Please Avoid detectCores() in your R Packages

This article is originally published at https://www.jottr.org/

The detectCores() function of the parallel package is probably one of the most used functions when it comes to setting the number of parallel workers to use in R. In this blog post, I’ll try to explain why using it is not always a good idea. Already now, I am going to make a bold request and ask you to:

Please avoid using parallel::detectCores() in your package!

By reading this blog post, I hope you become more aware of the different problems that arise from using detectCores() and how they might affect you and your users of your code.

TL;DR

If you don’t have time to read everything, but will take my word that we should avoid detectCores(), then the quick summary is that you basically have two choices for the number of parallel workers to use by default;

1. Have your code run with a single core by default (i.e. sequentially), or

2. replace all parallel::detectCores() with parallelly::availableCores().

I’m in the conservative camp and recommend the first alternative. Using sequential processing by default, where the user has to make an explicit choice to run in parallel, significantly lowers the risk for clogging up the CPUs (left panel in Figure 1), especially when there are other things running on the same machine.

The second alternative is useful if you’re not ready to make the move to run sequentially by default. The availableCores() function of the parallelly package is fully backward compatible with detectCores(), while it avoids the most common problems that comes with detectCores(), plus it is agile to a lot more CPU-related settings, including settings that the end-user, the systems administrator, job schedulers and Linux containers control. It is designed to take care of common overuse issues so that you do not have to spend time worry about them.

Background

There are several problems with using detectCores() from the parallel package for deciding how many parallel workers to use. But before we get there, I want you to know that we find this function commonly used in R script and R packages, and frequently suggested in tutorials. So, do not feel ashamed if you use it.

If we scan the code of the R packages on CRAN (e.g. by searching GitHub¹), or on Bioconductor (e.g. by searching Bioc::CodeSearch) we find many cases where detectCores() is used. Here are some variants we see in the wild:

cl <- makeCluster(detectCores())
cl <- makeCluster(detectCores() - 1)
y <- mclapply(..., mc.cores = detectCores())
registerDoParallel(detectCores())

We also find functions that let the user choose the number of workers via some argument, which defaults to detectCores(). Sometimes the default is explicit, as in:

fast_fcn <- function(x, ncores = parallel::detectCores()) {
  if (ncores > 1) {
    cl <- makeCluster(ncores)
    ...
  }
}

and sometimes it’s implicit, as in:

fast_fcn <- function(x, ncores = NULL) {
  if (is.null(ncores)) 
    ncores <- parallel::detectCores() - 1
  if (ncores > 1) {
    cl <- makeCluster(ncores)
    ...
  }
}

As we will see next, all the above examples are potentially buggy and might result in run-time errors.

Common mistakes when using detectCores()

Issue 1: detectCores() may return a missing value

A small, but important detail about detectCores() that is often missed is the following section in help("detectCores", package = "parallel"):

Value

An integer, NA if the answer is unknown.

Because of this, we cannot rely on:

ncores <- detectCores()

to always work, i.e. we might end up with errors like:

ncores <- detectCores()
workers <- parallel::makeCluster(ncores)
Error in makePSOCKcluster(names = spec, ...) : 
  numeric 'names' must be >= 1

We need to account for this, especially as package developers. One way to handle it is simply by using:

ncores <- detectCores()
if (is.na(ncores)) ncores <- 1L

or, by using the following shorter, but also harder to understand, one-liner:

ncores <- max(1L, detectCores(), na.rm = TRUE)

This construct is guaranteed to always return at least one core.

Shameless advertisement for the parallelly package: In contrast to detectCores(), parallelly::availableCores() handles the above case automatically, and it guarantees to always return at least one core.

Issue 2: detectCores() may return one

Although it’s rare to run into hardware with single-core CPUs these days, you might run into a virtual machine (VM) configured to have a single core. Because of this, you cannot reliably use:

ncores <- detectCores() - 1L

or

ncores <- detectCores() - 2L

in your code. If you use these constructs, a user of your code might end up with zero or a negative number of cores here, which another way we can end up with an error downstream. A real-world example of this problem can be found in continous integration (CI) services, e.g. detectCores() returns 2 in GitHub Actions jobs. So, we need to account also for this case, which we can do by using the above max() solution, e.g.

ncores <- max(1L, detectCores() - 2L, na.rm = TRUE)

This is guaranteed to always return at least one.

Shameless advertisement for the parallelly package: In contrast, parallelly::availableCores() handles this case via argument omit, which makes it easier to understand the code, e.g.

ncores <- availableCores(omit = 2L)

This construct is guaranteed to return at least one core, e.g. if there are one, two, or three CPU cores on this machine, ncores will be one in all three cases.

Issue 3: detectCores() does not give the number of “allowed” cores

There’s a note in help("detectCores", package = "parallel") that touches on the above problems, but also on other important limitations that we should know of:

Note

This [= detectCores()] is not suitable for use directly for the mc.cores argument of mclapply nor specifying the number of cores in makeCluster. First because it may return NA, second because it does not give the number of allowed cores, and third because on Sparc Solaris and some Windows boxes it is not reasonable to try to use all the logical CPUs at once.

When is this relevant? The answer is: Always! This is because as package developers, we cannot really know when this occurs, because we never know on what type of hardware and system our code will run on. So, we have to account for these unknowns too.

Let’s look at some real-world case where using detectCores() can become a real issue.

3a. A personal computer

A user might want to run other software tools at the same time while running the R analysis. A very common pattern we find in R code is to save one core for other purposes, say, browsing the web, e.g.

ncores <- detectCores() - 1L

This is a good start. It is the first step toward your software tool acknowledging that there might be other things running on the same machine. However, contrary to end-users, we as package developers cannot know how many cores the user needs, or wishes, to set aside. Because of this, it is better to let the user make this decision.

A related scenario is when the user wants to run two concurrent R sessions on the same machine, both using your code. If your code assumes it can use all cores on the machine (i.e. detectCores() cores), the user will end up running the machine at 200% of its capacity. Whenever we use over 100% of the available CPU resources, we get penalized and waste our computational cycles on overhead from context switching, sub-optimal memory access, and more. This is where we end up with the situation illustrated in the left part of Figure 1.

Note also that users might not know that they use an R function that runs on all cores by default. They might not even be aware that this is a problem. Now, imagine if the user runs three or four such R sessions, resulting in a 300-400% CPU load. This is when things start to run slowly. The computer will be sluggish, maybe unresponsive, and mostly likely going to get very hot (“we’re frying the computer”). By the time the four concurrent R processes complete, the user might have been able to finish six to eight similar processes if they would not have been fighting each other for the limited CPU resources.

A shared computer

In the academia and the industry, it is common that several users share the same compute server och set of compute nodes. It might be as simple as they SSH into a shared machine with many cores and large amounts of memory to run their analysis there. On such setups, load balancing between users is often based on an honor system, where each user checks how many resources are available before launching an analysis. This helps to make sure they don’t end up using too many cores, or too much memory, slowing down the computer for everyone else.

Now, imagine they run a software tool that uses all CPU cores by default. In that case, there is a significant risk they will step on the other users’ processes, slowing everything down for everyone, especially if there is already a big load on the machine. From my experience in academia, this happens frequently. The user causing the problem is often not aware, because they just launch the problematic software with the default settings, leave it running, with a plan to coming back to it a few hours or a few days later. In the meantime, other users might wonder why their command-line prompts become sluggish or even non-responsive, and their analyses suddenly take forever to complete. Eventually, someone or something alerts the systems administrators to the problem, who end up having to drop everything else and start troubleshooting. This often results in them terminating the wild-running processes and reaching out to the user who runs the problematic software, which leads to a large amount of time and resources being wasted among users and administrators. All this is only because we designed our R package to use all cores by default. This is not a made-up toy story; it is a very likely scenario that happens on shared servers if you make detectCores() the default in your R code.

Shameless advertisement for the parallelly package: In contrast to detectCores(), if you use parallelly::availableCores() the user, or the systems administrator, can limit the default number of CPU cores returned by setting environment variable R_PARALLELLY_AVAILABLECORES_FALLBACK. For instance, by setting it to R_PARALLELLY_AVAILABLECORES_FALLBACK=2 centrally, availableCores() will, unless there are other settings that allow the process to use more, return two cores regardless how many CPU cores the machine has. This will lower the damage any single process can inflict on the system. It will take many such processes running at the same time in order for them to have an overall a negative impact. The risk for that to happen by mistake is much lower than when using detectCores() by default.

A shared compute cluster with many machines

Other, larger compute systems, often referred to as high-performance compute (HPC) cluster, have a job scheduler for running scripts in batches distributed across multiple machines. When users submit their scripts to the scheduler’s job queue, they request how many cores and how much memory each job requires. For example, a user on a Slurm cluster can request that their run_my_rscript.sh script gets to run with 48 CPU cores and 256 GiB of RAM by submitting it to the scheduler as:

sbatch --cpus-per-task=48 --mem=256G run_my_rscript.sh

The scheduler keeps track of all running and queued jobs, and when enough compute slots are freed up, it will launch the next job in the queue, giving it the compute resources it requested. This is a very convenient and efficient way to batch process a large amount of analyses coming from many users.

However, just like with a shared server, it is important that the software tools running this way respect the compute resources that the job scheduler allotted to the job. The detectCores() function does not know about job schedulers - all it does is return the number of CPU cores on the current machine regardless of how many cores the job has been allotted by the scheduler. So, if your R package uses detectCores() cores by default, then it will overuse the CPUs and slow things down for everyone running on the same compute node. Again, when this happens, it often slows everything done and triggers lots of wasted user and admin efforts spent on troubleshooting and communication back and forth.

Shameless advertisement for the parallelly package: In contrast, parallelly::availableCores() respects the number of CPU slots that the job scheduler has given to the job. It recognizes environment variables set by our most common HPC schedulers, including Fujitsu Technical Computing Suite (PJM), Grid Engine (SGE), Load Sharing Facility (LSF), PBS/Torque, and Simple Linux Utility for Resource Management (Slurm).

Running R via CGroups on in a Linux container

This far, we have been concerned about the overuse of the CPU cores affecting other processes and other users running on the same machine. Some systems are configured to protect against misbehaving software from affecting other users. In Linux, this can be done with so-called control groups (“cgroups”), where a process gets allotted a certain amount of CPU cores. If the process uses too many parallel workers, they cannot break out from the sandbox set up by cgroups. From the outside, it will look like the process uses its maximum amount of allocated CPU cores. Some HPC job schedulers have this feature enabled, but not all of them. You find the same feature for Linux containers, e.g. we can limit the number of CPU cores, or throttle the CPU load, using command-line options when you launch a Docker container, e.g. docker run --cpuset-cpus=0-2,8 … or docker run --cpu=3.4 ….

So, if you are a user on a system where compute resources are compartmentalized this way, you run a much lower risk for wreaking havoc on a shared system. That is good news, but if you run too many parallel workers, that is, try to use more cores than available to you, then you will clog up your own analysis. The behavior would be the same as if you request 96 parallel workers on your local eight-core notebook (the scenario in the left panel of Figure 1), with the exception that you will not overheat the computer.

The problem with detectCores() is that it returns the number of CPU cores on the hardware, regardless of the cgroups settings. So, if your R process is limited to eight cores by cgroups, and you use ncores = detectCores() on a 96-core machine, you will end up running 96 parallel workers fighting for the resources on eight cores. A real-world example of this happens for those of you who have a free account on RStudio Cloud. In that case, you are given only a single CPU core to run your R code on, but the underlying machine typically has 16 cores. If you use detectCores() there, you will end up creating 16 parallel workers, running on the same CPU core, which is a very ineffecient way to run the code.

Shameless advertisement for the parallelly package: In contrast to detectCores(), parallelly::availableCores() respects cgroups, and will return eight cores instead of 96 in the above example, and a single core on a free RStudio Cloud account.

My opinionated recommendation

As developers, I think we should at least be aware of these problems, and acknowledge that they exist and they are indeed real problem that people run into “out there”. We should also accept that we cannot predict on what type of compute environment our R code will run on. Unfortunately, I don’t have a magic solution that addresses all the problems reported here. That said, I think the best we can do is to be conservative and don’t make hard-coded decisions on parallelization in our R packages and R scripts.

Because of this, I argue that the safest is to design your R package to run sequentially by default (e.g. ncores = 1L), and leave it to the user to decide on the number of parallel workers to use.

The second-best alternative that I can come up with, is to replace detectCores() with availableCores(), e.g. ncores = parallelly::availableCores(). It is designed to respect common system and R settings that control the number of allowed CPU cores. It also respects R options and environment variables commonly used to limit CPU usage, including those set by our most common HPC job schedulers. In addition, it is possible to control the fallback behavior so that it uses only a few cores when nothing else being set. For example, if the environment variable R_PARALLELLY_AVAILABLECORES_FALLBACK is set to 2, then availableCores() returns two cores by default, unless other settings allowing more are available. A conservative systems administrator may want to set export R_PARALLELLY_AVAILABLECORES_FALLBACK=1 in /etc/profile.d/single-core-by-default.sh. To see other benefits from using availableCores(), see https://parallelly.futureverse.org.

Believe it or not, there’s actually more to be said on this topic, but I think this is already more than a mouthful, so I will save that for another blog post. If you made it this far, I applaud you and I thank you for your interest. If you agree, or disagree, or have additional thoughts around this, please feel free to reach out on the Future Discussions Forum.

Over and out,

Henrik

¹ Searching code on GitHub, requires you to log in to GitHub.

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