[Mpi3-ft] MPI_Init / MPI_Finalize

Bronis R. de Supinski bronis at llnl.gov
Wed Aug 25 23:08:36 CDT 2010


There is no wiggle room. MPI_FINALIZE is collective across
MPI_COMM_WORLD. I do not understand why you would say otherwise.
Here is more of the passage I was quoting:


MPI_FINALIZE is collective over all connected processes. If no processes 
were spawned, accepted or connected then this means over  MPI_COMM_WORLD; 
otherwise it is collective over the union of all processes that have been 
and continue to be connected, as explained in Section Releasing 
Connections  on page Releasing Connections.


The "connected" terminology is used to handle dynamic process
management issues, for which the set of all processes cannot
easily be defined in terms of a single communicator.


On Wed, 25 Aug 2010, Fab Tillier wrote:

> What defines "connected"?  MPI_FINALIZE isn't collective across MPI_COMM_WORLD, as processes might never communicate with one another.  Even if they do, communication may not require a connection, so they may never be connected.
> It seems to me there might be enough wiggle room in the standard to allow MPI_Finalize to not be collective at all?
> -Fab
> Bronis R. de Supinski wrote on Wed, 25 Aug 2010 at 15:06:38
>> Josh:
>> On p293 of the 2.2 standard, it says "MPI_FINALIZE is collective
>> over all connected processes." I don't know that the call being
>> collective changes your analysis but your statement that the
>> call is not collective was incorrect...
>> Bronis
>> On Wed, 25 Aug 2010, Joshua Hursey wrote:
>>> During the discussion of the run-though stabilization proposal today
>>> on the teleconf, we spent a while discussing the expected behavior of
>>> MPI_Init and MPI_Finalize in the presence of process failures. I would
>>> like to broaden the discussion a bit to help pin down the expected
>>> behavior.
>>> MPI_Init(): ----------- Problem: If a process fails before or during
>>> MPI_Init, what should the MPI implementation do?
>>> The current standard says nothing about the return value of
>>> MPI_Init() (Ch. 8.7). To the greatest possible extent the application
>>> should not be put in danger if it wishes to ignore errors (assumes
>>> MPI_ERRORS_ARE_FATAL), so returning an error from this function (in
>>> contrast to aborting the job) might be dangerous. However, if the
>>> application is prepared to handle process failures, it is unable to
>>> communicate that information to the MPI implementation until after the
>>> completion of MPI_Init().
>>> So a couple of solutions were presented each with pros and cons (please
>>> fill in if I missed any): 1) If a process fails in MPI_Init() (default
>>> error handler is
>>> MPI_ERRORS_ARE_FATAL) then the entire job is aborted (similar to
>>> calling MPI_Abort on MPI_COMM_WORLD).
>>> 2) If a process fails in MPI_Init() the MPI implementation will
>>> return an appropriate error code/class (e.g., MPI_ERR_RANK_FAIL_STOP),
>>> and all subsequent calls into the MPI implementation will return the
>>> error class MPI_ERR_OTHER (should be create a MPI_ERR_NOT_ACTIVE?).
>>> Applications should eventually notice the error and terminate.
>>> 3) Allow the application to register only the MPI_ERRORS_RETURN
>>> handle on MPI_COMM_WORLD before MPI_Init() using the
>>> MPI_Errhandler_set() function. Errors that occur before the
>>> MPI_Errhandler_set() call are fatal. Errors afterward, including during
>>> MPI_Init() are not fatal.
>>> In the cases where MPI_Init() returns MPI_ERR_RANK_FAIL_STOP to
>>> indicate a process failure, is the library usable or not? If the
>>> application can continue running through the failure, then the MPI
>>> library should still be usable, thus MPI_Init() must be fault tolerant
>>> in its initialization to be able to handle process failures. If the MPI
>>> implementation finds itself in trouble and cannot continue it should
>>> return MPI_ERR_CANNOT_CONTINUE from all subsequent calls including
>>> MPI_Init, if possible.
>>> MPI_Finalize():
>>> ---------------
>>> Problem: If a process fails before or during MPI_Finalize (and the
>>> error handler is not MPI_ERRORS_ARE_FATAL), what should this function
>>> return? Should that return value be consistent to all processes?
>>> To preserve locality of fault handling, a local process should not be
>>> explicitly forced to recognize the failure of a peer process that they
>>> never interact with neither directly (e.g., point-to-point) or
>>> indirectly (e.g., collective). So MPI_Finalize should be fault tolerant
>>> and keep trying to complete even in the presence of failures.
>>> MPI_Finalize is not required to be a collective operation, though it
>>> is often implemented that way. An implementation may need to delay the
>>> return from MPI_Finalize until its role in the failure information
>>> distribution channel is complete. But we should not require a multi-
>>> phase commit protocol to ensure that everyone either succeeds or
>>> returns some error. Implementations may do so internally in order to
>>> ensure that MPI_Finalize does not hang.
>>> If MPI_Finalize returns an error (say MPI_ERR_RANK_FAIL_STOP
>>> indicating a 'new to this rank' failure), what good is this information
>>> to the application? It cannot query for which rank(s) failed since MPI
>>> has been finalized. Nor can it initiate recovery. The best it could do
>>> is assume that all other processes failed and take local action.
>>> MPI_Finalize: MPI_COMM_WORLD process rank 0:
>>> --------------------------------------------
>>> In chapter 8, Example 8.7 illustrates that "Although it is not
>>> required that all processes return from MPI_Finalize, it is required
>>> that at least process 0 in MPI_COMM_WORLD return, so that users can
>>> know that the MPI portion of the computation is over."
>>> We deduced that the reasoning for this explanation was to allow for
>>> MPI implementation that create and destroy MPI processes during
>>> init/finalize from rank 0. Or worded differently, rank 0 is the only
>>> rank that can be assumed to exist before MPI_Init and after
>>> MPI_Finalize.
>>> Problem: So what if rank 0 fails at some point during the computation
>>> (or just some point during MPI_Finalize)?
>>> In the proposal, I added an advice to users to tell them to not
>>> depend on any specific ranks to exist before MPI_Init or after
>>> MPI_Finalize. So, in a faulty environment, the example will produce
>>> incorrect results under certain failure scenarios (e.g., failure of
>>> rank 0).
>>> In an MPI environment that depends on rank 0 for process creation and
>>> destruction, the failure of rank 0 is (should be?) critical and the MPI
>>> implementation will either abort the job or return
>>> MPI_ERR_CANNOT_CONTINUE from all calls to the MPI implementation. So we
>>> believe that the advice to users was a sufficient addition to this
>>> section. What do others think?
>>> So MPI_Init seems to be a more complex issue than MPI_Finalize. What
>>> do folks think about the presented problems and possible solutions? Are
>>> there other issues not mentioned here that we should be addressing?
>>> -- Josh
>>> Run-Through Stabilization Proposal:
>>>  https://**svn.mpi-forum.org/trac/mpi-forum-
>>> web/wiki/ft/run_through_stabilization
>>> ------------------------------------
>>> Joshua Hursey
>>> Postdoctoral Research Associate
>>> Oak Ridge National Laboratory
>>> http://**www.**cs.indiana.edu/~jjhursey
>>> _______________________________________________
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>>> mpi3-ft at lists.mpi-forum.org
>>> http://**lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft
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