QMap holding memory not release to OS

QMap not release memory to OS after program running for a while.

I have create a simple situation.

• Create a 100 threads, each thread loop 10000 and doing 2 two function below

  inline void raiseMemory(const qint64 numLoops)
  {
      auto data = QMap<std::string, QByteArray>({});
      for (int i = 0; i < numLoops; ++i) {
          data.insert(std::to_string(i), QByteArray(1024, 'D'));
      }
  }
  inline void raiseCpu(const qint64 numLoops)
  {
      uint64_t sum = 0;
      for (uint64_t i = 0; i < numLoops; ++i) {
          sum += i * i % 123456789;
      }
  } 

• The main thread:

  // [1] Start threads
  qInfo() << "Sleeping for 3s before starting threads";
  QThread::msleep(3000);
  qInfo() << "Starting threads";
  QList<QThread*> threads;
  for (int i = 0; i < inArgs.numThreads; ++i) {
      QThread* thread = QThread::create([&inArgs]() {
          raiseCpu(inArgs.numLoops);
          raiseMemory(inArgs.numLoops);
      });
      threads.append(thread);
      thread->start();
  }// [2] Wait for threads to finish
  for (QThread* thread : threads) {
      thread->wait();
  }
  qInfo() << "All threads finished. Sleeping for 3s before deleting threads.";
  QThread::msleep(3000);// [3] Delete threads
  for (QThread* thread : threads) {
      delete thread;
  }
  qInfo() << "All threads deleted. Sleeping for 3s before finishing.";
  QThread::msleep(3000); 

After running I got a chart – the memory does not go down

I replaced QMap with QVector, QList and the memory will drop immediately when the threads finish their work.

I checked Qt documentation but couldn't find any documentation related to this issue.

And I think this is Qt BUG as the behavior of each container class is different.

Please check this out

My sample source code attached below