The parallelly package provides functions that
enhance the parallel packages. For example,
availableCores() gives the number of CPU cores available to
your R process as given by R options and environment variables,
including those set by job schedulers on high-performance compute (HPC)
clusters. If R runs under ‘cgroups’ or in a Linux container, then their
settings are acknowledges too. If nothing else is set, then it will fall
back to parallel::detectCores(). Another example is
makeClusterPSOCK(), which is backward compatible with
parallel::makePSOCKcluster() while doing a better job in
setting up remote cluster workers without having to know your local
public IP address and configuring the firewall to do port-forwarding to
your local computer. The functions and features added to this package
are written to be backward compatible with the parallel
package, such that they may be incorporated there later. The
parallelly package comes with an open invitation for
the R Core Team to adopt all or parts of its code into the
parallel package.
| parallelly | parallel | |
|---|---|---|
| remote clusters without knowing local public IP | ✓ | N/A |
| remote clusters without firewall configuration | ✓ | N/A |
| remote username in ~/.ssh/config | ✓ | R (>= 4.1.0) with
user = NULL |
| set workers’ library package path on startup | ✓ | N/A |
| set workers’ environment variables on startup | ✓ | N/A |
| custom workers startup code | ✓ | N/A |
| fallback to RStudio’s SSH and PuTTY’s plink | ✓ | N/A |
| faster, parallel setup of local workers (R >= 4.0.0) | ✓ | ✓ |
| faster, little-endian protocol by default | ✓ | N/A |
| faster, low-latency socket connections by default | ✓ | N/A |
| validation of cluster at setup | ✓ | ✓ |
| attempt to launch failed workers multiple times | ✓ | N/A |
| collect worker details at cluster setup | ✓ | N/A |
| termination of workers if cluster setup fails | ✓ | R (>= 4.0.0) |
| shutdown of cluster by garbage collector | ✓ | N/A |
| combining multiple, existing clusters | ✓ | N/A |
| more informative printing of cluster objects | ✓ | N/A |
| check if local and remote workers are alive | ✓ | N/A |
| restart local and remote workers | ✓ | N/A |
| defaults via options & environment variables | ✓ | N/A |
| respecting CPU resources allocated by cgroups, Linux containers, and HPC schedulers | ✓ | N/A |
| early error if requesting more workers than possible | ✓ | N/A |
| informative error messages | ✓ | N/A |
Any cluster created by the parallelly package is
fully compatible with the clusters created by the
parallel package and can be used by all of
parallel’s functions for cluster processing,
e.g. parallel::clusterEvalQ() and
parallel::parLapply(). The
parallelly::makeClusterPSOCK() function can be used as a
stand-in replacement of the parallel::makePSOCKcluster(),
or equivalently,
parallel::makeCluster(..., type = "PSOCK").
Most of parallelly functions apply also to clusters created by the parallel package. For example,
cl <- parallel::makeCluster(2)
cl <- parallelly::autoStopCluster(cl)makes the cluster created by parallel to shut down automatically when R’s garbage collector removes the cluster object. This lowers the risk for leaving stray R worker processes running in the background by mistake. Another way to achieve the above in a single call is to use:
cl <- parallelly::makeClusterPSOCK(2, autoStop = TRUE)The availableCores() function is designed as a better,
safer alternative to detectCores() of the
parallel package. It is designed to be a worry-free
solution for developers and end-users to query the number of available
cores - a solution that plays nice on multi-tenant systems, in Linux
containers, on high-performance compute (HPC) cluster, on CRAN and
Bioconductor check servers, and elsewhere.
Did you know that parallel::detectCores() might return
NA on some systems, or that parallel::detectCores() - 1
might return 0 on some systems, e.g. old hardware and virtual machines?
Because of this, you have to use
max(1, parallel::detectCores() - 1, na.rm = TRUE) to get it
correct. In contrast, parallelly::availableCores() is
guaranteed to return a positive integer, and you can use
parallelly::availableCores(omit = 1) to return all but one
core and always at least one.
Just like other software tools that “hijacks” all cores by default, R
scripts, and packages that defaults to detectCores() number
of parallel workers cause lots of suffering for fellow end-users and
system administrators. For instance, a shared server with 48 cores will
come to a halt already after a few users run parallel processing using
detectCores() number of parallel workers. This problem gets
worse on machines with many cores because they can host even more
concurrent users. If these R users would have used
availableCores() instead, then the system administrator can
limit the number of cores each user get to, say, two (2), by setting the
environment variable
R_PARALLELLY_AVAILABLECORES_FALLBACK=2. In contrast, it is
not possible to override what
parallel::detectCores() returns, cf. PR#17641 - WISH:
Make parallel::detectCores() agile to new env var
R_DEFAULT_CORES.
Similarly, availableCores() is also agile to CPU
limitations set by Unix control groups (cgroups), which is often used by
Linux containers (e.g. Docker, Apptainer / Singularity, and Podman) and
Kubernetes (K8s) environments. For example,
docker run --cpuset-cpus=0-2,8 ... sets the CPU affinity so
that the processes can only run on CPUs 0, 1, 2, and 8 on the host
system. In this case availableCores() detects this and
returns four (4). Another example is
docker run --cpu=3.4 ..., which throttles the CPU quota to
on average 3.4 CPUs on the host system. In this case
availableCores() detects this and returns three (3),
because it rounds to the nearest integer. In contrast,
parallel::detectCores() completely ignores such cgroups
settings and returns the number of CPUs on the host system, which
results in CPU overuse and degredated performance. Continous Integration
(CI) services (e.g. GitHub Actions, Travis CI, and Appveyor CI) and
cloud services (e.g. RStudio Cloud) use these types of cgroups settings
under the hood, which means availableCores() respects their
CPU allocations.
If running on an HPC cluster with a job scheduler, a script that uses
availableCores() will run the number of parallel workers
that the job scheduler has assigned to the job. For example, if we
submit a Slurm job as sbatch --cpus-per-task=16 ..., then
availableCores() returns 16, because it respects the
SLURM_* environment variables set by the scheduler. On Son
of Grid Engine (SGE), the scheduler sets NSLOTS when
submitting using qsub -pe smp 8 ... and
availableCores() returns eight (8). See
help("availableCores", package = "parallelly") for
currently supported job schedulers, which includes ‘Fujitsu Technical
Computing Suite’, ‘Load Sharing Facility’ (LSF), Simple Linux Utility
for Resource Management (Slurm), Sun Grid Engine/Oracle Grid Engine/Son
of Grid Engine (SGE), Univa Grid Engine (UGE), and TORQUE/PBS.
Of course, availableCores() respects also R options and
environment variables commonly used to specify the number of parallel
workers, e.g. R option mc.cores and Bioconductor
environment variable BIOCPARALLEL_WORKER_NUMBER. It will
also detect when running R CMD check and limit the number
of workers to two (2), which is the maximum number of parallel workers
allowed by the CRAN
Policies. This way you, as a package developer, know that your
package will always play by the rules on CRAN and Bioconductor.
If nothing is set that limits the number of cores, then
availableCores() falls back to
parallel::detectCores() and if that returns
NA_integer_ then one (1) is returned.
The below table summarize the benefits:
| availableCores() | parallel::detectCores() | |
|---|---|---|
| Guaranteed to return a positive integer | ✓ | no (may return
NA_integer_) |
| Safely use all but some cores | ✓ | no (may return zero or less) |
| Can be overridden, e.g. by a sysadm | ✓ | no |
| Respects cgroups and Linux containers | ✓ | no |
| Respects job scheduler allocations | ✓ | no |
| Respects CRAN policies | ✓ | no |
| Respects Bioconductor policies | ✓ | no |
The functions in this package originate from the future
package where we have used and validated them for several years. I moved
these functions to this separate package in 2020, because they are also
useful outside of the future framework. For backward-compatibility
reasons of the future framework, the R options and environment variables
that are prefixed with parallelly.* and
R_PARALLELLY_* can for the time being also be set with
future.* and R_FUTURE_* prefixes.
Submit parallelly to CRAN, with minimal changes compared to the corresponding functions in the future package (on CRAN as of 2020-10-20)
Update the future package to import and re-export the functions from the parallelly to maximize backward compatibility in the future framework (future 1.20.1 on CRAN as of 2020-11-03)
Switch to use 10-15% faster useXDR=FALSE
Implement same fast parallel setup of parallel PSOCK workers as in parallel (>= 4.0.0)
After having validated that there is no negative impact on the future
framework, allow for changes in the parallelly package,
e.g. renaming the R options and environment variable to be
parallelly.* and R_PARALLELLY_* while falling
back to future.* and R_FUTURE_*
Migrate, currently internal, UUID functions and export them,
e.g. uuid(), connectionUuid(), and
sessionUuid()
(https://github.com/HenrikBengtsson/Wishlist-for-R/issues/96). Because
R
does not have a built-in md5 checksum function that operates on
object, these functions require us adding a dependency on the
digest
package.
Add vignettes on how to set up cluster running on local or remote machines, including in Linux containers and on popular cloud services, and vignettes on common problems and how to troubleshoot them
Initially, backward compatibility for the future package is of top priority.
R package parallelly is available on CRAN and can be installed in R as:
install.packages("parallelly")To install the pre-release version that is available in Git branch
develop on GitHub, use:
remotes::install_github("HenrikBengtsson/parallelly", ref="develop")This will install the package from source.