[OMPI users] using specific algorithm for collective communication, and knowing the root cpu?

[George Markomanolis]

Dear all,

I would like to ask about collective communication. With debug mode 
enabled, I can see many info during the execution which algorithm is 
used etc. But my question is that I would like to use a specific 
algorithm (the simplest I suppose). I am profiling some applications and 
I want to simulate them with another program so I must be able to know 
for example what the mpi_allreduce is doing. I saw many algorithms that 
depend on the message size and the number of processors, so I would like 
to ask:

1) what is the way to say at open mpi to use a simple algorithm for 
allreduce (is there any way to say to use the simplest algorithm for all 
the collective communication?). Basically I would like to know the root 
cpu for every collective communication. What are the disadvantages for 
demanding the simplest algorithm?

2) Is there any overhead because I installed open mpi with debug mode 
even if I just run a program without any flag with --mca?

3) How you could describe allreduce by words? Can we say that the root 
cpu does reduce and then broadcast? I mean is that right for your 
implementation? I saw that it depends on the algorithm which cpu is the 
root, so is it possible to use an algorithm that I will know every time 
that cpu with rank 0 is the root?

Thanks a lot,
George