• Henry Weller's avatar
    Pstream: optimisation of data exchange · 56668b24
    Henry Weller authored
    Contributed by Mattijs Janssens.
    
    1. Any non-blocking data exchange needs to know in advance the sizes to
       receive so it can size the buffer.  For "halo" exchanges this is not
       a problem since the sizes are known in advance but or all other data
       exchanges these sizes need to be exchanged in advance.
    
       This was previously done by having all processors send the sizes of data to
       send to the master and send it back such that all processors
       - had the same information
       - all could work out who was sending what to where and hence what needed to
         be received.
    
       This is now changed such that we only send the size to the
       destination processor (instead of to all as previously). This means
       that
       - the list of sizes to send is now of size nProcs v.s. nProcs*nProcs before
       - we cut out the route to the master and back by using a native MPI
         call
    
       It causes a small change to the API of exchange and PstreamBuffers -
       they now return the sizes of the local buffers only (a labelList) and
       not the sizes of the buffers on all processors (labelListList)
    
    2. Reversing the order of the way in which the sending is done when
       scattering information from the master processor to the other
       processors. This is done in a tree like fashion. Each processor has a
       set of processors to receive from/ send to. When receiving it will
       first receive from the processors with the least amount of
       sub-processors (i.e. the ones which return first). When sending it
       needs to do the opposite: start sending to the processor with the
       most amount of sub-tree since this is the critical path.
    56668b24
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