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    INT: Integration of Mattijs' collocated parallel IO additions · d8d6030a
    Andrew Heather authored
    Original commit message:
    Parallel IO: New collated file format
    When an OpenFOAM simulation runs in parallel, the data for decomposed fields and
    mesh(es) has historically been stored in multiple files within separate
    directories for each processor.  Processor directories are named 'processorN',
    where N is the processor number.
    This commit introduces an alternative "collated" file format where the data for
    each decomposed field (and mesh) is collated into a single file, which is
    written and read on the master processor.  The files are stored in a single
    directory named 'processors'.
    The new format produces significantly fewer files - one per field, instead of N
    per field.  For large parallel cases, this avoids the restriction on the number
    of open files imposed by the operating system limits.
    The file writing can be threaded allowing the simulation to continue running
    while the data is being written to file.  NFS (Network File System) is not
    needed when using the the collated format and additionally, there is an option
    to run without NFS with the original uncollated approach, known as
    The controls for the file handling are in the OptimisationSwitches of
        //- Parallel IO file handler
        //  uncollated (default), collated or masterUncollated
        fileHandler uncollated;
        //- collated: thread buffer size for queued file writes.
        //  If set to 0 or not sufficient for the file size threading is not used.
        //  Default: 2e9
        maxThreadFileBufferSize 2e9;
        //- masterUncollated: non-blocking buffer size.
        //  If the file exceeds this buffer size scheduled transfer is used.
        //  Default: 2e9
        maxMasterFileBufferSize 2e9;
    When using the collated file handling, memory is allocated for the data in the
    thread.  maxThreadFileBufferSize sets the maximum size of memory in bytes that
    is allocated.  If the data exceeds this size, the write does not use threading.
    When using the masterUncollated file handling, non-blocking MPI communication
    requires a sufficiently large memory buffer on the master node.
    maxMasterFileBufferSize sets the maximum size in bytes of the buffer.  If the
    data exceeds this size, the system uses scheduled communication.
    The installation defaults for the fileHandler choice, maxThreadFileBufferSize
    and maxMasterFileBufferSize (set in etc/controlDict) can be over-ridden within
    the case controlDict file, like other parameters.  Additionally the fileHandler
    can be set by:
    - the "-fileHandler" command line argument;
    - a FOAM_FILEHANDLER environment variable.
    A foamFormatConvert utility allows users to convert files between the collated
    and uncollated formats, e.g.
        mpirun -np 2 foamFormatConvert -parallel -fileHandler uncollated
    An example case demonstrating the file handling methods is provided in:
    The work was undertaken by Mattijs Janssens, in collaboration with Henry Weller.