Enable multiple parallel seed trials for SabreSwap

qiskit/transpiler/passes/layout/sabre_layout.py

@@ -49,20 +49,42 @@ class SabreLayout(AnalysisPass):
     `arXiv:1809.02573 <https://arxiv.org/pdf/1809.02573.pdf>`_
     """
 
-    def __init__(self, coupling_map, routing_pass=None, seed=None, max_iterations=3):
+    def __init__(
+        self, coupling_map, routing_pass=None, seed=None, max_iterations=3, swap_trials=None
+    ):
         """SabreLayout initializer.
 
         Args:
             coupling_map (Coupling): directed graph representing a coupling map.
             routing_pass (BasePass): the routing pass to use while iterating.
+                This is mutually exclusive with the ``swap_trials`` argument and
+                if both are set an error will be raised.
             seed (int): seed for setting a random first trial layout.
             max_iterations (int): number of forward-backward iterations.
+            swap_trials (int): The number of trials to run of
+                :class:`~.SabreSwap` for each iteration. This is equivalent to
+                the ``trials`` argument on :class:`~.SabreSwap`. If this is not
+                specified (and ``routing_pass`` isn't set) by default the number
+                of physical CPUs on your local system will be used. For
+                reproducibility between environments it is best to set this
+                to an explicit number because the output will potentially depend
+                on the number of trials run. This option is mutually exclusive
+                with the ``routing_pass`` argument and an error will be raised
+                if both are used.
+
+        Raises:
+            TranspilerError: If both ``routing_pass`` and ``swap_trials`` are
+                specified
         """
         super().__init__()
         self.coupling_map = coupling_map
+        if routing_pass is not None and swap_trials is not None:
+            raise TranspilerError("Both routing_pass and swap_trials can't be set at the same time")
         self.routing_pass = routing_pass
         self.seed = seed
         self.max_iterations = max_iterations
+        self.trials = swap_trials
+        self.swap_trials = swap_trials
 
     def run(self, dag):
         """Run the SabreLayout pass on `dag`.
@@ -86,7 +108,9 @@ def run(self, dag):
         initial_layout = Layout({q: dag.qubits[i] for i, q in enumerate(physical_qubits)})
 
         if self.routing_pass is None:
-            self.routing_pass = SabreSwap(self.coupling_map, "decay", seed=self.seed, fake_run=True)
+            self.routing_pass = SabreSwap(
+                self.coupling_map, "decay", seed=self.seed, fake_run=True, trials=self.swap_trials
+            )
         else:
             self.routing_pass.fake_run = True
 

qiskit/transpiler/passes/routing/sabre_swap.py

@@ -23,6 +23,7 @@
 from qiskit.transpiler.exceptions import TranspilerError
 from qiskit.transpiler.layout import Layout
 from qiskit.dagcircuit import DAGOpNode
+from qiskit.tools.parallel import CPU_COUNT
 
 # pylint: disable=import-error
 from qiskit._accelerate.sabre_swap import (
@@ -61,20 +62,19 @@ class SabreSwap(TransformationPass):
     scored according to some heuristic cost function. The best SWAP is
     implemented and ``current_layout`` updated.
 
+    This transpiler pass adds onto the SABRE algorithm in that it will run
+    multiple trials of the algorithm with different seeds. The best output,
+    deteremined by the trial with the least amount of SWAPed inserted, will
+    be selected from the random trials.
+
     **References:**
 
     [1] Li, Gushu, Yufei Ding, and Yuan Xie. "Tackling the qubit mapping problem
     for NISQ-era quantum devices." ASPLOS 2019.
     `arXiv:1809.02573 <https://arxiv.org/pdf/1809.02573.pdf>`_
     """
 
-    def __init__(
-        self,
-        coupling_map,
-        heuristic="basic",
-        seed=None,
-        fake_run=False,
-    ):
+    def __init__(self, coupling_map, heuristic="basic", seed=None, fake_run=False, trials=None):
         r"""SabreSwap initializer.
 
         Args:
@@ -84,6 +84,12 @@ def __init__(
             seed (int): random seed used to tie-break among candidate swaps.
             fake_run (bool): if true, it only pretend to do routing, i.e., no
                 swap is effectively added.
+            trials (int): The number of seed trials to run sabre with. These will
+                be run in parallel (unless the PassManager is already running in
+                parallel). If not specified this defaults to the number of physical
+                CPUs on the local system. For reproducible results it is recommended
+                that you set this explicitly, as the output will be deterministic for
+                a fixed number of trials.
 
         Raises:
             TranspilerError: If the specified heuristic is not valid.
@@ -158,6 +164,11 @@ def __init__(
             self.seed = np.random.default_rng(None).integers(0, ii32.max, dtype=int)
         else:
             self.seed = seed
+        if trials is None:
+            self.trials = CPU_COUNT
+        else:
+            self.trials = trials
+
         self.fake_run = fake_run
         self._qubit_indices = None
         self._clbit_indices = None
@@ -216,6 +227,7 @@ def run(self, dag):
             self.heuristic,
             self.seed,
             layout,
+            self.trials,
         )
 
         layout_mapping = layout.layout_mapping()

qiskit/transpiler/preset_passmanagers/builtin_plugins.py

@@ -206,7 +206,9 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
             pass_manager_config.initial_layout,
         )
         if optimization_level == 0:
-            routing_pass = SabreSwap(coupling_map, heuristic="basic", seed=seed_transpiler)
+            routing_pass = SabreSwap(
+                coupling_map, heuristic="basic", seed=seed_transpiler, trials=5
+            )
             return common.generate_routing_passmanager(
                 routing_pass,
                 target,
@@ -215,7 +217,9 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
                 use_barrier_before_measurement=True,
             )
         if optimization_level == 1:
-            routing_pass = SabreSwap(coupling_map, heuristic="lookahead", seed=seed_transpiler)
+            routing_pass = SabreSwap(
+                coupling_map, heuristic="lookahead", seed=seed_transpiler, trials=5
+            )
             return common.generate_routing_passmanager(
                 routing_pass,
                 target,
@@ -227,7 +231,9 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
                 use_barrier_before_measurement=True,
             )
         if optimization_level == 2:
-            routing_pass = SabreSwap(coupling_map, heuristic="decay", seed=seed_transpiler)
+            routing_pass = SabreSwap(
+                coupling_map, heuristic="decay", seed=seed_transpiler, trials=10
+            )
             return common.generate_routing_passmanager(
                 routing_pass,
                 target,
@@ -238,7 +244,9 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
                 use_barrier_before_measurement=True,
             )
         if optimization_level == 3:
-            routing_pass = SabreSwap(coupling_map, heuristic="decay", seed=seed_transpiler)
+            routing_pass = SabreSwap(
+                coupling_map, heuristic="decay", seed=seed_transpiler, trials=20
+            )
             return common.generate_routing_passmanager(
                 routing_pass,
                 target,

qiskit/transpiler/preset_passmanagers/level0.py

@@ -86,7 +86,9 @@ def _choose_layout_condition(property_set):
     elif layout_method == "noise_adaptive":
         _choose_layout = NoiseAdaptiveLayout(backend_properties)
     elif layout_method == "sabre":
-        _choose_layout = SabreLayout(coupling_map, max_iterations=1, seed=seed_transpiler)
+        _choose_layout = SabreLayout(
+            coupling_map, max_iterations=1, seed=seed_transpiler, swap_trials=5
+        )
 
     toqm_pass = False
     # Choose routing pass

qiskit/transpiler/preset_passmanagers/level1.py

@@ -144,7 +144,9 @@ def _vf2_match_not_found(property_set):
     elif layout_method == "noise_adaptive":
         _improve_layout = NoiseAdaptiveLayout(backend_properties)
     elif layout_method == "sabre":
-        _improve_layout = SabreLayout(coupling_map, max_iterations=2, seed=seed_transpiler)
+        _improve_layout = SabreLayout(
+            coupling_map, max_iterations=2, seed=seed_transpiler, swap_trials=5
+        )
 
     toqm_pass = False
     routing_pm = None

qiskit/transpiler/preset_passmanagers/level2.py

@@ -127,7 +127,9 @@ def _vf2_match_not_found(property_set):
     elif layout_method == "noise_adaptive":
         _choose_layout_1 = NoiseAdaptiveLayout(backend_properties)
     elif layout_method == "sabre":
-        _choose_layout_1 = SabreLayout(coupling_map, max_iterations=2, seed=seed_transpiler)
+        _choose_layout_1 = SabreLayout(
+            coupling_map, max_iterations=2, seed=seed_transpiler, swap_trials=10
+        )
 
     toqm_pass = False
     routing_pm = None

qiskit/transpiler/preset_passmanagers/level3.py

@@ -138,7 +138,9 @@ def _vf2_match_not_found(property_set):
     elif layout_method == "noise_adaptive":
         _choose_layout_1 = NoiseAdaptiveLayout(backend_properties)
     elif layout_method == "sabre":
-        _choose_layout_1 = SabreLayout(coupling_map, max_iterations=4, seed=seed_transpiler)
+        _choose_layout_1 = SabreLayout(
+            coupling_map, max_iterations=4, seed=seed_transpiler, swap_trials=20
+        )
 
     toqm_pass = False
     # TODO: Remove when qiskit-toqm has it's own plugin and we can rely on just the plugin interface

releasenotes/notes/multiple-parallel-rusty-sabres-32bc93f79ae48a1f.yaml

@@ -0,0 +1,20 @@
+---
+features:
+  - |
+    The :class:`~.SabreSwap` transpiler pass has a new keyword argument on its
+    constructor, ``trials``. The ``trials`` argument is used to specify the
+    number of random seed trials to attempt. The output from the
+    `SABRE algorithm <https://arxiv.org/abs/1809.02573>`__  can differ greatly
+    based on the seed used for the random number. :class:`~.SabreSwap` will
+    now run the algorithm with ``trials`` number of random seeds and pick the
+    best (with the fewest swaps inserted). If ``trials`` is not specified the
+    pass will default to use the number of physical CPUs on the local system.
+  - |
+    The :class:`~.SabreLayout` transpiler pass has a new keyword argument on
+    its constructor, ``swap_trials``. The ``swap_trials`` argument is used
+    to specify how many random seed trials to run on the :class:`~.SabreSwap`
+    pass internally. It corresponds to the ``trials`` arugment on the
+    :class:`~.SabreSwap` pass. When set, each iteration of
+    :class:`~.SabreSwap` will be run internally ``swap_trials`` times. 
+    If ``swap_trials`` is not specified the will default to use
+    the number of physical CPUs on the local system.