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.

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

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')

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.

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”).

len(os.sched_getaffinity(0)) is what you usually want

https://docs.python.org/3/library/os.html#os.sched_getaffinity

os.sched_getaffinity(0) (added in Python 3) returns the set of CPUs available considering the sched_setaffinity Linux system call, which limits which CPUs a process and its children can run on.

0 means to get the value for the current process. The function returns a set() of allowed CPUs, thus the need for len().

multiprocessing.cpu_count() on the other hand just returns the total number of physical CPUs.

The difference is especially important because certain cluster management systems such as Platform LSF limit job CPU usage with sched_getaffinity.

Therefore, if you use multiprocessing.cpu_count(), your script might try to use way more cores than it has available, which may lead to overload and timeouts.

We can see the difference concretely by restricting the affinity with the taskset utility.

For example, if I restrict Python to just 1 core (core 0) in my 16 core system:

taskset -c 0 ./main.py

with the test script:

main.py

#!/usr/bin/env python3

import multiprocessing
import os

print(multiprocessing.cpu_count())
print(len(os.sched_getaffinity(0)))

then the output is:

16
1

nproc however does respect the affinity by default and:

taskset -c 0 nproc

outputs:

1

and man nproc makes that quite explicit:

print the number of processing units available

nproc has the --all flag for the less common case that you want to get the physical CPU count:

taskset -c 0 nproc --all

The only downside of this method is that this appears to be UNIX only. I supposed Windows must have a similar affinity API, possibly SetProcessAffinityMask, so I wonder why it hasn’t been ported. But I know nothing about Windows.

Tested in Ubuntu 16.04, Python 3.5.2.

platform independent:

psutil.cpu_count(logical=False)

https://github.com/giampaolo/psutil/blob/master/INSTALL.rst

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.

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.

Can’t figure out how to add to the code or reply to the message but here’s support for jython that you can tack in before you give up:

# jython
try:
    from java.lang import Runtime
    runtime = Runtime.getRuntime()
    res = runtime.availableProcessors()
    if res > 0:
        return res
except ImportError:
    pass

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))

You can also use “joblib” for this purpose.

import joblib
print joblib.cpu_count()

This method will give you the number of cpus in the system. joblib needs to be installed though. More information on joblib can be found here https://pythonhosted.org/joblib/parallel.html

Alternatively you can use numexpr package of python. It has lot of simple functions helpful for getting information about the system cpu.

import numexpr as ne
print ne.detect_number_of_cores()

Another option if you don’t have Python 2.6:

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