Fix performance regression from QuantumError assemble

Fixes issue with QuantumError.to_dict causing a huge performance regression in noisy simulations from its calling of `qiskit.assemble` inside Pybind11 code.

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 +1,7 @@
+---
+fixes:
+  - |
+    Fix performance regression in noisy simulations due to large increase in
+    serialization overhead for loading noise models from Python into C++.
+    See `issue 1407 <https://github.com/Qiskit/qiskit-aer/issues/1407>`_ for
+    details.

src/controllers/aer_controller.hpp

@@ -846,37 +846,31 @@ Result Controller::execute(const inputdata_t &input_qobj) {
     // Start QOBJ timer
     auto timer_start = myclock_t::now();
 
-    Noise::NoiseModel noise_model;
-    json_t config;
-
-    // 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);
-    }
+    // Initialize QOBJ
+    Qobj qobj(input_qobj, enable_truncation_);
+    auto qobj_time_taken =
+        std::chrono::duration<double>(myclock_t::now() - timer_start).count();
 
-    // 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_);
-    }
+    // Set config
+    set_config(qobj.config);
+
+    // 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 +1001,7 @@ 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 {
 
@@ -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
 };
 
 //============================================================================
@@ -73,9 +75,23 @@ 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);
+  // Get config
+  if (Parser<inputdata_t>::get_value(config, "config", input)) {
+    // Parse noise model
+    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