In similar situations, I've wondered the same thing. However, my experience in 'ghost zoning' in typical FEM simulations is that the applications tend to go out of there way to ensure that the numerical values in ghost zones (e.g. zones shared by more than one processor) are bit-for-bit identical.
In that case, the outcome should be the same, regardless of ultimate order and timing of operations, right? I mean even if for an 8 byte double, some of the bytes get written by one MPI task and some by another but both MPI tasks agree on what those bytes should be, the result is going to be the same regardless, right? Mark On Thu, 2010-07-08 at 09:43, Quincey Koziol wrote: > Hi Mark, > > On Jul 8, 2010, at 11:07 AM, Mark Howison wrote: > > > Hi all, > > > > I'm wondering what happens internally in parallel HDF5 when two or > > more procs have overlapping hyperslab selections, e.g. when each proc > > has a layer of ghost zones for a stencil calculation and it tries to > > write all of the data including the ghost zones. > > > > Does HDF5 issue redundant read/write requests for the elements in the > > overlapping region? Or does it try to shrink the regions until they > > are disjoint? In the MPI-IO VFD, will the behavior depend on > > collective vs. independent mode? > > The HDF5 library doesn't check for overlapping selections and the > behavior is dependent on the timing of the I/O operations (for serial and > parallel independent I/O) and whatever the MPI library does (for parallel > collective I/O), but the MPI standard says that this operation's outcome is > undefined (and thus will likely be subject to timing or process ordering > issues). > > Quincey > > > _______________________________________________ > Hdf-forum is for HDF software users discussion. > [email protected] > http://*mail.hdfgroup.org/mailman/listinfo/hdf-forum_hdfgroup.org -- Mark C. Miller, Lawrence Livermore National Laboratory ================!!LLNL BUSINESS ONLY!!================ [email protected] urgent: [email protected] T:8-6 (925)-423-5901 M/W/Th:7-12,2-7 (530)-753-8511 _______________________________________________ Hdf-forum is for HDF software users discussion. [email protected] http://mail.hdfgroup.org/mailman/listinfo/hdf-forum_hdfgroup.org
