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

[Simon Urbanek]

On Apr 18, 2008, at 12:03 PM, tyler wrote:

> 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?
>

As long as the code doesn't touch R during the time it's running  
threads, no (for all practical purposes). I.e. if you setup native  
structures to work on, you're free to spawn threads, wait for them to  
finish and then return to R. There are some issues that you could  
encounter: on some platforms you need special, reentrant versions of  
system libraries  which you don't get with R; there could be issues  
with error handling and interrupts - you will be on your own.


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

You can save yourself all the hassle and just use snow + Rmpi - that  
allows you to parallelize even R code, so you don' t need any of the  
above.

Cheers,
Simon