1. MPI Communications Point to Point Collective Communication Data Packaging

2. Point-to-Point Communication Send and Receive MPI_Send/MPI_Recv provide point-to- point communication –synchronization protocol is not fully specified. what are possibilities?

3. Fully Synchronized (Rendezvous) –Send and Receive complete simultaneously whichever code reaches the Send/Receive first waits –provides synchronization point (up to network delays) Buffered –Receive must wait until message is received –Send completes when message is moved to buffer clearing memory of message for reuse Send and Receive Synchronization

4. Asynchronous –Sending process may proceed immediately does not need to wait until message is copied to buffer must check for completion before using message memory –Receiving process may proceed immediately will not have message to use until it is received must check for completion before using message Send and Receive Synchronization

5. MPI_Send/MPI_Recv are synchronous, but buffering is unspecified –MPI_Recv suspends until message is received –MPI_Send may be fully synchronous or may be buffered implementation dependent Variations allow synchronous or buffering to be specified MPI Send and Receive

6. Asynchronous Send and Receive MPI_Isend() / MPI_Irecv() are non-blocking. Control returns to program after call is made. Syntax is the same as for Send and Recv, except a MPI_Request* parameter is added to Isend and replaces the MPI_Status* for receive.

7. Detecting Completion MPI_Wait(&request, &status) –request matches request on Isend or Irecv –status returns status equivalent to status for Recv when complete –Blocks for send until message is buffered or sent so message variable is free –Blocks for receive until message is received and ready

8. Detecting Completion MPI_Test(&request, flag, &status) –request, status as for MPI_Wait –does not block –flag indicates whether message is sent/received –enables code which can repeatedly check for communication completion

9. Collective Communications One to Many (Broadcast, Scatter) Many to One (Reduce, Gather) Many to Many (All Reduce, Allgather)

10. Broadcast A selected processor sends to all other processors in the communicator Any type of message can be sent Size of message should be known by all (it could be broadcast first) Can be optimized within system for any given architecture

11. MPI_Bcast() Syntax MPI_Bcast(mess, count, MPI_INT, root, MPI_COMM_WORLD); mess pointer to message buffer count number of items sent MPI_INT type of item sent Note: count and type should be the same on all processors root sending processor MPI_COMM_WORLD communicator within which broadcast takes place

12. MPI_Barrier() MPI_B arrier (MPI_COMM_WORLD); MPI_COMM_WORLD communicator within which broadcast takes place provides for barrier synchronization without message of broadcast

13. Reduce All Processors send to a single processor, the reverse of broadcast Information must be combined at receiver Several combining functions available –MAX, MIN, SUM, PROD, LAND, BAND, LOR, BOR, LXOR, BXOR, MAXLOC, MINLOC

14. MPI_Reduce() syntax MPI_Reduce(&dataIn, &result, count, MPI_DOUBLE, MPI_SUM, root, MPI_COMM_WORLD); dataIn data sent from each processor result stores result of combining operation count number of items in each of dataIn, result MPI_DOUBLE data type for dataIn, result MPI_SUM combining operation root rank of processor receiving data MPI_COMM_WORLD communicator

15. MPI_Reduce() Data and result may be arrays -- combining operation applied element-by-element Illegal to alias dataIn and result –avoids large overhead in function definition

16. MPI_Scatter() Spreads array to all processors Source is an array on the sending processor Each receiver, including sender, gets a piece of the array corresponding to their rank in the communicator

17. MPI_Gather() Opposite of Scatter Values on all processors (in the communicator) are collected into an array on the receiver Array locations correspond to ranks of processors

18. Collective Communications, underneath the hood

19. Many to Many Communications MPI_Allreduce –Syntax like reduce, except no root parameter –All nodes get result MPI_Allgather –Syntax like gather, except no root parameter –All nodes get resulting array Underneath -- virtual butterfly network

20. Data packaging Needed to combine irregular, non- contiguous data into single message pack -- unpack, explicitly pack data into a buffer, send, unpack data from buffer Derived data types, MPI heterogeneous data types which can be sent as a message

21. MPI_Pack() syntax MPI_Pack(Aptr, count, MPI_DOUBLE, buffer, size, &pos, MPI_COMM_WORLD); Aptr pointer to data to pack count number of items to pack type of items buffer buffer being packed size size of buffer (in bytes) pos position in buffer (in bytes), updated communicator

22. MPI_Unpack() reverses operation of MPI_Pack() MPI_Unpack(buffer, size, &pos, Aptr, count, MPI_DOUBLE, MPI_COMM_WORLD);