Fix parallezation in transpile.compile

qiskit/transpiler/_transpiler.py

@@ -70,49 +70,76 @@ def compile(circuits, backend,
     basis_gates = basis_gates or backend_conf['basis_gates']
     coupling_map = coupling_map or backend_conf['coupling_map']
 
-    # step 1: Making the list of dag circuits
-    dags = _circuits_2_dags(circuits)
-
-    # step 2: Transpile all the dags
-
-    # FIXME: Work-around for transpiling multiple circuits with different qreg names.
-    # Make compile take a list of initial_layouts.
-    _initial_layout = initial_layout
-
-    # Pick a good initial layout if coupling_map is not already satisfied
-    # otherwise keep it as q[i]->q[i].
-    # TODO: move this inside mapper pass.
-    initial_layouts = []
-    for dag in dags:
-        if (initial_layout is None and not backend.configuration()['simulator']
-                and not _matches_coupling_map(dag, coupling_map)):
-            _initial_layout = _pick_best_layout(dag, backend)
-        initial_layouts.append(_initial_layout)
-    dags = _transpile_dags(dags, basis_gates=basis_gates, coupling_map=coupling_map,
-                           initial_layouts=initial_layouts, seed=seed,
-                           pass_manager=pass_manager)
-
-    # step 3: Making a qobj
-    qobj = _dags_2_qobj(dags, backend_name=backend_name,
-                        config=config, shots=shots, max_credits=max_credits,
-                        qobj_id=qobj_id, basis_gates=basis_gates,
-                        coupling_map=coupling_map, seed=seed)
+    qobj_config = deepcopy(config or {})
+    qobj_config.update({'shots': shots,
+                        'max_credits': max_credits,
+                        'memory_slots': 0})
 
-    return qobj
+    qobj = Qobj(qobj_id=qobj_id or str(uuid.uuid4()),
+                config=QobjConfig(**qobj_config),
+                experiments=[],
+                header=QobjHeader(backend_name=backend_name))
+
+    if seed:
+        qobj.config.seed = seed
 
+    qobj.experiments = parallel_map(_build_exp_parallel, list(range(len(circuits))),
+                                    task_args=(circuits, backend),
+                                    task_kwargs={'initial_layout': initial_layout,
+                                                 'basis_gates': basis_gates,
+                                                 'config': config,
+                                                 'coupling_map': coupling_map,
+                                                 'seed': seed,
+                                                 'pass_manager': pass_manager})
+
+    qobj.config.memory_slots = max(experiment.config.memory_slots for
+                                   experiment in qobj.experiments)
 
-def _circuits_2_dags(circuits):
-    """Convert a list of circuits into a list of dags.
+    qobj.config.n_qubits = max(experiment.config.n_qubits for
+                               experiment in qobj.experiments)
 
-    Args:
-        circuits (list[QuantumCircuit]): circuit to compile
+    return qobj
 
+
+def _build_exp_parallel(idx, circuits, backend, initial_layout=None,
+                        basis_gates='u1,u2,u3,cx,id', config=None,
+                        coupling_map=None, seed=None, pass_manager=None):
+    """Builds a single Qobj experiment.  Usually called in parallel mode.
+    Args:
+        idx (int): Index of circuit in circuits list.
+        circuits (list): List of circuits passed.
+        backend (BaseBackend or str): a backend to compile for
+        initial_layout (list): initial layout of qubits in mapping
+        basis_gates (str): comma-separated basis gate set to compile to
+        config (dict): dictionary of parameters (e.g. noise) used by runner
+        coupling_map (list): coupling map (perhaps custom) to target in mapping
+        initial_layout (list): initial layout of qubits in mapping
+        seed (int): random seed for simulators
+        pass_manager (PassManager): pass manager instance for the tranpilation process
+            If None, a default set of passes are run.
+            Otherwise, the passes defined in it will run.
+            If contains no passes in it, no dag transformations occur.
     Returns:
-        list[DAGCircuit]: the dag representation of the circuits
-        to be used in the transpiler
+        experiment: An instance of an experiment to be added to a Qobj.
     """
-    dags = parallel_map(DAGCircuit.fromQuantumCircuit, circuits)
-    return dags
+
+    circuit = circuits[idx]
+    dag = DAGCircuit.fromQuantumCircuit(circuit)
+
+    if (initial_layout is None and not backend.configuration()['simulator']
+            and not _matches_coupling_map(dag, coupling_map)):
+        _initial_layout = _pick_best_layout(dag, backend)
+    else:
+        _initial_layout = initial_layout
+
+    dag = _transpile_dags_parallel(0, [dag], [_initial_layout],
+                                   basis_gates=basis_gates, coupling_map=coupling_map,
+                                   seed=seed, pass_manager=pass_manager)
+
+    experiment = _dags_2_qobj_parallel(
+        dag, basis_gates=basis_gates, config=config, coupling_map=coupling_map)
+
+    return experiment
 
 
 def _transpile_dags(dags, basis_gates='u1,u2,u3,cx,id', coupling_map=None,