[R] doParallel cores HPC

Henrik Bengtsson henr|k@bengt@@on @end|ng |rom gm@||@com
Sat Jun 27 05:20:05 CEST 2020

> On the other hand, [...], you can spawn 20 workers on each of the 10 hosts:
> makePSOCKcluster(names = rep(c('Host01', ..., 'Host10'), each = 20))

Unfortunately, this will most likely not work because it will require
200 open connections - one for each worker - but R limits you to 125
(see ?base::connections):

> A maximum of 128 connections can be allocated (not necessarily open) at any one time. Three of these are pre-allocated (see stdout). The OS will impose limits on the numbers of connections of various types, but these are usually larger than 125.

Depending on the system, you might be able to increase this by
rebuilding R from source after editing a hard-coded constant.  I've
verified that this worked on a local Ubuntu 16.04 system.  See
https://github.com/HenrikBengtsson/Wishlist-for-R/issues/28 for
details about this problem.


On Fri, Jun 26, 2020 at 12:11 AM Ivan Krylov <krylov.r00t using gmail.com> wrote:
> On Thu, 25 Jun 2020 00:29:42 +0000
> "Silva, Eder David Borges da" <eder.silva using corteva.com> wrote:
> > I have the HPC, with 10 nodes, and each node with 20 cores in UNIX OS.
> > cl <- makePSOCKcluster(names=c('Host01', ... , 'Host10)
> > This code is the best way for use all machine power?
> The code as written will create one worker _process_ on each of the
> hosts. What happens next depends on the code to be running and the way
> R is installed.
> The code may or may not be written to take advantage of multi-core CPUs
> (e.g. using OpenMP). In particular, if R is linked with a
> multi-threaded BLAS (such as OpenBLAS or MKL) and uses matrix algebra
> during the computation, it may spawn multiple _threads_ to utilise the
> CPU better. Whether it succeeds depends on multiple factors, including
> the size of the task. On occasion I noticed OpenBLAS threads spending
> most of their time in sched_yield() system call, making the kernel do a
> lot of unnecessary work, and set the environment variable
> OPENBLAS_NUM_THREADS=1 to use only one thread instead.
> On the other hand, if the computation is purely single-threaded (or you
> disabled the multi-threaded behaviour of OpenMP or BLAS for some
> reason), you can spawn 20 workers on each of the 10 hosts:
> makePSOCKcluster(names = rep(c('Host01', ..., 'Host10'), each = 20))
> You can also try to combine the two approaches by limiting the number
> of working threads to a sensible value which results in the threads
> spending most of the time computing things (instead of waiting for more
> work busy-looping on sched_yield()), then spawning as many processes as
> required to utilise all of the cores.
> --
> Best regards,
> Ivan
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