MPI alterate order of execution of master and slaves

Question

I have two programs master and slave. My master does data decomposition and slaves do computation on the part of decomposed data. MPI scaterv is implemented for distribution of work.I execute my master program first then it dynamically spawns child or slave processes and slave executes different code ie.computation. Now again master has to collect results from slaves and executes next level of decomposition. how do I do that using MPI? I actually wanted to execute my master and slave code alternately.. How can I implement this?

Thank you in advance..

Answer 1

MPI-2 (if I remember correctly) introduced mechanisms for dynamic process management, you might care to search for mpi_comm_spawn to start learning about those mechanisms. So it is certainly possible to write an MPI program which alternates between one process running the master task and multiple processes running the worker tasks (the term slave is deprecated). It's even possible to design your computation so that one program runs the master task and another program runs the (multiple) worker tasks and to use MPI for passing messages between the two.

BUT (that's a big but) I don't think that many resource managers (either the humans who manage parallel computer systems or the operating system and systems software such as job managers) support such dynamic process management. Imagine the complexities of scheduling, and managing, two or more programs with the basic design that you propose. Just as program A tries to fire up 2^10 worker processes so too does program B, and program C, while program D tries to drop 2^8 worker processes; all this on a cluster with only 2^10 processors (or cores). It's probably not too difficult to construct scenarios where the throughput of jobs on the cluster falls towards zero as multiple jobs contend for scarce resources.

If your platform supports dynamic process management, go right ahead. In the far more likely case that your platform does not you have at least two choices, which one you choose depends on the ratio of master:worker time and probably other factors too. You could:

1. Do what most of us have always done and continue to do and request a total number of processors for the entire job, leaving all but one of them idle during the master-only phases. Wasteful perhaps but easy for the resource managers to cope with. Relatively easy to program too.
2. If the master does a lot of work between worker phases you could modify your program so that the master and worker are separate programs. First have the master execute on one process and, as it finishes, submit a request to the job management system to initiate the first phase of the worker computation. Have that, in turn, initiate the execution of the next master phase, and so on and so on.