Fix maximum number of ProcessPool workers (#7385)

Set the default for the maximum number of workers to `None` which lets
the executor decide. Practically that makes it equal to the number of
cores or the internal limit (whichever is lower). That's lower than our
1.5 * cores (which I imagine is to leave room for a bit of IO) but at
least will never hit the limit.

The other usage of ProcessPoolExecutor (in Dependencies.py) always takes
a number rather than guessing a default, so should be safe.

Fixes #7384

Author: da-woods

Cython/Build/Cythonize.py

@@ -12,10 +12,8 @@
 
 try:
     import multiprocessing
-    parallel_compiles = int(multiprocessing.cpu_count() * 1.5)
 except ImportError:
     multiprocessing = None
-    parallel_compiles = 0
 
 
 def find_package_base(path):
@@ -85,7 +83,8 @@ def _build(ext_modules, parallel):
     if not modcount:
         return
 
-    serial_execution_mode = modcount == 1 or parallel < 2
+    serial_execution_mode = modcount == 1 or (
+        parallel is not None and parallel < 2)
 
     try:
         pool_cm = (
@@ -248,8 +247,8 @@ def create_args_parser():
                       help="use CODESTRING as pre-benchmark setup code for --bench")
 
     parser.add_argument('-j', '--parallel', dest='parallel', metavar='N',
-                      type=int, default=parallel_compiles,
-                      help=f'run builds in N parallel jobs (default: {parallel_compiles or 1})')
+                      type=int, default=None,
+                      help='run builds in N parallel jobs (default: CPU count)')
     parser.add_argument('-f', '--force', dest='force', action='store_true', default=None,
                       help='force recompilation')
     parser.add_argument('-q', '--quiet', dest='quiet', action='store_true', default=None,

Cython/Build/Tests/TestCythonizeArgsParser.py

@@ -1,6 +1,5 @@
 from Cython.Build.Cythonize import (
     create_args_parser, parse_args_raw, parse_args,
-    parallel_compiles
 )
 
 from Cython.Compiler import Options
@@ -22,7 +21,9 @@ def setUp(self):
     def are_default(self, options, skip):
         # empty containers
         empty_containers = ['directives', 'compile_time_env', 'options', 'excludes']
-        are_none = ['language_level', 'annotate', 'build', 'build_inplace', 'force', 'quiet', 'lenient', 'keep_going', 'no_docstrings']
+        are_none = [
+            'language_level', 'annotate', 'build', 'build_inplace', 'force', 'quiet', 'lenient', 'keep_going', 'no_docstrings', 'parallel'
+        ]
         for opt_name in empty_containers:
             if len(getattr(options, opt_name))!=0 and (opt_name not in skip):
                 self.assertEqual(opt_name,"", msg="For option "+opt_name)
@@ -31,8 +32,6 @@ def are_default(self, options, skip):
             if (getattr(options, opt_name) is not None) and (opt_name not in skip):
                 self.assertEqual(opt_name,"", msg="For option "+opt_name)
                 return False
-        if options.parallel!=parallel_compiles and ('parallel' not in skip):
-            return False
         return True
 
     # testing directives: