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

These give you the hyperthreaded CPU count

1. multiprocessing.cpu_count()
2. os.cpu_count()

These give you the virtual machine CPU count

1. psutil.cpu_count()
2. numexpr.detect_number_of_cores()

Only matters if you works on VMs.

Answer 2

Another option if you don't have Python 2.6:

import commands
n = commands.getoutput("grep -c processor /proc/cpuinfo")

Answer 3

Another option is to use the psutil library, which always turn out useful in these situations:

>>> import psutil
>>> psutil.cpu_count()
2

This should work on any platform supported by psutil(Unix and Windows).

Note that in some occasions multiprocessing.cpu_count may raise a NotImplementedError while psutil will be able to obtain the number of CPUs. This is simply because psutil first tries to use the same techniques used by multiprocessing and, if those fail, it also uses other techniques.

Answer 4

multiprocessing.cpu_count() will return the number of logical CPUs, so if you have a quad-core CPU with hyperthreading, it will return 8. If you want the number of physical CPUs, use the python bindings to hwloc:

#!/usr/bin/env python
import hwloc
topology = hwloc.Topology()
topology.load()
print topology.get_nbobjs_by_type(hwloc.OBJ_CORE)

hwloc is designed to be portable across OSes and architectures.

Answer 5

In Python 3.4+: os.cpu_count().

multiprocessing.cpu_count() is implemented in terms of this function but raises NotImplementedError if os.cpu_count() returns None ("can't determine number of CPUs").

Answer 6

This may work for those of us who use different os/systems, but want to get the best of all worlds:

import os
workers = os.cpu_count()
if 'sched_getaffinity' in dir(os):
    workers = len(os.sched_getaffinity(0))