ~10 CPU years, and memory requirements are ~64 GB. Little to no IO is required. My serial version of the code in question (written in C) is working well enough and I have to start thinking about how to best parallelize the code.

I have access to clusters with ~64 GB RAM and 16 cores per node. I will probably limit myself to using e.g. <= 8 nodes. I'm imagining a setup where memory is shared between threads on a single node, with separate memory used on different nodes and relatively little communication between nodes.

From what I've read so far, the solution I have come up with is to use a hybrid OpenMP + OpenMPI design, using OpenMP to manage threads on individual compute nodes, and OpenMPI to pass information between nodes, like this: https://www.rc.colorado.edu/crcdocs/openmpi-openmp

My question is whether this is the "best" way to implement this parallelization. I'm an experienced C programmer but have very limited experience in parallel programming (a little bit with OpenMP, none with OpenMPI; most of my jobs in the past were embarrassingly parallel). As an alternative suggestion, is it possible with OpenMPI to efficiently share memory on a single host? If so then I could avoid using OpenMP, which would make things slightly simpler (one API instead of two).

Answer 1

Hybrid OpenMP and MPI coding is most appropriate for problems where one can clearly identify two separate levels of parallelism - corase grained one and the fine grained one nested inside each coarse subdomain. Since fine grained parallelism requires lots of communication when implemented with message passing, it doesn't scale, because the communication overhead can become comparable to the amount of work being done. As OpenMP is a shared memory paradigm, no data communication is necessary, only access synchronisation, and it is more appropriate for finer grained parallel tasks. OpenMP also benefits from data sharing between threads (and the corresponding cache sharing on modern multi-core CPUs with shared last-level cache) and usually requires less memory than the equivalent message passing code, where some of the data might need to be replicated in all processes. MPI on the other side can run cross nodes and is not limited to running on a single shared-memory system.

Your words suggest that your parallelisation is very coarse grained or belongs to the so-called embarassingly parallel problems. If I were you, I would go hybrid. If you only employ OpenMP pragmas and don't use runtime calls (e.g. omp_get_thread_num()) your code can be compiled as both pure MPI (i.e. with non-threaded MPI processes) or as hybrid, depending on whether you enable OpenMP or not (you can also provide a dummy OpenMP runtime to enable code to be compiled as serial). This will give you both the benefits of OpenMP (data sharing, cache reusage) and MPI (transparent networking, scalability, easy job launching) with the added option to switch off OpenMP and run in an MPI-only mode. And as an added bonus, you will be able to meet the future, which looks like brining us interconnected many-many-core CPUs.