MPI or Sockets?

Question

I'm working on a loosely coupled cluster for some data processing. The network code and processing code is in place, but we are evaluating different methodologies in our approach. Right now, as we should be, we are I/O bound on performance issues, and we're trying to decrease that bottleneck. Obviously, faster switches like Infiniband would be awesome, but we can't afford the luxury of just throwing out what we have and getting new equipment.

My question posed is this. All traditional and serious HPC applications done on clusters is typically implemented with message passing versus sending over sockets directly. What are the performance benefits to this? Should we see a speedup if we switched from sockets?

Answer 1

MPI MIGHT use sockets. But there are also MPI implementation to be used with SAN (System area network) that use direct distributed shared memory. That of course if you have the hardware for that. So MPI allows you to use such resources in the future. On that case you can gain massive performance improvements (on my experience with clusters back at university time, you can reach gains of a few orders of magnitude). So if you are writting code that can be ported to higher end clusters, using MPI is a very good idea.

Even discarding performance issues, using MPI can save you a lot of time, that you can use to improve performance of other parts of your system or simply save your sanity.

Answer 2

I would recommend using MPI instead of rolling your own, unless you are very good at that sort of thing. Having wrote some distributed computing-esque applications using my own protocols, I always find myself reproducing (and poorly reproducing) features found within MPI.

Performance wise I would not expect MPI to give you any tangible network speedups - it uses sockets just like you. MPI will however provide you with much the functionality you would need for managing many nodes, i.e. synchronisation between nodes.

Answer 3

Performance is not the only consideration in this case, even on high performance clusters. MPI offers a standard API, and is "portable." It is relatively trivial to switch an application between the different versions of MPI.

Most MPI implementations use sockets for TCP based communication. Odds are good that any given MPI implementation will be better optimized and provide faster message passing, than a home grown application using sockets directly.

In addition, should you ever get a chance to run your code on a cluster that has InfiniBand, the MPI layer will abstract any of those code changes. This is not a trivial advantage - coding an application to directly use OFED (or another IB Verbs) implementation is very difficult.

Most MPI applications include small test apps that can be used to verify the correctness of the networking setup independently of your application. This is a major advantage when it comes time to debug your application. The MPI standard includes the "pMPI" interfaces, for profiling MPI calls. This interface also allows you to easily add checksums, or other data verification to all the message passing routines.