message: avoid using MPI_Rget to make SSC work on Summit
diff: @@ -60,16 +60,11 @@ void SscReader::GetDeferredCommon(Variable<std::string> &variable,
  totalDataSize += i.second.second;
  }
  m_Buffer.resize(totalDataSize);
  - std::vector<MPI_Request> requests;
  for (const auto &i : m_AllReceivingWriterRanks)
  {
  - requests.emplace_back();
  - MPI_Rget(m_Buffer.data() + i.second.first, i.second.second,
  - MPI_CHAR, i.first, 0, i.second.second, MPI_CHAR, m_MpiWin,
  - &requests.back());
  + MPI_Get(m_Buffer.data() + i.second.first, i.second.second, MPI_CHAR,
  + i.first, 0, i.second.second, MPI_CHAR, m_MpiWin);
  }
  - MPI_Status statuses[requests.size()];
  - MPI_Waitall(requests.size(), requests.data(), statuses);
  }
  for (const auto &i : m_AllReceivingWriterRanks)
  @@ -146,16 +141,11 @@ void SscReader::GetDeferredCommon(Variable<T> &variable, T *data)
  totalDataSize += i.second.second;
  }
  m_Buffer.resize(totalDataSize);
  - std::vector<MPI_Request> requests;
  for (const auto &i : m_AllReceivingWriterRanks)
  {
  - requests.emplace_back();
  - MPI_Rget(m_Buffer.data() + i.second.first, i.second.second,
  - MPI_CHAR, i.first, 0, i.second.second, MPI_CHAR, m_MpiWin,
  - &requests.back());
  + MPI_Get(m_Buffer.data() + i.second.first, i.second.second, MPI_CHAR,
  + i.first, 0, i.second.second, MPI_CHAR, m_MpiWin);
  }
  - MPI_Status statuses[requests.size()];
  - MPI_Waitall(requests.size(), requests.data(), statuses);
  }
  for (const auto &i : m_AllReceivingWriterRanks)
  