cpu_count() unreliable on Windows

[mark-summerfield]

BPO	23037
Nosy	@vstinner, @tjguk, @mark-summerfield, @zware, @zooba

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2014-12-12.13:13:57.454>
created_at = <Date 2014-12-12.12:36:11.806>
labels = ['type-bug', 'library', 'OS-windows']
title = 'cpu_count() unreliable on Windows'
updated_at = <Date 2014-12-12.13:13:57.452>
user = 'https://github.com/mark-summerfield'

bugs.python.org fields:

activity = <Date 2014-12-12.13:13:57.452>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2014-12-12.13:13:57.454>
closer = 'vstinner'
components = ['Library (Lib)', 'Windows']
creation = <Date 2014-12-12.12:36:11.806>
creator = 'mark'
dependencies = []
files = []
hgrepos = []
issue_num = 23037
keywords = []
message_count = 2.0
messages = ['232540', '232544']
nosy_count = 5.0
nosy_names = ['vstinner', 'tim.golden', 'mark', 'zach.ware', 'steve.dower']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue23037'
versions = ['Python 2.7', 'Python 3.2', 'Python 3.3', 'Python 3.4']

[mark-summerfield]

In message
http://bugs.python.org/issue17914#msg188626
Victor Stenner says

"On Windows, GetSystemInfo() is called instead of reading an environment variable. I suppose that this function is more reliable."

From my reading, and based on feedback from one of my customers, I believe he is correct and that GetSystemInfo() ought to be used on Windows. (It is available in pywin32 win32api.)

[vstinner]

The code getting the number of processors on Windows is different between the multiprocessing (Python 3.3) and os (Python 3.5) modules.

multiprocessing (old code):

    try:
        num = int(os.environ['NUMBER_OF_PROCESSORS'])
    except (ValueError, KeyError):
        num = 0

os (new code):

SYSTEM_INFO sysinfo;
GetSystemInfo(&sysinfo);
ncpu = sysinfo.dwNumberOfProcessors;

os.cpu_count() is already implemented with GetSystemInfo() in Python 3.5, so this issue can be closed. Thanks for the reminder Mark, it was useful to double check.

If you want to take a look at the source code:
https://hg.python.org/cpython/file/35b7dde7fd53/Modules/posixmodule.c#l16064