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MPI Communications. Point to Point Collective Communication Data Packaging. 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?. Send and Receive Synchronization.

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mpi communications

MPI Communications

Point to Point

Collective Communication

Data Packaging

point to point communication send and receive
Point-to-Point CommunicationSend and Receive
  • MPI_Send/MPI_Recv provide point-to-point communication
    • synchronization protocol is not fully specified.
      • what are possibilities?
send and receive synchronization
Send and Receive Synchronization
  • 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 synchronization4
Send and Receive Synchronization
  • 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
mpi send and receive
MPI Send and Receive
  • 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
asynchronous send and receive
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.
detecting completion
Detecting Completion
  • MPI_Wait(&request, &status)
    • requestmatches 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
detecting completion8
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
collective communications
Collective Communications
  • One to Many (Broadcast, Scatter)
  • Many to One (Reduce, Gather)
  • Many to Many (All Reduce, Allgather)
broadcast
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
mpi bcast syntax
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

mpi barrier
MPI_Barrier()

MPI_Barrier(MPI_COMM_WORLD);

MPI_COMM_WORLD communicator within which

broadcast takes place

provides for barrier synchronization without message of broadcast

reduce
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
mpi reduce syntax
MPI_Reduce() syntax

MPI_Reduce(&dataIn, &result, count,

MPI_DOUBLE, MPI_SUM, root,

MPI_COMM_WORLD);

dataIn data sent from each processor

resultstores result of combining operation

countnumber of items in each of dataIn, result

MPI_DOUBLEdata type for dataIn, result

MPI_SUMcombining operation

rootrank of processor receiving data

MPI_COMM_WORLDcommunicator

mpi reduce
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
mpi scatter
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
mpi gather
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
many to many communications
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
data packaging
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
mpi pack syntax
MPI_Pack() syntax

MPI_Pack(Aptr, count, MPI_DOUBLE,

buffer, size, &pos, MPI_COMM_WORLD);

Aptr pointer to data to pack

countnumber of items to pack

type of items

bufferbuffer being packed

sizesize of buffer (in bytes)

posposition in buffer (in bytes), updated

communicator

mpi unpack
MPI_Unpack()
  • reverses operation of MPI_Pack()

MPI_Unpack(buffer, size, &pos, Aptr, count, MPI_DOUBLE, MPI_COMM_WORLD);