Fix performance regression from QuantumError.to_dict (Qiskit#1408)

Fixes issue with QuantumError.to_dict causing a huge performance regression in noisy simulations from its calling of `qiskit.assemble` inside Pybind11 code, which resulted in unintended nested python multiprocessing processes.

qiskit/providers/aer/noise/errors/quantum_error.py

@@ -24,7 +24,6 @@
 from qiskit.circuit.exceptions import CircuitError
 from qiskit.circuit.library.generalized_gates import PauliGate
 from qiskit.circuit.library.standard_gates import IGate
-from qiskit.compiler import assemble
 from qiskit.exceptions import QiskitError
 from qiskit.extensions import UnitaryGate
 from qiskit.quantum_info.operators.base_operator import BaseOperator
@@ -453,13 +452,12 @@ def error_term(self, position):
 
     def to_dict(self):
         """Return the current error as a dictionary."""
-        qobj = assemble(self.circuits)
-        instructions = [exp.to_dict()['instructions'] for exp in qobj.experiments]
         error = {
             "type": "qerror",
             "id": self.id,
             "operations": [],
-            "instructions": instructions,
+            "instructions": [[op[0].assemble().to_dict() for op in circ.data]
+                             for circ in self._circs],
             "probabilities": list(self.probabilities)
         }
         return error

releasenotes/notes/fix-qerror-assemble-9919a93b210ca776.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fix performance regression in noisy simulations due to large increase in
+    serialization overhead for loading noise models from Python into C++
+    resulting from unintended nested Python multiprocessing calls.
+    See `issue 1407 <https://github.com/Qiskit/qiskit-aer/issues/1407>`__
+    for details.

src/controllers/aer_controller.hpp

@@ -332,8 +332,6 @@ class Controller {
     }
     return max_memory_mb_;
   }
-  // Truncate and remap input qubits
-  bool enable_truncation_ = true;
 
   // The maximum number of threads to use for various levels of parallelization
   int max_parallel_threads_;
@@ -390,8 +388,6 @@ class Controller {
 //-------------------------------------------------------------------------
 
 void Controller::set_config(const json_t &config) {
-  // Load validation threshold
-  JSON::get_value(enable_truncation_, "enable_truncation", config);
 
   // Load validation threshold
   JSON::get_value(validation_threshold_, "validation_threshold", config);
@@ -846,37 +842,31 @@ Result Controller::execute(const inputdata_t &input_qobj) {
     // Start QOBJ timer
     auto timer_start = myclock_t::now();
 
-    Noise::NoiseModel noise_model;
-    json_t config;
+    // Initialize QOBJ
+    Qobj qobj(input_qobj);
+    auto qobj_time_taken =
+        std::chrono::duration<double>(myclock_t::now() - timer_start).count();
 
-    // Check for config
-    if (Parser<inputdata_t>::get_value(config, "config", input_qobj)) {
-      // Set config
-      set_config(config);
-      // Load noise model
-      Parser<json_t>::get_value(noise_model, "noise_model", config);
-    }
+    // Set config
+    set_config(qobj.config);
 
-    // Initialize qobj
-    Qobj qobj;
-    if (noise_model.has_nonlocal_quantum_errors()) {
-       // Non-local noise does not work with optimized initialization
-       qobj = Qobj(input_qobj, false);
-    } else {
-      qobj = Qobj(input_qobj, enable_truncation_);
-    }
+    // Run qobj circuits
+    auto result = execute(qobj.circuits, qobj.noise_model, qobj.config);
+
+    // Add QOBJ loading time
+    result.metadata.add(qobj_time_taken, "time_taken_load_qobj");
 
-    auto result = execute(qobj.circuits, noise_model, config);
     // Get QOBJ id and pass through header to result
     result.qobj_id = qobj.id;
     if (!qobj.header.empty()) {
       result.header = qobj.header;
     }
+
     // Stop the timer and add total timing data including qobj parsing
-    auto timer_stop = myclock_t::now();
     auto time_taken =
-        std::chrono::duration<double>(timer_stop - timer_start).count();
+        std::chrono::duration<double>(myclock_t::now() - timer_start).count();
     result.metadata.add(time_taken, "time_taken");
+
     return result;
   } catch (std::exception &e) {
     // qobj was invalid, return valid output containing error message
@@ -1007,7 +997,7 @@ Result Controller::execute(std::vector<Circuit> &circuits,
     auto timer_stop = myclock_t::now();
     auto time_taken =
         std::chrono::duration<double>(timer_stop - timer_start).count();
-    result.metadata.add(time_taken, "time_taken");
+    result.metadata.add(time_taken, "time_taken_execute");
   }
   // If execution failed return valid output reporting error
   catch (std::exception &e) {

src/framework/qobj.hpp

@@ -21,6 +21,7 @@
 #include <vector>
 
 #include "framework/circuit.hpp"
+#include "noise/noise_model.hpp"
 
 namespace AER {
 
@@ -40,7 +41,7 @@ class Qobj {
 
   // Deserialization constructor
   template <typename inputdata_t>
-  Qobj(const inputdata_t &input, bool truncation = false);
+  Qobj(const inputdata_t &input);
 
   //----------------------------------------------------------------
   // Data
@@ -50,6 +51,7 @@ class Qobj {
   std::vector<Circuit> circuits;  // List of circuits
   json_t header;                  // (optional) passed through to result
   json_t config;                  // (optional) qobj level config data
+  Noise::NoiseModel noise_model;  // (optional) noise model
 };
 
 //============================================================================
@@ -60,7 +62,7 @@ class Qobj {
 inline void from_json(const json_t &js, Qobj &qobj) { qobj = Qobj(js); }
 
 template <typename inputdata_t>
-Qobj::Qobj(const inputdata_t &input, bool truncation) {
+Qobj::Qobj(const inputdata_t &input) {
   // Check required fields
   if (Parser<inputdata_t>::get_value(id, "qobj_id", input) == false) {
     throw std::invalid_argument(R"(Invalid qobj: no "qobj_id" field)");
@@ -73,9 +75,29 @@ Qobj::Qobj(const inputdata_t &input, bool truncation) {
     throw std::invalid_argument(R"(Invalid qobj: no "experiments" field.)");
   }
 
-  // Get header and config;
-  Parser<inputdata_t>::get_value(config, "config", input);
-  Parser<inputdata_t>::get_value(header, "header", input);
+  // Apply qubit truncation
+  bool truncation = true;
+
+  // Parse config
+  if (Parser<inputdata_t>::get_value(config, "config", input)) {
+    // Check for truncation option
+    Parser<json_t>::get_value(truncation, "enable_truncation", config);
+
+    // Load noise model
+    if (Parser<json_t>::get_value(noise_model, "noise_model", config)) {
+      // If noise model has non-local errors disable trunction
+      if (noise_model.has_nonlocal_quantum_errors()) {
+        truncation = false;
+      }
+    }
+  } else {
+    config = json_t::object();
+  }
+
+  // Parse header
+  if (!Parser<inputdata_t>::get_value(header, "header", input)) {
+    header = json_t::object();
+  }
 
   // Check for fixed simulator seed
   // If simulator seed is set, each experiment will be set to a fixed (but different) seed

test/terra/backends/aer_simulator/test_noise.py

@@ -17,9 +17,11 @@
 from qiskit.providers.aer import noise
 
 import qiskit.quantum_info as qi
+from qiskit import transpile
 from qiskit.circuit import QuantumCircuit, Reset
 from qiskit.circuit.library import QFT
 from qiskit.circuit.library.standard_gates import IGate, HGate
+from qiskit.quantum_info.states.densitymatrix import DensityMatrix
 from test.terra.backends.simulator_test_case import (
     SimulatorTestCase, supported_methods)
 from test.terra.reference import ref_kraus_noise
@@ -156,15 +158,35 @@ def test_kraus_gate_noise(self, method, device):
     def test_kraus_gate_noise_on_QFT(self, method, device):
         """Test Kraus noise on a QFT circuit"""
         shots = 10000
-        noise_model = ref_kraus_noise.kraus_gate_error_noise_models_full()
+
+        # Build noise model
+        error1 = noise.amplitude_damping_error(0.2)
+        error2 = error1.tensor(error1)
+        noise_model = noise.NoiseModel()
+        noise_model.add_all_qubit_quantum_error(error1, ['h'])
+        noise_model.add_all_qubit_quantum_error(error2, ['cp', 'swap'])
+
         backend = self.backend(
             method=method, device=device, noise_model=noise_model)
-        circuit = QFT(3).decompose()
-        circuit.measure_all()
-        ref_target = ref_kraus_noise.kraus_gate_error_counts_on_QFT(shots)
-        result = backend.run(circuit, shots=shots).result()
+        ideal_circuit = transpile(QFT(3), backend)
+
+        # manaully build noise circuit
+        noise_circuit = QuantumCircuit(3)
+        for inst, qargs, cargs in ideal_circuit.data:
+            noise_circuit.append(inst, qargs, cargs)
+            if inst.name == "h":
+                noise_circuit.append(error1, qargs)
+            elif inst.name in ["cp", "swap"]:
+                noise_circuit.append(error2, qargs)
+        # compute target counts
+        noise_state = DensityMatrix(noise_circuit)
+        ref_target = {i: shots * p for i, p in noise_state.probabilities_dict().items()}
+
+        # Run sim
+        ideal_circuit.measure_all()
+        result = backend.run(ideal_circuit, shots=shots).result()
         self.assertSuccess(result)
-        self.compare_counts(result, [circuit], [ref_target], delta=0.1 * shots)
+        self.compare_counts(result, [ideal_circuit], [ref_target], hex_counts=False, delta=0.1 * shots)
 
     @supported_methods(ALL_METHODS)
     def test_clifford_circuit_noise(self, method, device):

test/terra/backends/aer_simulator/test_truncate.py

@@ -40,7 +40,7 @@ def create_circuit_for_truncate(self):
         circuit.measure(1, 1)
         return circuit
 
-    def deveice_backend(self):
+    def device_backend(self):
         return mock.FakeQuito()
 
     def test_truncate_ideal_sparse_circuit(self):
@@ -65,7 +65,7 @@ def test_truncate_default(self):
             [0, 1], [1, 2], [2, 3], [3, 4], [4, 5],
             [5, 6], [6, 7], [7, 8], [8, 9], [9, 0]
         ]
-        noise_model = NoiseModel.from_backend(self.deveice_backend())
+        noise_model = NoiseModel.from_backend(self.device_backend())
         backend = self.backend(noise_model=noise_model)
         circuit = transpile(
             self.create_circuit_for_truncate(),
@@ -78,7 +78,7 @@ def test_truncate_default(self):
 
     def test_truncate_non_measured_qubits(self):
         """Test truncation of non-measured uncoupled qubits."""
-        noise_model = NoiseModel.from_backend(self.deveice_backend())
+        noise_model = NoiseModel.from_backend(self.device_backend())
         backend = self.backend(noise_model=noise_model)
         circuit = transpile(
             self.create_circuit_for_truncate(),
@@ -95,7 +95,7 @@ def test_truncate_disable_noise(self):
             [0, 1], [1, 2], [2, 3], [3, 4], [4, 5],
             [5, 6], [6, 7], [7, 8], [8, 9], [9, 0]
         ]
-        noise_model = NoiseModel.from_backend(self.deveice_backend())
+        noise_model = NoiseModel.from_backend(self.device_backend())
         backend = self.backend(noise_model=noise_model, enable_truncation=False)
         circuit = transpile(
             self.create_circuit_for_truncate(),