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Jonathan Carroll-Nellenback CIRC Summer School MESSAGE PASSING INTERFACE (MPI)
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Global Communicator MPI_COMM_WORLD Global Communication Routines: [All]Gather[v] Scatter[v] [All]Reduce[v] Alltoall[v] BCast Barrier Reduction Operators MPI_[MAX,MIN,SUM,PROD], MPI[B,L][AND,OR,XOR] Basic Data Types (put MPI_ in front of name of data type) Fortran - MPI_[CHARACTER,INTEGER,REAL,LOGICAL,...] C – MPI_[CHAR,SHORT,INT,LONG,...] REVIEW
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Send Buffer – The starting address of the data to be sent Send Count – The number of elements in the send buffer Send Type – The type of elements in the send buffer Recv Buffer – The starting address of the recv buffer Recv Count – The number of elements to recv Recv Type – The type of element to recv Displacements – The offsets for Gatherv & Scatterv etc... Tag – A message identifier Root – The '1' in all-to-1 or 1-to-all communication Dest – The destination for a point to point send Source – The source for a point to point recv Communicator – An independent collection of mpi tasks Request – A handle to keep track of non-blocking sends or receives Status – The status of a non-blocking send or any receive TYPES OF MPI ARGUMENTS
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VARIOUS WAY TO PARALLELIZE DO LOOPS DO i=1,n a(i)=f(i) END DO a=0 DO i=rank+1,n,procs a(i)=f(i) END DO CALL MPI_Allreduce(MPI_IN_PLACE, a, n, MPi_REAL, MPI_SUM, MPI_COMM_WORLD, err) m=n/procs ALLOCATE(b(m)) DO i=1,m b(i)=f(m*rank+i) END DO CALL MPI_ALLGather(b, m, MPI_REAL, a, m, MPi_REAL, MPI_COMM_WORLD, err) DEALLOCATE(b)
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GATHER VS. GATHERV m=n/procs rem=mod(n,procs) ALLOCATE(sizes(procs), displacements(procs+1)) sizes=(/(m+1,i=1,rem),(m,i=rem+1,procs)/) displacements=(/0,(sum(sizes(1:i)), i=1,procs)/) ALLOCATE(b(sizes(rank))) DO i=1,sizes(rank) b(i)=f(displacements(rank)+i) END DO CALL MPI_ALLGatherv(b, sizes(rank), MPI_REAL, a, sizes, displacements, & MPI_REAL, MPI_COMM_WORLD, err) DEALLOCATE(b,sizes,displacements) m=n/procs ALLOCATE(b(m)) DO i=1,m b(i)=f(m*rank+i) END DO CALL MPI_ALLGather(b, m, MPI_REAL, a, m, MPi_REAL, MPI_COMM_WORLD, err) DEALLOCATE(b)
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AHMDAL'S LAW (THE HARD TRUTH) Glass half full Glass half empty Speed up S expected for a program run on n processors where P is the fraction of the program that runs in parallel
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Fortran: CALL MPI_WTIME(time) C: time=MPI_WTIME() Measure the performance of exercise2p.f90 or exercise2p.c MEASURING PERFORMANCE
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Parallelize exercise3.f90 using an MPI_Reduce and measure the scaling with N=512, and N=1024 and 1, 4, and 16 procs. EXERCISE 3
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/public/jcarrol5/mpi/example4.f90 Tags – additional identifiers on messages MPI_Send MPI_Recv BASIC SENDING AND RECEIVING
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Modify your program from exercise2 to only use point to point communication routines. (You can start with exercise2p.f90 or exercise2p.c) EXERCISE 4
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Blocking vs Non-blocking Non Blocking sends and receives will immediately return control to the calling routine. However, they usually will require buffering and testing later on to see whether the send/recv has completed. Good for overlapping communication with computation May lead to extra buffering Synchronous vs Asynchronous Synchronous sends require a matching recv to be called before returning. Blocking only if recv has not been posted. Does not require any additional buffering. Buffered vs NonBuffered Buffered sends explicitly buffer the data to be sent so that the calling routine can release the memory. Ready send Assumes that the receiver has already posted the recv. SENDING MODES
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/public/jcarrol5/mpi/example4.f90 MPI_Send – May or may not block MPI_Bsend – May buffer – returns immediately MPI_Ssend – Synchronous Send (returns after matching recv posted) MPi_Rsend – Ready send (matching recv must be posted) MPI_Isend – Nonblocking send (must check for completion) MPI_Ibsend – Nonblocking buffered send MPI_Issend – Nonblocking synchronous send MPI_Irsend- Nonblocking ready send MPI_Recv – Blocking receive MPI_IRecv – Nonblocking receive SEND ROUTINES
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Rewrite exercise 3 using ready sends(rsend), synchronous sends (ssend), and nonblocking sends (isend) and see if it is any faster. EXERCISE 5
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/public/jcarrol5/mpi/example5.f90 MPI starts with one communicator (MPI_COMM_WORLD) Separate communicator groups can be formed using MPI_Comm_split Or you can extract the group belonging to mpi_comm_world and create subgroups through various routines. Multiple communicators can use the same group. COMMUNICATORS AND GROUPS
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