Detect number of CPUs installed

EDIT: Now rails ships with concurrent-ruby as a dependency so it's probably the best solution;

$ gem install concurrent-ruby
$ irb
irb(main):001:0> require 'concurrent'
=> true
irb(main):002:0> Concurrent.processor_count
=> 8
irb(main):003:0> Concurrent.physical_processor_count
=> 4

see http://ruby-concurrency.github.io/concurrent-ruby/master/Concurrent.html for more info. Because it does both physical and logical cores, it's better than the inbuilt Etc.nprocessors.

and here is the previous answer;

$ gem install facter
$ irb
irb(main):001:0> require 'facter'
=> true
irb(main):002:0> puts Facter.value('processors')['count']
4
=> nil
irb(main):003:0> 

This facter gem is the best if you want other facts about the system too, it's not platform specific and designed to do this exact thing.

UPDATE: updated to include Nathan Kleyn's tip on the api change.

with JRuby you can check it with the following Java code:

 Runtime runtime = Runtime.getRuntime();   
 int numberOfProcessors = runtime.availableProcessors(); 

Surely if you can cat it, you can open, read and close it using the standard features of the language without resorting to a system()-type call.

You may just need to detect what platform you're on dynamically and either:

• use the /proc/cpuinfo "file" for Linux; or
• communicate with WMI for Windows.

That last line can use:

require 'win32ole'
wmi = WIN32OLE.connect("winmgmts://")
info = wmi.ExecQuery ("select * from Win32_ComputerSystem")

Then use info's NumberOfProcessors item.

on Mac:

thiago-pradis-macbook:~ tchandy$ hwprefs cpu_count

2