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1.2 Current Parallel Programming Paradigms - page 19 / 33

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COMMUNICATOR MODES

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Rationale

Although

in

MPI,

they

contain

groups

are

usually

a

considered to be local objects list of participating processes.

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Since this list might have changed during recovery, all user fined groups are considered to be potentially out of date.

de-

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After the recovery operation, the user has access to the same non-local operations like after MPI Init. These are:

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Groups: none

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Communicators: MPI COMM WORLD and MPI COMM SELF.

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Rationale It would be theoretically possible to modify non-local objects on the surviving processes such, that they contain the up-to-date information of the run-time environment. However, assuming that failed processes are replaced by the run-time envi- ronment (see the following section) there is no MPI function call to pass the additional handles to the re-spawned processes in a portable, MPI conforming manner.

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Groups and Communicators can have di erent formats after the recovery procedure, depending on the communicator mode. The communicator mode specifies, how the run-time environment should treat failed processes. Four modes are currently defined:

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1. FTMPI COMM MODE ABORT: like in MPI-1 and MPI-2, the MPI library will abort the execution if one or several processes have failed. This mode is available for backward compatibility.

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2. FTMPI COMM MODE REBUILD: failed processes will be replaced by the run-time environment. Surviving processes will retain their rank in MPI COMM WORLD. No assumptions are made within the FT-MPI specification where the new processes are placed.

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3. FTMPI COMM MODE BLANK: failed processes will not be replaced, the size of MPI COMM WORLD will remain unchanged. However, the failed processes are blanked out and treated similarly to MPI- PROC NULL. Detailed specifications about operations using blank processes can be found in the next subsections.

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4. FTMPI COMM MODE SHRINK: failed processes will not be replaced. The size of MPI COMM WORLD will be adjusted to the number of

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