Comp 422: Parallel Programming Lecture 8: Message Passing (MPI)

Explicit Parallelism Same thing as multithreading for shared memory. Explicit parallelism is more common with message passing. –User has explicit control over processes. –Good: control can be used to performance benefit. –Bad: user has to deal with it.

Distributed Memory - Message Passing proc1proc2proc3procN mem1mem2mem3memN network

Distributed Memory - Message Passing A variable x, a pointer p, or an array a[] refer to different memory locations, depending of the processor. In this course, we discuss message passing as a programming model (can be on any hardware)

What does the user have to do? This is what we said for shared memory: –Decide how to decompose the computation into parallel parts. –Create (and destroy) processes to support that decomposition. –Add synchronization to make sure dependences are covered. Is the same true for message passing?

Another Look at SOR Example for some number of timesteps/iterations { for (i=0; i<n; i++ ) for( j=0; j<n, j++ ) temp[i][j] = 0.25 * ( grid[i-1][j] + grid[i+1][j] grid[i][j-1] + grid[i][j+1] ); for( i=0; i<n; i++ ) for( j=0; j<n; j++ ) grid[i][j] = temp[i][j]; }

Shared Memory proc1proc2proc3procN grid 11 temp

Message-Passing Data Distribution (only middle processes) proc2proc grid temp

Is this going to work? Same code as we used for shared memory for( i=from; i<to; i++ ) for( j=0; j<n; j++ ) temp[i][j] = 0.25*( grid[i-1][j] + grid[i+1][j] + grid[i][j-1] + grid[i][j+1]); No, we need extra boundary elements for grid.

Data Distribution (only middle processes) proc2proc grid temp

Is this going to work? Same code as we used for shared memory for( i=from; i<to; i++) for( j=0; j<n; j++ ) temp[i][j] = 0.25*( grid[i-1][j] + grid[i+1][j] + grid[i][j-1] + grid[i][j+1]); No, on the next iteration we need boundary elements from our neighbors.

Data Communication (only middle processes) proc2proc3 grid

Is this now going to work? Same code as we used for shared memory for( i=from; i<to; i++ ) for( j=0; j<n; j++ ) temp[i][j] = 0.25*( grid[i-1][j] + grid[i+1][j] + grid[i][j-1] + grid[i][j+1]); No, we need to translate the indices.

Index Translation for( i=0; i<n/p; i++) for( j=0; j<n; j++ ) temp[i][j] = 0.25*( grid[i-1][j] + grid[i+1][j] + grid[i][j-1] + grid[i][j+1]); Remember, all variables are local.

Index Translation is Optional Allocate the full arrays on each processor. Leave indices alone. Higher memory use. Sometimes necessary (see later).

What does the user need to do? Divide up program in parallel parts. Create and destroy processes to do above. Partition and distribute the data. Communicate data at the right time. (Sometimes) perform index translation. Still need to do synchronization? –Sometimes, but many times goes hand in hand with data communication.

Message Passing Systems Provide process creation and destruction. Provide message passing facilities (send and receive, in various flavors) to distribute and communicate data. Provide additional synchronization facilities.

MPI (Message Passing Interface) Is the de facto message passing standard. Available on virtually all platforms, including public domain versions (MPICH). Grew out of an earlier message passing system, PVM, now outdated.

MPI Process Creation/Destruction MPI_Init( int argc, char **argv ) Initiates a computation. MPI_Finalize() Terminates a computation.

MPI Process Identification MPI_Comm_size( comm, &size ) Determines the number of processes. MPI_Comm_rank( comm, &pid ) Pid is the process identifier of the caller.

MPI Basic Send MPI_Send(buf, count, datatype, dest, tag, comm) buf: address of send buffer count: number of elements datatype: data type of send buffer elements dest: process id of destination process tag: message tag (ignore for now) comm: communicator (ignore for now)

MPI Basic Receive MPI_Recv(buf, count, datatype, source, tag, comm, &status) buf: address of receive buffer count: size of receive buffer in elements datatype: data type of receive buffer elements source: source process id or MPI_ANY_SOURCE tag and comm: ignore for now status: status object

MPI Matrix Multiply (w/o Index Translation) main(int argc, char *argv[]) { MPI_Init (&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &myrank); MPI_Comm_size(MPI_COMM_WORLD, &p); from = (myrank * n)/p; to = ((myrank+1) * n)/p; /* Data distribution */... /* Computation */... /* Result gathering */... MPI_Finalize(); } Willy Zwaenepoel: missing initialization of a and b Willy Zwaenepoel: missing initialization of a and b

MPI Matrix Multiply (w/o Index Translation) /* Data distribution */ if( myrank != 0 ) { MPI_Recv( &a[from], n*n/p, MPI_INT, 0, tag, MPI_COMM_WORLD, &status ); MPI_Recv( &b, n*n, MPI_INT, 0, tag, MPI_COMM_WORLD, &status ); } else { for( i=1; i<p; i++ ) { MPI_Send( &a[from], n*n/p, MPI_INT, i, tag, MPI_COMM_WORLD ); MPI_Send( &b, n*n, MPI_INT, I, tag, MPI_COMM_WORLD ); }

MPI Matrix Multiply (w/o Index Translation) /* Computation */ for ( i=from; i<to; i++) for (j=0; j<n; j++) { C[i][j]=0; for (k=0; k<n; k++) C[i][j] += A[i][k]*B[k][j]; }

MPI Matrix Multiply (w/o Index Translation) /* Result gathering */ if (myrank!=0) MPI_Send( &c[from], n*n/p, MPI_INT, 0, tag, MPI_COMM_WORLD); else for (i=1; i<p; i++) MPI_Recv( &c[from], n*n/p, MPI_INT, i, tag, MPI_COMM_WORLD, &status);

MPI Matrix Multiply (with Index Translation) main(int argc, char *argv[]) { MPI_Init (&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &myrank); MPI_Comm_size(MPI_COMM_WORLD, &p); from = (myrank * n)/p; to = ((myrank+1) * n)/p; /* Data distribution */... /* Computation */... /* Result gathering */... MPI_Finalize(); } Willy Zwaenepoel: missing initialization of a and b Willy Zwaenepoel: missing initialization of a and b

MPI Matrix Multiply (with Index Translation) /* Data distribution */ if( myrank != 0 ) { MPI_Recv( &a, n*n/p, MPI_INT, 0, tag, MPI_COMM_WORLD, &status ); MPI_Recv( &b, n*n, MPI_INT, 0, tag, MPI_COMM_WORLD, &status ); } else { for( i=1; i<p; i++ ) { MPI_Send( &a[from], n*n/p, MPI_INT, i, tag, MPI_COMM_WORLD ); MPI_Send( &b, n*n, MPI_INT, I, tag, MPI_COMM_WORLD ); }

MPI Matrix Multiply (with Index Translation) /* Computation */ for ( i=0; i<n/p; i++) for (j=0; j<n; j++) { C[i][j]=0; for (k=0; k<n; k++) C[i][j] += A[i][k]*B[k][j]; }

MPI Matrix Multiply (with Index Translation) /* Result gathering */ if (myrank!=0) MPI_Send( &c, n*n/p, MPI_INT, 0, tag, MPI_COMM_WORLD); else for( i=1; i<p; i++ ) MPI_Recv( &c[from], n*n/p, MPI_INT, i, tag, MPI_COMM_WORLD, &status);

Running a MPI Program mpirun Interacts with a daemon process on the hosts. Causes a Unix process to be run on each of the hosts. Currently: only runs in interactive mode on our Itanium (batch mode blocked by ssh)