Fix default batching in variational algorithms (Qiskit#9038)

* Fix default batching in variational algorithms

* fix test

* reduce batching to only SPSA

* fix tests

* Apply suggestions from code review

Co-authored-by: Matthew Treinish <[email protected]>

Co-authored-by: Matthew Treinish <[email protected]>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/algorithms/eigensolvers/vqd.py

@@ -38,6 +38,9 @@
 from ..exceptions import AlgorithmError
 from ..observables_evaluator import estimate_observables
 
+# private function as we expect this to be updated in the next release
+from ..utils.set_batching import _set_default_batchsize
+
 logger = logging.getLogger(__name__)
 
 
@@ -264,10 +267,18 @@ def compute_eigenvalues(
                     fun=energy_evaluation, x0=initial_point, bounds=bounds
                 )
             else:
+                # we always want to submit as many estimations per job as possible for minimal
+                # overhead on the hardware
+                was_updated = _set_default_batchsize(self.optimizer)
+
                 opt_result = self.optimizer.minimize(
                     fun=energy_evaluation, x0=initial_point, bounds=bounds
                 )
 
+                # reset to original value
+                if was_updated:
+                    self.optimizer.set_max_evals_grouped(None)
+
             eval_time = time() - start_time
 
             self._update_vqd_result(result, opt_result, eval_time, self.ansatz.copy())

qiskit/algorithms/minimum_eigensolvers/sampling_vqe.py

@@ -39,6 +39,9 @@
 from ..observables_evaluator import estimate_observables
 from ..utils import validate_initial_point, validate_bounds
 
+# private function as we expect this to be updated in the next released
+from ..utils.set_batching import _set_default_batchsize
+
 
 logger = logging.getLogger(__name__)
 
@@ -208,10 +211,18 @@ def compute_minimum_eigenvalue(
             # pylint: disable=not-callable
             optimizer_result = self.optimizer(fun=evaluate_energy, x0=initial_point, bounds=bounds)
         else:
+            # we always want to submit as many estimations per job as possible for minimal
+            # overhead on the hardware
+            was_updated = _set_default_batchsize(self.optimizer)
+
             optimizer_result = self.optimizer.minimize(
                 fun=evaluate_energy, x0=initial_point, bounds=bounds
             )
 
+            # reset to original value
+            if was_updated:
+                self.optimizer.set_max_evals_grouped(None)
+
         optimizer_time = time() - start_time
 
         logger.info(

qiskit/algorithms/minimum_eigensolvers/vqe.py

@@ -35,6 +35,9 @@
 from ..observables_evaluator import estimate_observables
 from ..utils import validate_initial_point, validate_bounds
 
+# private function as we expect this to be updated in the next released
+from ..utils.set_batching import _set_default_batchsize
+
 logger = logging.getLogger(__name__)
 
 
@@ -181,10 +184,18 @@ def compute_minimum_eigenvalue(
                 fun=evaluate_energy, x0=initial_point, jac=evaluate_gradient, bounds=bounds
             )
         else:
+            # we always want to submit as many estimations per job as possible for minimal
+            # overhead on the hardware
+            was_updated = _set_default_batchsize(self.optimizer)
+
             optimizer_result = self.optimizer.minimize(
                 fun=evaluate_energy, x0=initial_point, jac=evaluate_gradient, bounds=bounds
             )
 
+            # reset to original value
+            if was_updated:
+                self.optimizer.set_max_evals_grouped(None)
+
         optimizer_time = time() - start_time
 
         logger.info(

qiskit/algorithms/optimizers/optimizer.py

@@ -180,7 +180,7 @@ def __init__(self):
         self._bounds_support_level = self.get_support_level()["bounds"]
         self._initial_point_support_level = self.get_support_level()["initial_point"]
         self._options = {}
-        self._max_evals_grouped = 1
+        self._max_evals_grouped = None
 
     @abstractmethod
     def get_support_level(self):
@@ -205,7 +205,7 @@ def set_options(self, **kwargs):
 
     # pylint: disable=invalid-name
     @staticmethod
-    def gradient_num_diff(x_center, f, epsilon, max_evals_grouped=1):
+    def gradient_num_diff(x_center, f, epsilon, max_evals_grouped=None):
         """
         We compute the gradient with the numeric differentiation in the parallel way,
         around the point x_center.
@@ -214,11 +214,14 @@ def gradient_num_diff(x_center, f, epsilon, max_evals_grouped=1):
             x_center (ndarray): point around which we compute the gradient
             f (func): the function of which the gradient is to be computed.
             epsilon (float): the epsilon used in the numeric differentiation.
-            max_evals_grouped (int): max evals grouped
+            max_evals_grouped (int): max evals grouped, defaults to 1 (i.e. no batching).
         Returns:
             grad: the gradient computed
 
         """
+        if max_evals_grouped is None:  # no batching by default
+            max_evals_grouped = 1
+
         forig = f(*((x_center,)))
         grad = []
         ei = np.zeros((len(x_center),), float)

qiskit/algorithms/optimizers/spsa.py

@@ -719,7 +719,7 @@ def _batch_evaluate(function, points, max_evals_grouped, unpack_points=False):
     """
 
     # if the function cannot handle lists of points as input, cover this case immediately
-    if max_evals_grouped == 1:
+    if max_evals_grouped is None or max_evals_grouped == 1:
         # support functions with multiple arguments where the points are given in a tuple
         return [
             function(*point) if isinstance(point, tuple) else function(point) for point in points

qiskit/algorithms/utils/set_batching.py

@@ -0,0 +1,27 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2022.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Set default batch sizes for the optimizers."""
+
+from qiskit.algorithms.optimizers import Optimizer, SPSA
+
+
+def _set_default_batchsize(optimizer: Optimizer) -> bool:
+    """Set the default batchsize, if None is set and return whether it was updated or not."""
+    if isinstance(optimizer, SPSA):
+        updated = optimizer._max_evals_grouped is None
+        if updated:
+            optimizer.set_max_evals_grouped(50)
+    else:  # we only set a batchsize for SPSA
+        updated = False
+
+    return updated

test/python/algorithms/minimum_eigensolvers/test_vqe.py

@@ -300,6 +300,35 @@ def run_check():
             vqe.optimizer = L_BFGS_B()
             run_check()
 
+    def test_default_batch_evaluation_on_spsa(self):
+        """Test the default batching works."""
+        ansatz = TwoLocal(2, rotation_blocks=["ry", "rz"], entanglement_blocks="cz")
+
+        wrapped_estimator = Estimator()
+        inner_estimator = Estimator()
+
+        callcount = {"estimator": 0}
+
+        def wrapped_estimator_run(*args, **kwargs):
+            kwargs["callcount"]["estimator"] += 1
+            return inner_estimator.run(*args, **kwargs)
+
+        wrapped_estimator.run = partial(wrapped_estimator_run, callcount=callcount)
+
+        spsa = SPSA(maxiter=5)
+
+        vqe = VQE(wrapped_estimator, ansatz, spsa)
+        _ = vqe.compute_minimum_eigenvalue(Pauli("ZZ"))
+
+        # 1 calibration + 5 loss + 1 return loss
+        expected_estimator_runs = 1 + 5 + 1
+
+        with self.subTest(msg="check callcount"):
+            self.assertEqual(callcount["estimator"], expected_estimator_runs)
+
+        with self.subTest(msg="check reset to original max evals grouped"):
+            self.assertIsNone(spsa._max_evals_grouped)
+
     def test_batch_evaluate_with_qnspsa(self):
         """Test batch evaluating with QNSPSA works."""
         ansatz = TwoLocal(2, rotation_blocks=["ry", "rz"], entanglement_blocks="cz")