New feature: intra-node write aggregation #156
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This new feature is designed to reduce the possible congestion of all-to-many
personalized communication inside of MPI-IO when the number of allocated MPI
processes per compute node is large, for example larger than 64 per node, and
the non-contiguous requests per process spread over a wide range of file space.
Such congestion have been observed when running the E3SM-IO F case using 21600
MPI processes on DOE parallel computers, Perlmutter at NRSC, Frontier at OLCF,
and Aurora at ALCF.
When this feature enabled, a subset of MPI processes running on the same
compute node will be selected to be the aggregator. The non-aggregators send
their write requests to the assigned aggregators and then only the aggregators
make the MPI file write calls to write to the file.
To enable this new feature, a PnetCDF I/O hint 'nc_num_aggrs_per_node' can set
by the users. When setting its value to a positive integral value, PnetCDF will
select this number of processes per node to be the aggregators. Setting it to 0
disables the aggregation (the default mode.)