Add config option to leverage all cores for sabre (#12780)

* Add config option to leverage all cores for sabre

By default when running sabre in parallel we use a fixed number of
threads (depending on optimization level). This was a tradeoff made for
having deterministic results across multiple systems with a fixed seed
set. However when running qiskit on systems with a lot of CPUs
available we're leaving potential performance on the table by not using
all the available cores. This new flag lets users opt-in to running
sabre with n trials for n CPUs to potentially get better output results
from the transpiler, with minimal to no runtime overhead, at the cost of
the results not necessarily being reproducible when run on a different
computer.

* Apply suggestions from code review

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

* Rework logic to use the default if larger than CPU_COUNT

This commit refactors the logic added in the previous commit to a single
helper function. This reduces the code duplication and makes it easier
to work with. While doing this the logic has been updated so that when
the flag is set and the default number of trials is larger than the
CPU_COUNT we use the default. This means the logic when the flag is set
is to run `max(default_trials, CPU_COUNT)` which should better match
user expectations around the flag.

---------

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

qiskit/transpiler/preset_passmanagers/builtin_plugins.py

@@ -12,6 +12,8 @@
 
 """Built-in transpiler stage plugins for preset pass managers."""
 
+import os
+
 from qiskit.transpiler.passmanager import PassManager
 from qiskit.transpiler.exceptions import TranspilerError
 from qiskit.transpiler.passes import BasicSwap
@@ -63,6 +65,10 @@
     SXGate,
     SXdgGate,
 )
+from qiskit.utils.parallel import CPU_COUNT
+from qiskit import user_config
+
+CONFIG = user_config.get_config()
 
 
 class DefaultInitPassManager(PassManagerStagePlugin):
@@ -397,11 +403,12 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
             pass_manager_config.initial_layout,
         )
         if optimization_level == 0:
+            trial_count = _get_trial_count(5)
             routing_pass = SabreSwap(
                 coupling_map_routing,
                 heuristic="basic",
                 seed=seed_transpiler,
-                trials=5,
+                trials=trial_count,
             )
             return common.generate_routing_passmanager(
                 routing_pass,
@@ -411,11 +418,12 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
                 use_barrier_before_measurement=True,
             )
         if optimization_level == 1:
+            trial_count = _get_trial_count(5)
             routing_pass = SabreSwap(
                 coupling_map_routing,
                 heuristic="decay",
                 seed=seed_transpiler,
-                trials=5,
+                trials=trial_count,
             )
             return common.generate_routing_passmanager(
                 routing_pass,
@@ -429,11 +437,13 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
                 use_barrier_before_measurement=True,
             )
         if optimization_level == 2:
+            trial_count = _get_trial_count(20)
+
             routing_pass = SabreSwap(
                 coupling_map_routing,
                 heuristic="decay",
                 seed=seed_transpiler,
-                trials=10,
+                trials=trial_count,
             )
             return common.generate_routing_passmanager(
                 routing_pass,
@@ -446,11 +456,12 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
                 use_barrier_before_measurement=True,
             )
         if optimization_level == 3:
+            trial_count = _get_trial_count(20)
             routing_pass = SabreSwap(
                 coupling_map_routing,
                 heuristic="decay",
                 seed=seed_transpiler,
-                trials=20,
+                trials=trial_count,
             )
             return common.generate_routing_passmanager(
                 routing_pass,
@@ -737,12 +748,15 @@ def _swap_mapped(property_set):
                 max_trials=2500,  # Limits layout scoring to < 600ms on ~400 qubit devices
             )
             layout.append(ConditionalController(choose_layout_1, condition=_layout_not_perfect))
+
+            trial_count = _get_trial_count(5)
+
             choose_layout_2 = SabreLayout(
                 coupling_map,
                 max_iterations=2,
                 seed=pass_manager_config.seed_transpiler,
-                swap_trials=5,
-                layout_trials=5,
+                swap_trials=trial_count,
+                layout_trials=trial_count,
                 skip_routing=pass_manager_config.routing_method is not None
                 and pass_manager_config.routing_method != "sabre",
             )
@@ -769,12 +783,15 @@ def _swap_mapped(property_set):
             layout.append(
                 ConditionalController(choose_layout_0, condition=_choose_layout_condition)
             )
+
+            trial_count = _get_trial_count(20)
+
             choose_layout_1 = SabreLayout(
                 coupling_map,
                 max_iterations=2,
                 seed=pass_manager_config.seed_transpiler,
-                swap_trials=20,
-                layout_trials=20,
+                swap_trials=trial_count,
+                layout_trials=trial_count,
                 skip_routing=pass_manager_config.routing_method is not None
                 and pass_manager_config.routing_method != "sabre",
             )
@@ -801,12 +818,15 @@ def _swap_mapped(property_set):
             layout.append(
                 ConditionalController(choose_layout_0, condition=_choose_layout_condition)
             )
+
+            trial_count = _get_trial_count(20)
+
             choose_layout_1 = SabreLayout(
                 coupling_map,
                 max_iterations=4,
                 seed=pass_manager_config.seed_transpiler,
-                swap_trials=20,
-                layout_trials=20,
+                swap_trials=trial_count,
+                layout_trials=trial_count,
                 skip_routing=pass_manager_config.routing_method is not None
                 and pass_manager_config.routing_method != "sabre",
             )
@@ -902,42 +922,50 @@ def _swap_mapped(property_set):
         layout = PassManager()
         layout.append(_given_layout)
         if optimization_level == 0:
+            trial_count = _get_trial_count(5)
+
             layout_pass = SabreLayout(
                 coupling_map,
                 max_iterations=1,
                 seed=pass_manager_config.seed_transpiler,
-                swap_trials=5,
-                layout_trials=5,
+                swap_trials=trial_count,
+                layout_trials=trial_count,
                 skip_routing=pass_manager_config.routing_method is not None
                 and pass_manager_config.routing_method != "sabre",
             )
         elif optimization_level == 1:
+            trial_count = _get_trial_count(5)
+
             layout_pass = SabreLayout(
                 coupling_map,
                 max_iterations=2,
                 seed=pass_manager_config.seed_transpiler,
-                swap_trials=5,
-                layout_trials=5,
+                swap_trials=trial_count,
+                layout_trials=trial_count,
                 skip_routing=pass_manager_config.routing_method is not None
                 and pass_manager_config.routing_method != "sabre",
             )
         elif optimization_level == 2:
+            trial_count = _get_trial_count(20)
+
             layout_pass = SabreLayout(
                 coupling_map,
                 max_iterations=2,
                 seed=pass_manager_config.seed_transpiler,
-                swap_trials=20,
-                layout_trials=20,
+                swap_trials=trial_count,
+                layout_trials=trial_count,
                 skip_routing=pass_manager_config.routing_method is not None
                 and pass_manager_config.routing_method != "sabre",
             )
         elif optimization_level == 3:
+            trial_count = _get_trial_count(20)
+
             layout_pass = SabreLayout(
                 coupling_map,
                 max_iterations=4,
                 seed=pass_manager_config.seed_transpiler,
-                swap_trials=20,
-                layout_trials=20,
+                swap_trials=trial_count,
+                layout_trials=trial_count,
                 skip_routing=pass_manager_config.routing_method is not None
                 and pass_manager_config.routing_method != "sabre",
             )
@@ -957,3 +985,9 @@ def _swap_mapped(property_set):
         embed = common.generate_embed_passmanager(coupling_map)
         layout.append(ConditionalController(embed.to_flow_controller(), condition=_swap_mapped))
         return layout
+
+
+def _get_trial_count(default_trials=5):
+    if CONFIG.get("sabre_all_threads", None) or os.getenv("QISKIT_SABRE_ALL_THREADS"):
+        return max(CPU_COUNT, default_trials)
+    return default_trials

qiskit/user_config.py

@@ -35,6 +35,7 @@ class UserConfig:
     transpile_optimization_level = 1
     parallel = False
     num_processes = 4
+    sabre_all_threads = true
 
     """
 
@@ -168,6 +169,13 @@ def read_config_file(self):
                     )
                 self.settings["num_processes"] = num_processes
 
+            # Parse sabre_all_threads
+            sabre_all_threads = self.config_parser.getboolean(
+                "default", "sabre_all_threads", fallback=None
+            )
+            if sabre_all_threads is not None:
+                self.settings["sabre_all_threads"] = sabre_all_threads
+
 
 def set_config(key, value, section=None, file_path=None):
     """Adds or modifies a user configuration
@@ -208,6 +216,7 @@ def set_config(key, value, section=None, file_path=None):
         "transpile_optimization_level",
         "parallel",
         "num_processes",
+        "sabre_all_threads",
     }
 
     if section in [None, "default"]:

releasenotes/notes/add-sabre-all-threads-option-ad4ff7a4d045cb2b.yaml

@@ -0,0 +1,26 @@
+---
+features_transpiler:
+  - |
+    Added a new user config file option ``sabre_all_threads`` and a
+    corresponding environment variable ``QISKIT_SABRE_ALL_THREADS``. When this
+    flag is set the preset pass managers will run the :class:`.SabreLayout`
+    and :class:`.SabreSwap` transpiler passes using all the available
+    CPUs on the local system. Using this option is a tradeoff between
+    determinism of output between different computers and potentially better
+    output with fewer :class:`.SwapGate`\s.
+
+    These transpiler passes run multiple random trials in parallel and pick
+    the output which results in the fewest :class:`.SwapGate`\s. As a rule of
+    thumb, if you run more trials, this provides the algorithm more opportunities
+    to find a better result. By default, the preset pass managers use a fixed
+    number of trials, in this release 5 trials for levels 0 and 1, and 20
+    trials for levels 2 and 3, but these numbers may change in future releases
+    (and were different in historical releases). Using a fixed number of
+    trials results in deterministic results regardless of the local system,
+    because even with a fixed seed if you were to default to the number of
+    local CPUs available the results would different when running between
+    different computers.
+
+    If the default number of trials for a given optimization level is higher
+    than the number of local CPUs it will use the optimization level default
+    which is higher.