How do I debug an MPI program?

Question

I have an MPI program which compiles and runs, but I would like to step through it to make sure nothing bizarre is happening. Ideally, I would like a simple way to attach GDB to any particular process, but I'm not really sure whether that's possible or how to do it. An alternative would be having each process write debug output to a separate log file, but this doesn't really give the same freedom as a debugger.

Are there better approaches? How do you debug MPI programs?

Answer 1

I have found gdb quite useful. I use it as

mpirun -np <NP> xterm -e gdb ./program  

This the launches xterm windows in which I can do

run <arg1> <arg2> ... <argN> 

usually works fine

You can also package these commands together using:

mpirun -n <NP> xterm -hold -e gdb -ex run --args ./program [arg1] [arg2] [...] 

Answer 2

As someone else said, TotalView is the standard for this. But it will cost you an arm and a leg.

The OpenMPI site has a great FAQ on MPI debugging. Item #6 in the FAQ describes how to attach GDB to MPI processes. Read the whole thing, there are some great tips.

If you find that you have far too many processes to keep track of, though, check out Stack Trace Analysis Tool (STAT). We use this at Livermore to collect stack traces from potentially hundreds of thousands of running processes and to represent them intelligently to users. It's not a full-featured debugger (a full-featured debugger would never scale to 208k cores), but it will tell you which groups of processes are doing the same thing. You can then step through a representative from each group in a standard debugger.