detectCores: Detect the Number of CPU Cores

Description Usage Arguments Details Value Note Author(s) Examples


Attempt to detect the number of CPU cores on the current host.


detectCores(all.tests = FALSE, logical = TRUE)



Logical: if true apply all known tests.


Logical: if possible, use the number of physical CPUs/cores (if FALSE) or logical CPUs (if TRUE). Currently this is honoured only on Linux, macOS, Sparc Solaris and Windows.


This attempts to detect the number of available CPU cores.

It has methods to do so for Linux, macOS, FreeBSD, OpenBSD, Solaris, Irix and Windows. detectCores(TRUE) could be tried on other Unix-alike systems.

Prior to R 3.3.0 the default was logical = FALSE except on Windows, but logical = TRUE had an effect only on Sparc Solaris and Windows (where it was the default).


An integer, NA if the answer is unknown.

Exactly what this represents is OS-dependent: where possible by default it counts logical (e.g., hyperthreaded) CPUs and not physical cores or packages.

Under macOS there is a further distinction between ‘available in the current power management mode’ and ‘could be available this boot’, and this function returns the first.

On Sparc Solaris logical = FALSE returns the number of physical cores and logical = TRUE returns the number of available hardware threads. (Some Sparc CPUs which do have multiple cores per CPU, others have multiple threads per core and some have both.) For example, the UltraSparc T2 CPU in the CRAN check server is a single physical CPU with 8 cores, and each core supports 8 hardware threads. So detectCores(logical = FALSE) returns 8, and detectCores(logical = TRUE) returns 64.

Where virtual machines are in use, one would hope that the result represents the number of CPUs available (or potentially available) to that particular VM.


This 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.


Simon Urbanek and Brian Ripley



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