Bump minimum supported symengine version for built-in pickle support

qiskit/circuit/parameterexpression.py

@@ -506,34 +506,6 @@ def __eq__(self, other):
             return len(self.parameters) == 0 and complex(self._symbol_expr) == other
         return False
 
-    def __getstate__(self):
-        if _optionals.HAS_SYMENGINE:
-            from sympy import sympify
-
-            symbols = {k: sympify(v) for k, v in self._parameter_symbols.items()}
-            expr = sympify(self._symbol_expr)
-            return {"type": "symengine", "symbols": symbols, "expr": expr, "names": self._names}
-        else:
-            return {
-                "type": "sympy",
-                "symbols": self._parameter_symbols,
-                "expr": self._symbol_expr,
-                "names": self._names,
-            }
-
-    def __setstate__(self, state):
-        if state["type"] == "symengine":
-            import symengine
-
-            self._symbol_expr = symengine.sympify(state["expr"])
-            self._parameter_symbols = {k: symengine.sympify(v) for k, v in state["symbols"].items()}
-            self._parameters = set(self._parameter_symbols)
-        else:
-            self._symbol_expr = state["expr"]
-            self._parameter_symbols = state["symbols"]
-            self._parameters = set(self._parameter_symbols)
-        self._name_map = state["names"]
-
     def is_real(self):
         """Return whether the expression is real"""
 

qiskit/tools/parallel.py

@@ -65,9 +65,6 @@
     # Default False on Windows
     if sys.platform == "win32":
         PARALLEL_DEFAULT = False
-    # 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:

releasenotes/notes/bump-symengine-8ca362f5b9fef199.yaml

@@ -0,0 +1,19 @@
+---
+upgrade:
+  - |
+    The minimum supported version of ``symengine`` is now 0.9.0. This was
+    necessary to improve compatibility with Python's ``pickle`` module which
+    is used internally as part of parallel dispatch with :func:`.parallel_map`.
+  - |
+    The default value of ``QISKIT_PARALLEL`` when running with Python 3.9 on
+    Linux is now set to ``TRUE``. This means when running :func:`.parallel_map`
+    or functions that call it internally such as :func:`.transpile` and
+    :func:`.assemble` the function will be executed in multiple processes and
+    should have better run time performance. This change was made because the
+    issues with reliability of parallel dispatch appear to have been resolved
+    (see `#6188 <https://github.com/Qiskit/qiskit-terra/issues/6188>`__ for
+    more details). If you still encounter issues because of this you can disable
+    multiprocessing and revert to the previous default behavior by setting the
+    ``QISKIT_PARALLEL`` environment variable to ``FALSE`` or setting the
+    ``parallel`` option to ``False`` in your user config file (also please file
+    an issue so we can track any issues related to multiprocessing).

requirements.txt

@@ -8,5 +8,5 @@ dill>=0.3
 python-constraint>=1.4
 python-dateutil>=2.8.0
 stevedore>=3.0.0
-symengine>=0.8 ; platform_machine == 'x86_64' or platform_machine == 'aarch64' or platform_machine == 'ppc64le' or platform_machine == 'amd64' or platform_machine == 'arm64'
+symengine>=0.9 ; platform_machine == 'x86_64' or platform_machine == 'aarch64' or platform_machine == 'ppc64le' or platform_machine == 'amd64' or platform_machine == 'arm64'
 tweedledum>=1.1,<2.0