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Subject: Re: [OMPI devel] failure with zero-length Reduce() andbothsbuf=rbuf=NULL
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2010-02-10 11:42:43


Here is a related case.

If I remember correctly, the HPCC pingpong test synchronizes
occasionally by having one process send a zero-byte broadcast to all
other processes. What's a zero-byte broadcast? Well, some MPIs
apparently send no data, but do have synchronization semantics. (No
non-root process can exit before the root process has entered.) Other
MPIs treat the zero-byte broadcasts as no-ops; there is no
synchronization and then timing results from the HPCC pingpong test are
very misleading. So far as I can tell, the MPI standard doesn't address
which behavior is correct. The test strikes me as deficient: it would
have been just as easy to have a single-word broadcast to implement the
synchronization they were looking for.

Sigh.