[Rd] SUGGESTION: Settings to disable forked processing in R, e.g. parallel::mclapply()

[Tomas Kalibera]

On 1/10/20 7:33 AM, Henrik Bengtsson wrote:
> I'd like to pick up this thread started on 2019-04-11
> (https://hypatia.math.ethz.ch/pipermail/r-devel/2019-April/077632.html).
> Modulo all the other suggestions in this thread, would my proposal of
> being able to disable forked processing via an option or an
> environment variable make sense?

I don't think R should be doing that. There are caveats with using fork, 
and they are mentioned in the documentation of the parallel package, so 
people can easily avoid functions that use it, and this all has been 
discussed here recently.

If it is the case, we can expand the documentation in parallel package, 
add a warning against the use of forking with RStudio, but for that I it 
would be good to know at least why it is not working. From the github 
issue I have the impression that it is not really known why, whether it 
could be fixed, and if so, where. The same github issue reflects also 
that some people want to use forking for performance reasons, and even 
with RStudio, at least on Linux. Perhaps it could be fixed? Perhaps it 
is just some race condition somewhere?

Tomas

>   I've prototyped a working patch that
> works like:
>
>> options(fork.allowed = FALSE)
>> unlist(parallel::mclapply(1:2, FUN = function(x) Sys.getpid()))
> [1] 14058 14058
>> parallel::mcmapply(1:2, FUN = function(x) Sys.getpid())
> [1] 14058 14058
>> parallel::pvec(1:2, FUN = function(x) Sys.getpid() + x/10)
> [1] 14058.1 14058.2
>> f <- parallel::mcparallel(Sys.getpid())
> Error in allowFork(assert = TRUE) :
>    Forked processing is not allowed per option ‘fork.allowed’ or
> environment variable ‘R_FORK_ALLOWED’
>> cl <- parallel::makeForkCluster(1L)
> Error in allowFork(assert = TRUE) :
>    Forked processing is not allowed per option ‘fork.allowed’ or
> environment variable ‘R_FORK_ALLOWED’
>
> The patch is:
>
> Index: src/library/parallel/R/unix/forkCluster.R
> ===================================================================
> --- src/library/parallel/R/unix/forkCluster.R (revision 77648)
> +++ src/library/parallel/R/unix/forkCluster.R (working copy)
> @@ -30,6 +30,7 @@
>
>   newForkNode <- function(..., options = defaultClusterOptions, rank)
>   {
> +    allowFork(assert = TRUE)
>       options <- addClusterOptions(options, list(...))
>       outfile <- getClusterOption("outfile", options)
>       port <- getClusterOption("port", options)
> Index: src/library/parallel/R/unix/mclapply.R
> ===================================================================
> --- src/library/parallel/R/unix/mclapply.R (revision 77648)
> +++ src/library/parallel/R/unix/mclapply.R (working copy)
> @@ -28,7 +28,7 @@
>           stop("'mc.cores' must be >= 1")
>       .check_ncores(cores)
>
> -    if (isChild() && !isTRUE(mc.allow.recursive))
> +    if (!allowFork() || (isChild() && !isTRUE(mc.allow.recursive)))
>           return(lapply(X = X, FUN = FUN, ...))
>
>       ## Follow lapply
> Index: src/library/parallel/R/unix/mcparallel.R
> ===================================================================
> --- src/library/parallel/R/unix/mcparallel.R (revision 77648)
> +++ src/library/parallel/R/unix/mcparallel.R (working copy)
> @@ -20,6 +20,7 @@
>
>   mcparallel <- function(expr, name, mc.set.seed = TRUE, silent =
> FALSE, mc.affinity = NULL, mc.interactive = FALSE, detached = FALSE)
>   {
> +    allowFork(assert = TRUE)
>       f <- mcfork(detached)
>       env <- parent.frame()
>       if (isTRUE(mc.set.seed)) mc.advance.stream()
> Index: src/library/parallel/R/unix/pvec.R
> ===================================================================
> --- src/library/parallel/R/unix/pvec.R (revision 77648)
> +++ src/library/parallel/R/unix/pvec.R (working copy)
> @@ -25,7 +25,7 @@
>
>       cores <- as.integer(mc.cores)
>       if(cores < 1L) stop("'mc.cores' must be >= 1")
> -    if(cores == 1L) return(FUN(v, ...))
> +    if(cores == 1L || !allowFork()) return(FUN(v, ...))
>       .check_ncores(cores)
>
>       if(mc.set.seed) mc.reset.stream()
>
> with a new file src/library/parallel/R/unix/allowFork.R:
>
> allowFork <- function(assert = FALSE) {
>      value <- Sys.getenv("R_FORK_ALLOWED")
>      if (nzchar(value)) {
>          value <- switch(value,
>             "1"=, "TRUE"=, "true"=, "True"=, "yes"=, "Yes"= TRUE,
>             "0"=, "FALSE"=,"false"=,"False"=, "no"=, "No" = FALSE,
>              stop(gettextf("invalid environment variable value: %s==%s",
>             "R_FORK_ALLOWED", value)))
> value <- as.logical(value)
>      } else {
>          value <- TRUE
>      }
>      value <- getOption("fork.allowed", value)
>      if (is.na(value)) {
>          stop(gettextf("invalid option value: %s==%s", "fork.allowed", value))
>      }
>      if (assert && !value) {
>        stop(gettextf("Forked processing is not allowed per option %s or
> environment variable %s", sQuote("fork.allowed"),
> sQuote("R_FORK_ALLOWED")))
>      }
>      value
> }
>
> /Henrik
>
> On Mon, Apr 15, 2019 at 3:12 AM Tomas Kalibera <tomas.kalibera using gmail.com> wrote:
>> On 4/15/19 11:02 AM, Iñaki Ucar wrote:
>>> On Mon, 15 Apr 2019 at 08:44, Tomas Kalibera <tomas.kalibera using gmail.com> wrote:
>>>> On 4/13/19 12:05 PM, Iñaki Ucar wrote:
>>>>> On Sat, 13 Apr 2019 at 03:51, Kevin Ushey <kevinushey using gmail.com> wrote:
>>>>>> I think it's worth saying that mclapply() works as documented
>>>>> Mostly, yes. But it says nothing about fork's copy-on-write and memory
>>>>> overcommitment, and that this means that it may work nicely or fail
>>>>> spectacularly depending on whether, e.g., you operate on a long
>>>>> vector.
>>>> R cannot possibly replicate documentation of the underlying operating
>>>> systems. It clearly says that fork() is used and readers who may not
>>>> know what fork() is need to learn it from external sources.
>>>> Copy-on-write is an elementary property of fork().
>>> Just to be precise, copy-on-write is an optimization widely deployed
>>> in most modern *nixes, particularly for the architectures in which R
>>> usually runs. But it is not an elementary property; it is not even
>>> possible without an MMU.
>> Yes, old Unix systems without virtual memory had fork eagerly copying.
>> Not relevant today, and certainly not for systems that run R, but indeed
>> people interested in OS internals can look elsewhere for more precise
>> information.
>>
>> Tomas
>>