Obtain the number of CPU cores in Julia

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

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

length(Sys.cpu_info())

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.