parallel nnetar fitting

[dashaub]

What do you think about adding parallelization to nnetar? The code in avnnet looks very easy to parallelize. For long timeseries with large repeat and large number of CPU cores I imagine it would create a good speedup.

[robjhyndman]

Yes, good idea.

[dashaub]

I've put together a PR #349 implementing this. After messing around with parallel::parLapply() for a while and getting argument name collision errors, I settled with foreach::foreach(). The performance improvements look good on the long taylor series but bad for short series, so I set the default parallel = FALSE. An adaptive rule based on the series length like used in tbats() could make sense here.

Pros: great performance on long series
Cons: imports foreach::foreach() and doParallel::registerDoParallel(). These are common packages and probably installed on most systems already, however.

# current implementation
library(microbenchmark)
library(devtools)
install_github("robjhyndman/forecast")
library(forecast)
microbenchmark(nnetar(taylor),
               nnetar(AirPassengers),
               nnetar(AirPassengers, repeats = 500), times = 5)
Unit: seconds
                                 expr        min         lq       mean
                       nnetar(taylor) 837.956053 841.290937 841.430421
                nnetar(AirPassengers)   1.009523   1.030911   1.027371
 nnetar(AirPassengers, repeats = 500)  24.779071  24.818174  24.841770
     median         uq        max neval
 842.189486 842.570680 843.144946     5
   1.031746   1.032079   1.032597     5
  24.845551  24.863318  24.902737     5

# parallel implementation with parallel on by default
install_github("dashaub/forecast")
library(forecast)
microbenchmark(nnetar(taylor, parallel = TRUE),
               nnetar(AirPassengers, parallel = FALSE),
               nnetar(AirPassengers, parallel = TRUE, num.cores = 1),
               nnetar(AirPassengers, parallel = TRUE, num.cores = 2),
               nnetar(AirPassengers, parallel = TRUE, num.cores = 4),
               nnetar(AirPassengers, repeats = 500, parallel = TRUE, num.cores = 4),
               times = 5)
Unit: seconds
                                                                 expr
                                      nnetar(taylor, parallel = TRUE)
                              nnetar(AirPassengers, parallel = FALSE)
                nnetar(AirPassengers, parallel = TRUE, num.cores = 1)
                nnetar(AirPassengers, parallel = TRUE, num.cores = 2)
                nnetar(AirPassengers, parallel = TRUE, num.cores = 4)
 nnetar(AirPassengers, repeats = 500, parallel = TRUE, num.cores = 4)
        min         lq       mean     median         uq        max neval
 424.717306 426.561738 427.136085 427.486460 428.064305 428.850618     5
   1.018690   1.018691   1.022518   1.022630   1.025139   1.027438     5
   6.375443   6.402015   6.401181   6.404755   6.405817   6.417876     5
   7.482467   7.518907   7.545821   7.519844   7.529500   7.678389     5
  10.714595  10.798919  10.816837  10.811451  10.834412  10.924808     5
  18.562486  18.579964  18.624954  18.595774  18.641525  18.745022     5


# some more tests on the taylor series
microbenchmark(nnetar(taylor, parallel = FALSE),
               nnetar(taylor, parallel = TRUE, num.cores = 1),
               nnetar(taylor, parallel = TRUE, num.cores = 2),
               nnetar(taylor, parallel = TRUE, num.cores = 4),
               nnetar(taylor, repeats = 100, parallel = TRUE, num.cores = 1),
               nnetar(taylor, repeats = 100, parallel = TRUE, num.cores = 2),
               nnetar(taylor, repeats = 100, parallel = TRUE, num.cores = 4),
               times = 5)

Unit: seconds
                                                          expr       min
                              nnetar(taylor, parallel = FALSE)  839.0974
                nnetar(taylor, parallel = TRUE, num.cores = 1)  839.0866
                nnetar(taylor, parallel = TRUE, num.cores = 2)  426.8741
                nnetar(taylor, parallel = TRUE, num.cores = 4)  222.6650
 nnetar(taylor, repeats = 100, parallel = TRUE, num.cores = 1) 4179.6068
 nnetar(taylor, repeats = 100, parallel = TRUE, num.cores = 2) 2106.8621
 nnetar(taylor, repeats = 100, parallel = TRUE, num.cores = 4) 1067.8230
        lq      mean    median        uq       max neval
  843.1898  844.2343  844.1039  846.0222  848.7580     5
  839.7962  842.2116  842.8739  843.8619  845.4396     5
  426.9788  427.9270  427.8327  428.3260  429.6235     5
  222.7813  223.3000  223.0341  223.9712  224.0484     5
 4182.5776 4184.9711 4184.7301 4186.4020 4191.5392     5
 2109.5174 2109.8424 2110.0868 2111.0205 2111.7252     5
 1070.3660 1072.8482 1070.8182 1073.7582 1081.4759     5

[robjhyndman]

I would like to avoid adding additional package dependencies. What were the issues with using parallel?

[dashaub]

nnet() uses the x and y arguments for the data, and it seems that parLapply() is also passing a differentx argument in the ... arguments down the function call somewhere that conflicts with this. There might be a way around this by setting up a wrapper function for avnnet()

[robjhyndman]

You could try using do.call