How to find out the number of CPUs using python

Question

I want to know the number of CPUs on the local machine using Python. The result should be user/real as output by time(1) when called with an optimally scaling userspace-only program.

Answer 1

If you have python with a version >= 2.6 you can simply use

import multiprocessing  multiprocessing.cpu_count() 

http://docs.python.org/library/multiprocessing.html#multiprocessing.cpu_count

Answer 2

If you're interested into the number of processors available to your current process, you have to check cpuset first. Otherwise (or if cpuset is not in use), multiprocessing.cpu_count() is the way to go in Python 2.6 and newer. The following method falls back to a couple of alternative methods in older versions of Python:

import os import re import subprocess   def available_cpu_count():     """ Number of available virtual or physical CPUs on this system, i.e.     user/real as output by time(1) when called with an optimally scaling     userspace-only program"""      # cpuset     # cpuset may restrict the number of *available* processors     try:         m = re.search(r'(?m)^Cpus_allowed:\s*(.*)$',                       open('/proc/self/status').read())         if m:             res = bin(int(m.group(1).replace(',', ''), 16)).count('1')             if res > 0:                 return res     except IOError:         pass      # Python 2.6+     try:         import multiprocessing         return multiprocessing.cpu_count()     except (ImportError, NotImplementedError):         pass      # https://github.com/giampaolo/psutil     try:         import psutil         return psutil.cpu_count()   # psutil.NUM_CPUS on old versions     except (ImportError, AttributeError):         pass      # POSIX     try:         res = int(os.sysconf('SC_NPROCESSORS_ONLN'))          if res > 0:             return res     except (AttributeError, ValueError):         pass      # Windows     try:         res = int(os.environ['NUMBER_OF_PROCESSORS'])          if res > 0:             return res     except (KeyError, ValueError):         pass      # jython     try:         from java.lang import Runtime         runtime = Runtime.getRuntime()         res = runtime.availableProcessors()         if res > 0:             return res     except ImportError:         pass      # BSD     try:         sysctl = subprocess.Popen(['sysctl', '-n', 'hw.ncpu'],                                   stdout=subprocess.PIPE)         scStdout = sysctl.communicate()[0]         res = int(scStdout)          if res > 0:             return res     except (OSError, ValueError):         pass      # Linux     try:         res = open('/proc/cpuinfo').read().count('processor\t:')          if res > 0:             return res     except IOError:         pass      # Solaris     try:         pseudoDevices = os.listdir('/devices/pseudo/')         res = 0         for pd in pseudoDevices:             if re.match(r'^cpuid@[0-9]+$', pd):                 res += 1          if res > 0:             return res     except OSError:         pass      # Other UNIXes (heuristic)     try:         try:             dmesg = open('/var/run/dmesg.boot').read()         except IOError:             dmesgProcess = subprocess.Popen(['dmesg'], stdout=subprocess.PIPE)             dmesg = dmesgProcess.communicate()[0]          res = 0         while '\ncpu' + str(res) + ':' in dmesg:             res += 1          if res > 0:             return res     except OSError:         pass      raise Exception('Can not determine number of CPUs on this system')