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Status of PIO library

The Status of PIO library design, goals, and relevant concepts in the parallel I/O domain including features such as high performance I/O, backward compatibility, support for various formats, complex encapsulation, and more.

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Status of PIO library

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  1. Status of PIO library

  2. Design goals • Provide parallel IO for all component models • Reduce memory usage • Potentially better performance • Encapsulate complexity into library • Simple interface for component developers to implement

  3. Design goals (con’t) • Extensible for future I/O technology • Backward compatible (node=0) • Support for binary(direct) and netcdf formats • Preserve format of input/output files • Support 2D and 3D arrays • Currently XY • Extensible to XZ or YZ

  4. Terms • PnetCDF: [ANL] • High performance I/O • Different interface • Stable • netCDF4 + HDF5 [NCSA] • Same interface • Needs HDF5 library • Less stable • Lower performance • No support on Blue Gene

  5. Terms (con’t) • Aggregators: • Number of IO tasks the MPI-IO subsystem will use • Accessible through hints • Not currently supported on BGL[?] • Processor stride: • Allows matching of subset of MPI IO nodes to system hardware

  6. Component Model ‘issues’ • POP & CICE: • Missing blocks • Update of neighbors halo • Who writes missing blocks? • Asymmetry between read/write • ‘sub-block’ decompositions not rectangular • CLM • Decomposition not rectangular • Who writes missing data?

  7. Some Concepts • IO decomp vs. COMP decomp • IO decomp == COMP decomp • MPI-IO + message aggregation • IO decomp != COMP decomp • Rearranger needed • MCT [Nov 2006] • ESMF [???] • No component specific info in library • Pair with existing communication tech • 1D arrays in library; component must flatten 2D and 3D arrays

  8. What works • Binary I/O [direct] • Rearrange w/MCT + MPI-IO • MPI-IO no rearrangement • Test on POWER5 + BGL • Issues: • Some configurations fail • MCT errors • memory usage [ 277 vs 18 Mbytes/processor]

  9. What works (con’t) • PnetCDF • MPI-IO no rearrangement • Some configurations work • Prototype added to POP2 • Reads restart and forcing files properly • POPIO benchmark • Test code for correctness and performance • Tested on 30K BGL processors in Oct 06 • Performance issues

  10. POPIO benchmark on BGW

  11. Complexity • Mulitple ways to express decomposition • GDOF: global degree of freedom --> (MCT) • MPI datatypes: offsets + counts (MPI-IO) • Subarrays: start + count (pNetCDF) • Limited expressiveness • Will not support ‘sub-block’ in POP & CICE, CLM • Need common language for interface • Interface between component model and library

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