[Rd] mclapply memory leak?

[Toby Hocking]

Dear R-devel,

I am running mclapply with many iterations over a function that modifies
nothing and makes no copies of anything. It is taking up a lot of memory,
so it seems to me like this is a bug. Should I post this to
bugs.r-project.org?

A minimal reproducible example can be obtained by first starting a memory
monitoring program such as htop, and then executing the following code
while looking at how much memory is being used by the system

library(parallel)
seconds <- 5
N <- 100000
result.list <- mclapply(1:N, function(i)Sys.sleep(1/N*seconds))

On my system, memory usage goes up about 60MB on this example. But it does
not go up at all if I change mclapply to lapply. Is this a bug?

For a more detailed discussion with a figure that shows that the memory
overhead is linear in N, please see
https://github.com/tdhock/mclapply-memory

> sessionInfo()
R version 3.2.2 (2015-08-14)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu precise (12.04.5 LTS)

locale:
 [1] LC_CTYPE=en_CA.UTF-8       LC_NUMERIC=C
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_CA.UTF-8
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_CA.UTF-8
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
 [9] LC_ADDRESS=C               LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C

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

other attached packages:
[1] ggplot2_1.0.1      RColorBrewer_1.0-5 lattice_0.20-33

loaded via a namespace (and not attached):
 [1] Rcpp_0.11.6             digest_0.6.4            MASS_7.3-43
 [4] grid_3.2.2              plyr_1.8.1              gtable_0.1.2
 [7] scales_0.2.3            reshape2_1.2.2          proto_1.0.0
[10] labeling_0.2            tools_3.2.2             stringr_0.6.2
[13] dichromat_2.0-0         munsell_0.4.2           PeakSegJoint_2015.08.06
[16] compiler_3.2.2          colorspace_1.2-4

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