[Rd] naive question regarding running parallel C code from R

[tyler]

Hi,

I have only the vaguest notions of what parallel programing, but I think
I have a situation where it might be of use to me, or at least provide
me with the opportunity to learn more about it. Before I invest in
figuring out the nuts and bolts, can anyone confirm that this is a sane
approach, or provide alternatives that I could pursue?

I'm running stochastic simulations, with the actual simulation in C
code, with an R interface to set up the parameters, format the output,
and save the resulting objects periodically through the run. The basic
layout is:

R function sets up the run
R for(c = 0; c < CYCLES; c++)
  call C function
  C for(i = 0; i < TIME; i++)
    immigration loop adds individuals to the recruit vector
    birth loop adds individuals to the recruit vector
    recruit vector is added to the community vector
    death loop removes excess individuals
  Return results to R, which processes and saves the objects
  Repeat

A typical run has 20 cycles, each with 500 time steps, and takes about
an hour. The immigration and birth loops are independent of each other,
and so could run simultaneously. They both add to the recruit vector,
but the order of the addition doesn't matter so long as both finish
before the recruit vector is added to the community vector. The
immigration, birth, and death loop iterate over arrays in a way that the
outcome at different locations is independent. i.e., the impact of the
birth vector on recruit vector position 0 has no influence on what the
birth vector does to recruit vector position 1.

What I'm thinking of doing is running the birth and immigration loops as
separate threads, and possibly running each of those threads as a group
of threads - so a thread for a birth loop that iterates over the first N
positions, another thread for the second N positions and so on.

I'm keen to learn about parallel programming, but I don't understand
enough yet to make sense of the information in the R extensions manual
and the various discussions on this list about R being thread-safe. Does
it matter if R is thread-safe if the actual simulation is being computed
in separate, shared C code?

I'm running my current, sequential code, on a cluster that supports both
OpenMP and MPI, should I figure out how to use it.

Thanks for your patience,

Tyler

-- 
Don't learn the tricks of the trade; learn the trade.