Remove hard time limit from vf2 passes in preset passmanagers (#8021)

* Remove hard time limit from vf2 passes in preset passmanagers

This commit removes the hard time limit from the preset passmanager. We
were previously setting two limits on the pass a call_limit parameter,
which is used to set the limit of internal state visits the vf2
implementation in retworkx will attempt before giving up, and a time
limit which sets a hard wall time limit that is checked after each
mapping to ensure we don't spend more than a specific amount of time on
trying to find an optimal mapping. As subgraph isomorphism is
NP-complete we could spend a very large amount of time trying to find
the next isomorphic mapping and both these limits were set to avoid a
situation where we spend hours in optimization level 1 trying to find a
better layout. However, as we've seen in #8017 setting a hard time limit
limits the level of reproducibility on the output of the transpiler. It
means that setting a fixed seed is no longer sufficient to exactly
reproduce the output as it's also partially a function of the
performance of the local system we're running on. If the local
environment is slower we might get different results because we've hit
the hard time limit.

To address this, this commit removes the hard time limit so that we only
rely on the call limit parameter on the preset pass manager. While this
parameter is a bit more opaque in how it should be used because it
requires an understanding of how the VF2 algorithm works (and is
implemented in retworkx) it should be sufficient to avoid the runaway
execution problem but also means the limits are determinitisically
applied. This should improve the reproducability of the transpiler because
the pass will always try exactly up until a given point regardless of
how fast the local environment is.

For people manually instatiating the pass they can still rely on the
hard time limit parameter if they want it, but for the preset pass
managers and the transpile() function we no longer set this.

Fixes #8017

* Add release note

* Fix tense

Co-authored-by: Jake Lishman <jake@binhbar.com>

qiskit/transpiler/preset_passmanagers/level1.py

@@ -156,7 +156,6 @@ def level_1_pass_manager(pass_manager_config: PassManagerConfig) -> PassManager:
             coupling_map,
             seed=seed_transpiler,
             call_limit=int(5e4),  # Set call limit to ~100ms with retworkx 0.10.2
-            time_limit=0.1,
             properties=backend_properties,
             target=target,
         )
@@ -325,7 +324,6 @@ def level_1_pass_manager(pass_manager_config: PassManagerConfig) -> PassManager:
                     backend_properties,
                     seed_transpiler,
                     call_limit=int(5e4),  # Set call limit to ~100ms with retworkx 0.10.2
-                    time_limit=0.1,
                     strict_direction=False,
                 ),
                 condition=_trivial_not_perfect,

qiskit/transpiler/preset_passmanagers/level2.py

@@ -154,7 +154,6 @@ def level_2_pass_manager(pass_manager_config: PassManagerConfig) -> PassManager:
             coupling_map,
             seed=seed_transpiler,
             call_limit=int(5e6),  # Set call limit to ~10 sec with retworkx 0.10.2
-            time_limit=10.0,
             properties=backend_properties,
             target=target,
         )
@@ -312,7 +311,6 @@ def level_2_pass_manager(pass_manager_config: PassManagerConfig) -> PassManager:
                     backend_properties,
                     seed_transpiler,
                     call_limit=int(5e6),  # Set call limit to ~10 sec with retworkx 0.10.2
-                    time_limit=10.0,
                     strict_direction=False,
                 )
             )

qiskit/transpiler/preset_passmanagers/level3.py

@@ -157,7 +157,6 @@ def level_3_pass_manager(pass_manager_config: PassManagerConfig) -> PassManager:
             coupling_map,
             seed=seed_transpiler,
             call_limit=int(3e7),  # Set call limit to ~60 sec with retworkx 0.10.2
-            time_limit=60,
             properties=backend_properties,
             target=target,
         )
@@ -323,7 +322,6 @@ def level_3_pass_manager(pass_manager_config: PassManagerConfig) -> PassManager:
                     backend_properties,
                     seed_transpiler,
                     call_limit=int(3e7),  # Set call limit to ~60 sec with retworkx 0.10.2
-                    time_limit=60,
                     strict_direction=False,
                 )
             )

qiskit/transpiler/preset_passmanagers/releasenotes/notes/remove-hard-time-limit-vf2-be83830ecc71f72c.yaml

@@ -0,0 +1,25 @@
+---
+upgrade:
+  - |
+    The preset pass managers generated by :func:`~.level_1_pass_manager`,
+    :func:`~.level_2_pass_manager`, and :func:`~.level_3_pass_manager` and used
+    by the :func:`~.transpile` function's ``optimization_level`` argument at
+    1, 2, and 3 respectively no longer set a hard time limit on the
+    :class:`~.VF2Layout` transpiler pass. This means that the pass will no
+    longer stop trying to find a better alternative perfect layout up until a
+    fixed time limit (100ms for level 1, 10 sec for level 2, and 60 sec for
+    level 3) as doing this limited the reproducibility of compilation when a
+    perfect layout was available. This means that the output when using the pass
+    might be different than before, although in all cases it would only change
+    if a lower noise set of qubits can be found over the previous output. If
+    you wish to retain the previous behavior you can create a custom
+    :class:`~.PassManager` that sets the ``time_limit`` argument on the
+    constructor for the :class:`~VF2Layout` pass.
+fixes:
+  - |
+    Fixed an issue with reproducibility of the :func:`~.transpile` function
+    when running with ``optimization_level`` 1, 2, and 3. Previously, under
+    some conditions when there were multiple perfect layouts (a layout that
+    doesn't require any SWAP gates) available the selected layout and output
+    circuit could vary regardless of whether the ``seed_transpiler`` argument
+    was set.