On Jul 8, 2010, at 12:06 PM, Mark Miller wrote:
> 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?
Probably, but the MPI standard doesn't guarantee it. (And I can't
think of a realistic scenario where an MPI implementation would end up with
different bytes)
Quincey
> 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
>>
>>
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> --
> Mark C. Miller, Lawrence Livermore National Laboratory
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> [email protected] urgent: [email protected]
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