How can I label my sub-processes for logging when using multicore and doMC in R

I have started using the doMC package for R as the parallel backend for parallelised plyr routines.

The parallelisation itself seems to be working fine (though I have yet to properly benchmark the speedup), my problem is that the logging is now asynchronous and messages from different cores are getting mixed in together. I could created different logfiles for each core, but I think I neater solution is to simply add a different label for each core. I am currently using the log4r package for my logging needs.

I remember when using MPI that each processor got a rank, which was a way of distinguishing each process from one another, so is there a way to do this with doMC? I did have the idea of extracting the PID, but this does seem messy and will change for every iteration.

I am open to ideas though, so any suggestions are welcome.

EDIT (2011-开发者_Python百科04-08): Going with the suggestion of one answer, I still have the issue of correctly identifying which subprocess I am currently inside, as I would either need separate closures for each log() call so that it writes to the correct file, or I would have a single log() function, but have some logic inside it determining which logfile to append to. In either case, I would still need some way of labelling the current subprocess, but I am not sure how to do this.

Is there an equivalent of the mpi_rank() function in the MPI library?

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I think having multiple process write to the same file is a recipe for a disaster (it's just a log though, so maybe "disaster" is a bit strong).

Often times I parallelize work over chromosomes. Here is an example of what I'd do (I've mostly been using foreach/doMC):

foreach(chr=chromosomes, ...) %dopar% {
  cat("+++", chr, "+++\n")
  ## ... some undoubtedly amazing code would then follow ...
}

And it wouldn't be unusual to get output that tramples over each other ... something like (not exactly) this:

+++chr1+++
+++chr2+++
++++chr3++chr4+++

... you get the idea ...

If I were in your shoes, I think I'd split the logs for each process and set their respective filenames to be unique with respect to something happening in that process's loop (like chr in my case above). Collate them later if you must ... ie. map/reduce your log files :-)