fix a bug in multiple_chunk_required (Qiskit#1282)

releasenotes/notes/bug-fix-required-memory-check-a970c4dc6eb2896f.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    When required memory is smaller than the system memory, multi-chunk was enabled
+    and simulation was started though memory is short. This fix corrects this behavior
+    to throw an exception.

src/controllers/controller.hpp

@@ -467,16 +467,23 @@ void Controller::set_parallelization_circuit(const Circuit &circ,
 }
 
 bool Controller::multiple_chunk_required(const Circuit &circ,
-                                const Noise::NoiseModel &noise) const
-{
-  if(circ.num_qubits < 3)
+                                const Noise::NoiseModel &noise) const {
+  if (circ.num_qubits < 3)
     return false;
 
-  if(num_process_per_experiment_ > 1 || Controller::get_min_memory_mb() < required_memory_mb(circ, noise))
+  if (cache_block_qubit_ >= 2 && cache_block_qubit_ < circ.num_qubits)
     return true;
 
-  if(cache_block_qubit_ >= 2 && cache_block_qubit_ < circ.num_qubits)
-    return true;
+  if(num_process_per_experiment_ == 1 && cache_block_qubit_ >= 2 && num_gpus_ > 0)
+    return (max_gpu_memory_mb_ / num_gpus_ < required_memory_mb(circ, noise));
+
+  if(num_process_per_experiment_ > 1) {
+    size_t total_mem = max_memory_mb_;
+    if(cache_block_qubit_ >= 2)
+      total_mem += max_gpu_memory_mb_;
+    if(total_mem*num_process_per_experiment_ > required_memory_mb(circ, noise))
+      return true;
+  }
 
   return false;
 }