Can people run this script and see if it gives accurate count of CPUs on their system. Report results in the comments please. Thanks! Oh! And don't forget to report what kind of system you are running this on (linux, windows, mac, etc.). UPDATE: Revised version that uses the Java runtime if running under JRuby.

cpu.rb

require 'rbconfig'

# Based on a script at:
#   http://stackoverflow.com/questions/891537/ruby-detect-number-of-cpus-installed
class CpuCounter
  def self.count
    new.count
  end

  def count
    if defined?(Java::Java)
      count_via_java_runtime
    else
      case RbConfig::CONFIG['host_os']
      when /darwin9/
        count_via_hwprefs_cpu_count
      when /darwin/
        count_via_hwprefs_thread_count || count_via_sysctl
      when /linux/
        count_via_cpuinfo
      when /freebsd/
        count_via_sysctl
      when /mswin|mingw/
        count_via_win32
      end
    end
  end

  def count_via_java_runtime
    Java::Java.lang.Runtime.getRuntime.availableProcessors
  end

  def count_via_win32
    require 'win32ole'
    wmi = WIN32OLE.connect("winmgmts://")
    cpu = wmi.ExecQuery("select NumberOfCores from Win32_Processor") # TODO count hyper-threaded in this
    cpu.to_enum.first.NumberOfCores
  end

  def count_via_cpuinfo
    open('/proc/cpuinfo') { |f| f.readlines }.grep(/processor/).size
  end

  def count_via_hwprefs_thread_count
    run 'hwprefs', 'thread_count'
  end

  def count_via_hwprefs_cpu_count
    run 'hwprefs', 'cpu_count'
  end

  def count_via_sysctl
    run 'sysctl', '-n hw.ncpu'
  end

  def run(command, args)
    cmd = resolve_command(command)
    if cmd
      `#{cmd} #{args}`.to_i
    else
      nil
    end
  end

  def resolve_command(command)
    try_command("/sbin", command) ||
      try_command("/usr/sbin", command) ||
      in_path_command(command)
  end

  def try_command(dir, command)
    path = File.join(dir, command)
    File.exist?(path) ? path : nil
  end

  def in_path_command(command)
    `which #{command}` != '' ? command : nil
  end
end

if $0 == __FILE__
  puts "Number of CPUs = #{CpuCounter.count}"
end

Looks correct here too:

$ ruby cpu.rb
Number of CPUs = 8
$ uname -a
Linux notebook 3.2.0-29-generic #46-Ubuntu SMP Fri Jul 27 17:03:23 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux