parRF on caret not working for more than one core

Question

parRF from the caret R package is not working for me with more than one core, which is quite ironic, given the par in parRF stands for parallel. I'm on a windows machine, if that is a relevant piece of information. I checked that I'm using the latest an greatest regarding caret and doParallel.

I made a minimal example and and give the results below. Any ideas?

Source code

library(caret)
library(doParallel)

trCtrl <- trainControl(
  method = "repeatedcv"
  , number = 2
  , repeats = 5
  , allowParallel = TRUE
)

# WORKS
registerDoParallel(1)
train(form = Species~., data=iris, trControl = trCtrl, method="parRF")
closeAllConnections()

# FAILS
registerDoParallel(2)
train(form = Species~., data=iris, trControl = trCtrl, method="parRF")
closeAllConnections()

Output

> library(caret)
> library(doParallel)
> 
> trCtrl <- trainControl(
+   method = "repeatedcv"
+   , number = 2
+   , repeats = 5
+   , allowParallel = TRUE
+ )
> 
> 
> # WORKS
> registerDoParallel(1)
> train(form = Species~., data=iris, trControl = trCtrl, method="parRF")
Parallel Random Forest 

150 samples
  4 predictors
  3 classes: 'setosa', 'versicolor', 'virginica' 

... some more model output, works fine!
> closeAllConnections()
> 
> # FAILS
> registerDoParallel(2)
> train(form = Species~., data=iris, trControl = trCtrl, method="parRF")
Error in train.default(x, y, weights = w, ...) : 
  final tuning parameters could not be determined
In addition: Warning messages:
1: In nominalTrainWorkflow(x = x, y = y, wts = weights, info = trainInfo,  :
  There were missing values in resampled performance measures.
2: In train.default(x, y, weights = w, ...) :
  missing values found in aggregated results
> closeAllConnections()

Session Info

> sessionInfo()
R version 3.1.0 (2014-04-10)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252    LC_MONETARY=German_Germany.1252 LC_NUMERIC=C                   
[5] LC_TIME=German_Germany.1252    

attached base packages:
[1] parallel  stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] doParallel_1.0.8   iterators_1.0.7    foreach_1.4.2      e1071_1.6-3        randomForest_4.6-7 caret_6.0-30       ggplot2_1.0.0     
[8] lattice_0.20-29   

loaded via a namespace (and not attached):
 [1] BradleyTerry2_1.0-4 brglm_0.5-9         car_2.0-20          class_7.3-10        codetools_0.2-8     colorspace_1.2-4   
 [7] compiler_3.1.0      digest_0.6.4        gnm_1.0-7           grid_3.1.0          gtable_0.1.2        gtools_3.4.1       
[13] lme4_1.1-6          MASS_7.3-31         Matrix_1.1-3        minqa_1.2.3         munsell_0.4.2       nlme_3.1-117       
[19] nnet_7.3-8          plyr_1.8.1          proto_0.3-10        qvcalc_0.8-8        Rcpp_0.11.2         RcppEigen_0.3.2.1.2
[25] relimp_1.0-3        reshape2_1.4        scales_0.2.4        splines_3.1.0       stringr_0.6.2       tcltk_3.1.0        
[31] tools_3.1.0   

Update

• Tried it with 3.1.1 (same packages versions), same result.
• Tried it with 3.0.2 and some older Version of caret a doParallel, it worked (see session Info)

Session Info 2:

R version 3.0.2 (2013-09-25)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252    LC_MONETARY=German_Germany.1252
[4] LC_NUMERIC=C                    LC_TIME=German_Germany.1252    

attached base packages:
[1] parallel  stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] e1071_1.6-1        class_7.3-9        randomForest_4.6-7 doParallel_1.0.6   iterators_1.0.6   
 [6] caret_5.17-7       reshape2_1.2.2     plyr_1.8           lattice_0.20-24    foreach_1.4.1     
[11] cluster_1.14.4    

loaded via a namespace (and not attached):
[1] codetools_0.2-8 compiler_3.0.2  grid_3.0.2      stringr_0.6.2   tools_3.0.2    

Answer 1

This is clearly a bug in caret 6.0-30 that was introduced sometime after version 5.17-7. It's also another problem that is more likely to hit Windows users, since the doParallel "mclapply mode" works, while the "clusterApplyLB mode" fails.

I've run some tests, and it appears that the problem is due to the cluster workers not being properly initialized to perform nested parallel computations, so you can work-around the bug by loading the foreach package in the cluster workers before calling "train". To do this, you need to explicitly create the cluster object, rather than letting the "registerDoParallel" function create it for you (which it does on Windows). For example:

cl <- makePSOCKcluster(2)
clusterEvalQ(cl, library(foreach))
registerDoParallel(cl)

I'll contact the author of caret to discuss a solution to the problem.