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Is there a way to modify how R foreach loop does load balancing with doParallel backend ? When parallelizing tasks that have very different execution time, it can happen all nodes but one have finished their tasks while the last one still have several tasks to do. Here is a toy example: 

library(foreach)
library(doParallel)

registerDoParallel(4)

waittime = c(10,1,1,1,10,1,1,1,10,1,1,1,10,1,1,1)

w = iter(waittime)

foreach(i=w) %dopar% {
    message(paste("waiting",i, "on",Sys.getpid()))
    Sys.sleep(i)
}

Basically, the code register 4 cores. For each loop i, the task is to wait for waittime[i] seconds. However, because the load balancing in the foreach loop seems to be, by default, to split the total number of tasks into sets having a length of the number of registered cores, in the above example, the first core receives all the tasks with waittime = 10, while the 3 others receive tasks with waittime = 1 so that these 3 cores will have finished all their tasks before the first one have finished its first.

Is there a way to make foreach() distribute tasks one at a time ? i.e. in the above case, I'd like that the first 4 tasks are distributed among the 4 cores, and then that each next task is distributed to the next available core.

Thanks.

coletl
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xraynaud
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2 Answers2

4

I haven't tested it myself, but the doParallel backend provides a preschedule option akin to the mc.preschedule argument in mclapply(). (See section 7 of the doParallel vignette.)

You might try:

mcoptions <- list(preschedule = FALSE)
foreach(i = w, .options.multicore = mcoptions)
coletl
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1

Apologies for posting as an answer but I have insufficient rep to comment. Is it possible that you could rewrite your code to make use of parLapplyLB or parSapplyLB?

parLapplyLB, parSapplyLB are load-balancing versions, intended for use when applying FUN to different elements of X takes quite variable amounts of time, and either the function is deterministic or reproducible results are not required.

Lespied
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  • if possible, I'd definitely prefer staying wihth foreach. In my application, what's inside the foreach loop is way more complicated than the two commands I have put in my example so converting my code to a single function might be complicated. Thanks anyway. – xraynaud Mar 09 '17 at 10:06
  • I had guessed you were prodcuing a very limited example, if you could write your code inside the forloop in the form of a function using parLapply is trivial. – Lespied Mar 09 '17 at 12:35