Obtain the number of CPU cores in Julia

Question

I want to obtain the number of cores available in Julia. Currently I am doing the following:

using PyCall
@pyimport psutil
nCores = psutil.cpu_count()

This calls a Python function. I would like, however, to use some Julia procedure. How can it be done?

Blindfish

The constant Base.CPU_CORES may do the trick (but it counts hyper-threading cores as full-cores). – Xanthin

psutil.cpu_count() reports 8 cores on my Intel i7-2720QM-powered laptop so it also counts hyper-threading cores as full cores. – Litchi

This question needs a new answer. All the current answers are deprecated. – Cotter

Answer 1

I'm not 100% certain about this, but CPU_CORES returns the number of (hyper-threading) cores on my machine (OSX 10.9.5 and Julia 0.3.5), even when I start Julia in serial mode. I've been checking the number of available cores using nworkers() and nprocs(). Starting up Julia without the -p flag this returns 1 for both.

When I start julia as julia -p 4

julia> nprocs()
5

julia> nworkers()
4

In both cases CPU_CORES returns 8.

Orvalorvan

Base.CPU_CORES is deprecated. In order to obtain this information, you should try Sys.CPU_CORES. More details could be found here – Imp

For logical cores Sys.CPU_CORES, and for physical cores you can use CpuId.jl or Hwloc.jl – Frodina

Answer 2

Sys.CPU_CORES is not defined in Julia v1.1.0. However, the following does the job.

length(Sys.cpu_info())

Branks

It will bring the number of logical cores and not physical cores. – Mozellemozes

Answer 3

In recent versions of Julia, you can use Sys.CPU_CORES (and not Base.CPU_CORES as some answers mentioned). Tested on 0.6.

Moorman

Answer 4

According to the documentation, the "number of cores available" can be limited by the JULIA_NUM_THREADS environment variable.

To see the number of threads available to Julia, use

Threads.nthreads()

Joses

Answer 5

Sys.CPU_CORES is undefined in Julia 1.0.0 (at least, running on a macbook—I don't imagine that would make a difference). Instead, use Sys.CPU_THREADS.

Belk

Note, however, that cpu threads != cores, in particular for modern intel cpus with hyper threading. – Tallboy

But I think the old Sys.CPU_CORES did count HT as cores as well. – Tallboy

Answer 6

I don't know Julia but "psutil.cpu_count(logical=False)" in Python gives you the number of physical CPUs (hyper threaded ones are not counted).

Kiaochow

Answer 7

In Julia 1.10, you can do

length(Sys.cpu_info())

Cotter

Answer 8

Hwloc is a useful package for this purpose.

using Hwloc
num_physical_cores()
num_virtual_cores()

Osmanli

Why do you prefer this over previous answers? What benefits does it provide that they don’t offer? – Bernadette

Because, assuming that Hwloc is well implemented, it gives you directly the number of physical and virtual cores. – Osmanli