Use pathos for multiprocessing

qiskit/tools/parallel.py

@@ -49,8 +49,8 @@
 """
 
 import os
-from concurrent.futures import ProcessPoolExecutor
 import sys
+from pathos.pools import ProcessPool
 
 from qiskit.exceptions import QiskitError
 from qiskit.utils.multiprocessing import local_hardware_info
@@ -68,12 +68,6 @@
     # On python 3.9 default false to avoid deadlock issues
     elif sys.version_info[0] == 3 and sys.version_info[1] == 9:
         PARALLEL_DEFAULT = False
-    # On macOS default false on Python >=3.8
-    elif sys.platform == "darwin":
-        if sys.version_info[0] == 3 and sys.version_info[1] >= 8:
-            PARALLEL_DEFAULT = False
-        else:
-            PARALLEL_DEFAULT = True
     # On linux (and other OSes) default to True
     else:
         PARALLEL_DEFAULT = True
@@ -146,12 +140,10 @@ def _callback(_):
     ):
         os.environ["QISKIT_IN_PARALLEL"] = "TRUE"
         try:
-            results = []
-            with ProcessPoolExecutor(max_workers=num_processes) as executor:
-                param = map(lambda value: (task, value, task_args, task_kwargs), values)
-                future = executor.map(_task_wrapper, param)
+            pool = ProcessPool(nodes=num_processes)
+            param = map(lambda value: (task, value, task_args, task_kwargs), values)
+            results = pool.map(_task_wrapper, param)
 
-            results = list(future)
             Publisher().publish("terra.parallel.done", len(results))
 
         except (KeyboardInterrupt, Exception) as error:

qiskit/utils/multiprocessing.py

@@ -11,13 +11,14 @@
 # that they have been altered from the originals.
 
 """Multiprocessing utilities"""
-
+import os
 import multiprocessing as mp
 import platform
-import sys
 
 import psutil
 
+MAIN_PROCESS = os.getpid()
+
 
 def local_hardware_info():
     """Basic hardware information about the local machine.
@@ -38,17 +39,4 @@ def local_hardware_info():
 
 def is_main_process():
     """Checks whether the current process is the main one"""
-    if platform.system() == "Windows":
-        return not isinstance(mp.current_process(), mp.context.SpawnProcess)
-    else:
-        return not (
-            isinstance(mp.current_process(), (mp.context.ForkProcess, mp.context.SpawnProcess))
-            # In python 3.5 and 3.6, processes created by "ProcessPoolExecutor" are not
-            # mp.context.ForkProcess or mp.context.SpawnProcess. As a workaround,
-            # "name" of the process is checked instead.
-            or (
-                sys.version_info[0] == 3
-                and (sys.version_info[1] == 5 or sys.version_info[1] == 6)
-                and mp.current_process().name != "MainProcess"
-            )
-        )
+    return mp.current_process().pid == MAIN_PROCESS

requirements.txt

@@ -6,6 +6,7 @@ psutil>=5
 scipy>=1.4
 sympy>=1.3
 dill>=0.3
+pathos
 python-constraint>=1.4
 python-dateutil>=2.8.0
 symengine>0.7 ; platform_machine == 'x86_64' or platform_machine == 'aarch64' or platform_machine == 'ppc64le' or platform_machine == 'amd64' or platform_machine == 'arm64'