Bug 1476390 - [python-test] Default number of processes to multiprocessing.cpu_count() r=davehunt

Differential Revision: https://phabricator.services.mozilla.com/D2201

python/mach_commands.py

@@ -9,6 +9,7 @@ import logging
 import os
 import sys
 import tempfile
+from multiprocessing import cpu_count
 
 from concurrent.futures import (
     ThreadPoolExecutor,
@@ -73,9 +74,10 @@ class MachCommands(MachCommandBase):
                           'Python version, Pipenv will automatically scan your '
                           'system for a Python that matches that given version.')
     @CommandArgument('-j', '--jobs',
-                     default=1,
+                     default=None,
                      type=int,
-                     help='Number of concurrent jobs to run. Default is 1.')
+                     help='Number of concurrent jobs to run. Default is the number of CPUs '
+                          'in the system.')
     @CommandArgument('--subsuite',
                      default=None,
                      help=('Python subsuite to run. If not specified, all subsuites are run. '
@@ -97,7 +99,7 @@ class MachCommands(MachCommandBase):
                          test_objects=None,
                          subsuite=None,
                          verbose=False,
-                         jobs=1,
+                         jobs=None,
                          python=None,
                          **kwargs):
         self.activate_pipenv(pipfile=None, populate=True, python=python)
@@ -143,7 +145,7 @@ class MachCommands(MachCommandBase):
             else:
                 parallel.append(test)
 
-        self.jobs = jobs
+        self.jobs = jobs or cpu_count()
         self.terminate = False
         self.verbose = verbose