CS 420 Design of Algorithms

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CS 420 Design of Algorithms

• MPI Data Types
• Basic Message Passing - sends/receives

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MPI Data Types

• MPI has a set of its own data type definitions
• Most MPI types correspond to C types although
  some are unique to MPI
• C types may vary by implementation
• MPI types are consistent across implementations
• Supports portability in parallel applications

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MPI types and C types
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MPI types and C types
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Other MPI Data Types

• MPI_BYTE
• MPI_PACKED
• May be others depending on the implementation

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MPI send and receive

• MPI_Send and MPI_Recv are the most elemental
  forms of MPI data communications
• Provide the core set of functions
• MPI_Send and MPI_Recv are blocking communications
• Processing cannot proceed until the communication
  process is complete.

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MPI_Send send a message
MPI_Send( void message, int count, MPI_
Datatype datatype, int dest, int tag, M
PI_Comm comm)
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MPI_Send
MPI_Send(a,1,MPI_FLOAT,myrank1,11,MPI_COMM_WORLD)
sends a single float to the next process in
MPI_COMM_WORLD and attaches a tag of 11.
MPI_Send(vect,100, MPI_FLOAT,5,12,
MPI_COMM_WORLD) sends a vector of 100
floats to process 5 in MPI_COMM_WORLD and uses a
tag of 12.
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MPI_Recv
int MPI_Recv( void message, int count, MP
I_Datatype datatype, int source, int tag, M
PI_Comm comm, MPI_Status status)
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MPI_Recv
MPI_Recv(x, 1, MPI_FLOAT, lsource, 11,
MPI_COMM_WORLD, status) picks up a message
with a tag11,from the source lsource in
MPI_COMM_WORLD. The status of the transaction is
stored in status. MPI_Recv(xarray, 100,
MPI_FLOAT, xproc, 12, MPI_COMM_WORLD,
status) picks up a message tagged 12 from the
source xproc in MPI_COMM_WORLD. That status of
the transaction is stored in status.
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MPI_Recv - wildcards

• MPI_ANY_SOURCE
• lets MPI_Recv take a message from any source.
  Use as the source parameter
• MPI_ANY_TAG
• lets MPI_Recv take a message regardless of its
  tag.

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Wildcards

• No wildcards for MPI_Send
• No wildcards for communicator
• Send must specify a tag
• Recv must specify a matching tag or use a
  wildcard

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MPI_ANY_SOURCE

• To receive from any process
• MPI_Recv(x, 1, MPI_FLOAT, MPI_ANY_SOURCE, 11,
  MPI_COMM_WORLD, status)
• and if you dont care what the tag is-
• MPI_Recv(x, 1, MPI_FLOAT, lsource, MPI_ANY_TAG,
  MPI_COMM_WORLD,status)

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MPI_Status

• Return variable for the results of the
  communication process.
• A structure
• Roughly
• struct MPI_STATUS
• MPI_SOURCE source
• MPI_TAG tag
• MPI_ERROR errormsg

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MPI_Status

• Note no byte count in status variable (how many
  bytes received in the message)
• MPI_Get_count(
• MPI_Status status,
• MPI_Datatype datatype,
• int count_ptr)

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MPI Basic Core Functions

• MPI_Init
• MPI_Comm_rank
• MPI_Comm_size
• MPI_Send
• MPI_Recv
• MPI_Finalize

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