Always run vf2 scoring serially

qiskit/transpiler/passes/layout/vf2_layout.py

@@ -12,7 +12,6 @@
 
 
 """VF2Layout pass to find a layout using subgraph isomorphism"""
-import os
 from enum import Enum
 import itertools
 import logging
@@ -172,10 +171,6 @@ def run(self, dag):
         chosen_layout_score = None
         start_time = time.time()
         trials = 0
-        run_in_parallel = (
-            os.getenv("QISKIT_IN_PARALLEL", "FALSE").upper() != "TRUE"
-            or os.getenv("QISKIT_FORCE_THREADS", "FALSE").upper() == "TRUE"
-        )
 
         def mapping_to_layout(layout_mapping):
             return Layout({reverse_im_graph_node_map[k]: v for k, v in layout_mapping.items()})
@@ -204,7 +199,6 @@ def mapping_to_layout(layout_mapping):
                 reverse_im_graph_node_map,
                 im_graph,
                 self.strict_direction,
-                run_in_parallel,
             )
             # If the layout score is 0 we can't do any better and we'll just
             # waste time finding additional mappings that will at best match

qiskit/transpiler/passes/layout/vf2_post_layout.py

@@ -12,7 +12,6 @@
 
 
 """VF2PostLayout pass to find a layout after transpile using subgraph isomorphism"""
-import os
 from enum import Enum
 import logging
 import inspect
@@ -253,10 +252,6 @@ def run(self, dag):
                 call_limit=self.call_limit,
             )
         chosen_layout = None
-        run_in_parallel = (
-            os.getenv("QISKIT_IN_PARALLEL", "FALSE").upper() != "TRUE"
-            or os.getenv("QISKIT_FORCE_THREADS", "FALSE").upper() == "TRUE"
-        )
         try:
             if self.strict_direction:
                 initial_layout = Layout({bit: index for index, bit in enumerate(dag.qubits)})
@@ -276,7 +271,6 @@ def run(self, dag):
                     reverse_im_graph_node_map,
                     im_graph,
                     self.strict_direction,
-                    run_in_parallel,
                 )
         # Circuit not in basis so we have nothing to compare against return here
         except KeyError:
@@ -309,7 +303,6 @@ def run(self, dag):
                     reverse_im_graph_node_map,
                     im_graph,
                     self.strict_direction,
-                    run_in_parallel,
                 )
             logger.debug("Trial %s has score %s", trials, layout_score)
             if layout_score < chosen_layout_score:

qiskit/transpiler/passes/layout/vf2_utils.py

@@ -102,7 +102,7 @@ def score_layout(
     _reverse_bit_map,
     im_graph,
     strict_direction=False,
-    run_in_parallel=True,
+    run_in_parallel=False,
 ):
     """Score a layout given an average error map."""
     if layout_mapping:

releasenotes/notes/vf2-threading-b778a36de5b8832a.yaml

@@ -0,0 +1,8 @@
+---
+other:
+  - |
+    The :class:`.VF2Layout` and :class:`.VF2PostLayout` transpiler passes previously would
+    potentially run their internal scoring using multithreading if the input
+    circuit's were sufficiently large. However, the multithreading usage has
+    been removed from the passes as it was shown to cause a performance
+    regression instead of an improvement like originally intended.