nested parallel workers

On 03/25/2015 07:48 PM, Simon Urbanek wrote:

On Mar 25, 2015, at 3:46 PM, Valerie Obenchain <vobencha at fredhutch.org> wrote:

Hi Simon,

I'm having trouble with nested parallel workers, specifically, forking inside socket connections.

You simply can't by definition - when you fork *all* the workers share the same connection inherited from the parent, so you cannot use any I/O operations that you didn't start in the worker since reading in one worker affects all the workers.

Sorry if I'm missing the obvious here -
I thought since the fork workers were shut down by the time the SOCK worker returned to its master conflicting I/O wouldn't be a problem.

There are quite a few examples floating around where SOCK workers are spawned on a cluster and multicore workers are called within them. If I understand correctly this should not be done (or at least not encouraged). Instead, nested parallel should only be done with distributed memory workers, SOCK, MPI etc.

Thanks.
Valerie

Cheers,
Simon

When mclapply is called inside a SOCK, PSOCK or FORK worker I get an
error in unserialize().

cl <- makeCluster(1, "SOCK")

fun = function(i) {
 library(parallel)
 mclapply(1:2, sqrt)
}

Failure occurs after multiple calls to clusterApply:

clusterApply(cl, 1, fun)

[[1]]
[[1]][[1]]
[1] 1

[[1]][[2]]
[1] 1.414214

clusterApply(cl, 1, fun)

[[1]]
[[1]][[1]]
[1] 1

[[1]][[2]]
[1] 1.414214

clusterApply(cl, 1, fun)

Error in unserialize(node$con) : error reading from connection


This example is from Martin and may be a different problem.

~/tmp >cat test1.R
## like mclapply
## should run 'forever' but terminates semi-randomly
library(parallel)
children <- parallel:::children

while (TRUE) {
   n <- 8            ## n == dectectCores()
   jobs <- lapply(seq_len(n), function(i) mcparallel(Sys.sleep(20)))
   mccollect(children(jobs), FALSE)
   parallel:::mckill(children(jobs), tools::SIGTERM)
   leni <- length(mccollect(children(jobs)))
   message("leni: ", leni)
}

~/tmp >R-dev --vanilla --slave -f test1.R
leni: 6
leni: 7
leni: 7
leni: 7
leni: 7
leni: 7
leni: 7
leni: 7
leni: 8
leni: 7
leni: 7
leni: 7
~/tmp >


Thanks.
Valerie

sessionInfo()

R Under development (unstable) (2015-03-18 r68009)
Platform: x86_64-unknown-linux-gnu (64-bit)
Running under: Fedora 21 (Twenty One)

locale:
[1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
[3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
[5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.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  stats     graphics  grDevices utils     datasets  methods
[8] base

loaded via a namespace (and not attached):
[1] snow_0.3-13


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Computational Biology / Fred Hutchinson Cancer Research Center
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Email: vobencha at fredhutch.org
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