Lower limits on VF2PostLayout in exact mode (Qiskit#15068) (Qiskit#15075)

We currently run `VF2PostLayout(strict=True)` after the optimisation
loop at optimisation level 3.  We use the same values for `call_limit`
and `max_trials` as we do for the averaging methods, but the averaging
methods are pure Rust with no semantic matching, whereas the exact-match
have Python-space semantic matching, so are far slower.

We can look at raising the limits back to much higher values when these
paths are in Rust and use the branch-and-bound scoring.

(cherry picked from commit 7052dfe)

Co-authored-by: Jake Lishman <[email protected]>

Author: mergify[bot]

qiskit/transpiler/preset_passmanagers/builtin_plugins.py

@@ -616,6 +616,7 @@ def _unroll_condition(property_set):
                     optimization_level,
                     pass_manager_config.layout_method,
                     pass_manager_config.initial_layout,
+                    exact_match=True,
                 )
                 is_vf2_fully_bounded = vf2_call_limit and vf2_max_trials
                 if pass_manager_config.target is not None and is_vf2_fully_bounded:

qiskit/transpiler/preset_passmanagers/common.py

@@ -728,6 +728,7 @@ def get_vf2_limits(
     optimization_level: int,
     layout_method: Optional[str] = None,
     initial_layout: Optional[Layout] = None,
+    exact_match: bool = False,
 ) -> VF2Limits:
     """Get the VF2 limits for VF2-based layout passes.
 
@@ -739,13 +740,20 @@ def get_vf2_limits(
     if layout_method is None and initial_layout is None:
         if optimization_level in {1, 2}:
             limits = VF2Limits(
-                int(5e4),  # Set call limit to ~100ms with rustworkx 0.10.2
-                2500,  # Limits layout scoring to < 600ms on ~400 qubit devices
+                50_000,  # Set call limit to ~100ms with rustworkx 0.10.2
+                2_500,  # Limits layout scoring to < 600ms on ~400 qubit devices
             )
         elif optimization_level == 3:
             limits = VF2Limits(
-                int(3e7),  # Set call limit to ~60 sec with rustworkx 0.10.2
-                250000,  # Limits layout scoring to < 60 sec on ~400 qubit devices
+                30_000_000,  # Set call limit to ~60 sec with rustworkx 0.10.2
+                250_000,  # Limits layout scoring to < 60 sec on ~400 qubit devices
+            )
+        # In Qiskit 2.2, strict mode still includes heavy Python usage for the semantics and
+        # scoring, so we dial the limits way down.
+        if exact_match:
+            limits = VF2Limits(
+                ((limits.call_limit // 100) or 1) if limits.call_limit is not None else None,
+                ((limits.max_trials // 100) or 1) if limits.max_trials is not None else None,
             )
     return limits
 

releasenotes/notes/reduce-vf2post-limits-6a6ff25491f8ac4f.yaml

@@ -0,0 +1,6 @@
+---
+upgrade_transpiler:
+  - |
+    The maximum call and trial limits for the exact-matching run of :class:`.VF2PostLayout` at
+    ``optimization_level=3`` have been reduced to avoid excessive runtimes for highly symmetric
+    trial circuits being mapped to large coupling maps.