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Non-Blocking Collective I/O Routines

Non-Blocking Collective I/O Routines. Introduction. I/O is the main bottleneck in HPC applications. To mitigate that performance, several approaches have been taken: Collective I/O for aggregation Non-blocking individual I/O Higher level libraries (PHDF5 , ADIOS, etc…). Motivation.

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Non-Blocking Collective I/O Routines

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  1. Non-Blocking Collective I/O Routines

  2. Introduction • I/O is the main bottleneck in HPC applications. • To mitigate that performance, several approaches have been taken: • Collective I/O for aggregation • Non-blocking individual I/O • Higher level libraries (PHDF5 , ADIOS, etc…)

  3. Motivation • Routines for non-blocking individual I/O operations exist (MPI_File_i(read/write)(_at) • Non-blocking point-to-point (existing) and collective (to be added) communication operations have demonstrated benefits. • Split collective I/O operations have their restrictions and limitations. • What’s keeping us from adding non-blocking collective I/O operations? • Implementation

  4. New Routines • MPI_File_iread_all (MPI_File file, void *buf, int count, MPI_Datatype type, MPI_Status *status, MPI_Request *req); • MPI_File_iwrite_all (MPI_File file, void *buf, int count, MPI_Datatype type, MPI_Status *status, MPI_Request *req); • MPI_File_iread_at_all (MPI_File file, MPI_Offset offset, void *buf, int count, MPI_Datatype type, MPI_Status *status, MPI_Request *req); • MPI_File_iwrite_at_all (MPI_File file, MPI_Offset offset, void *buf, int count, MPI_Datatype type, MPI_Status *status, MPI_Request *req);

  5. Implementation • Major difference between collective communication and collective I/O operations: • each process is allowed to provide different volumes of data to a collective I/O operation, without having knowledge on the data volumes provided by other processes • Can’t post entire I/O operation to disk directly • Have to read/write data in cycles • Have to do Aggregation

  6. Implementation • Need non-blocking collective communication • Fortunately will be available • Integrate with the progress engine • Test/Wait on the request like other non-blocking operations • Explicit or Implicit progress? • Different collective I/O algorithms

  7. Conclusion • The need for non-blocking collective I/O is fairly high. • Implementation is the non-easy part. • Performance benefits can be substantial.

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