/* MPI sample program; NOT INTENDED TO BE EFFICIENT as a prime finder, either in algorithm or implementation MPI (Message Passing Interface) is a popular package using the "message passing" paradigm for communicating between processors in parallel applications; as the name implies, processors communicate by passing messages using "send" and "receive" functions finds and reports the number of primes less than or equal to N uses a pipeline approach: node 0 looks at all the odd numbers (i.e. has already done filtering out of multiples of 2) and filters out those that are multiples of 3, passing the rest to node 1; node 1 filters out the multiples of 5, passing the rest to node 2; in this simple example, we just have node 2 filter out all the rest and then report the number of primes note that we should NOT have a node run through all numbers before passing them on to the next node, since we would then have no parallelism at all; on the other hand, passing on just one number at a time isn't efficient either, due to the high overhead of sending a message if it is a network (tens of microseconds until the first bit reaches the wire, due to software delay); thus efficiency would be greatly improved if each node saved up a chunk of numbers before passing them to the next node */ // this include file is mandatory #include #define MAX_N 100000 #define PIPE_MSG 0 // type of message containing a number to be checked #define END_MSG 1 // type of message indicating no more data will be coming int NNodes, /* number of nodes in computation*/ N, /* find all primes from 2 to N */ Me, /* my node number */ ToCheck; /* current number to check for passing on to next node; stylistically this might be nicer as a local in Node*(), but I have placed it here to dramatize the fact that the globals are NOT shared among the nodes */ double T1,T2; /* start and finish times */ Init(Argc,Argv) int Argc; char **Argv; { int DebugWait; N = atoi(Argv[1]); DebugWait = atoi(Argv[2]); /* this loop is here to synchronize all nodes for debugging; if DebugWait is specified as 1 on the command line, all nodes wait here until the debugging programmer starts GDB at all nodes and within GDB sets DebugWait to 0 to then proceed */ while (DebugWait) ; /* mandatory to begin any MPI program */ MPI_Init(&Argc,&Argv); /* puts the number of nodes in NNodes */ MPI_Comm_size(MPI_COMM_WORLD,&NNodes); /* puts the node number of this node in Me */ MPI_Comm_rank(MPI_COMM_WORLD,&Me); /* OK, get started; first record current time in T1 */ if (Me == 2) T1 = MPI_Wtime(); } Node0() { int I,Dummy, Error; /* not checked in this example */ for (I = 1; I <= N/2; I++) { ToCheck = 2 * I + 1; if (ToCheck > N) break; /* MPI_Send -- send a message parameters: pointer to place where message is to be drawn from number of items in message item type destination node message type ("tag") programmer-defined node group number (in this case all nodes) */ if (ToCheck % 3 > 0) Error = MPI_Send(&ToCheck,1,MPI_INT,1,PIPE_MSG,MPI_COMM_WORLD); } Error = MPI_Send(&Dummy,1,MPI_INT,1,END_MSG,MPI_COMM_WORLD); } Node1() { int Error, /* not checked in this example */ Dummy; MPI_Status Status; /* see below */ while (1) { /* MPI_Recv -- receive a message parameters: pointer to place to store message number of items in message (see notes on this at the end of this file) item type accept message from which node(s) message type ("tag"), programmer-defined (in this case any type) node group number (in this case all nodes) status (see notes on this at the end of this file) */ Error = MPI_Recv(&ToCheck,1,MPI_INT,0,MPI_ANY_TAG, MPI_COMM_WORLD,&Status); if (Status.MPI_TAG == END_MSG) break; if (ToCheck % 5 > 0) Error = MPI_Send(&ToCheck,1,MPI_INT,2,PIPE_MSG,MPI_COMM_WORLD); } /* now send our end-of-data signal, which is conveyed in the message type, not the message (we have a dummy message just as a placeholder */ Error = MPI_Send(&Dummy,1,MPI_INT,2,END_MSG,MPI_COMM_WORLD); } Node2() { int ToCheck, /* current number to check from Node 0 */ Error, /* not checked in this example */ PrimeCount,I,IsComposite; MPI_Status Status; /* see below */ PrimeCount = 3; /* must account for the primes 2, 3 and 5, which won't be detected below */ while (1) { Error = MPI_Recv(&ToCheck,1,MPI_INT,1,MPI_ANY_TAG, MPI_COMM_WORLD,&Status); if (Status.MPI_TAG == END_MSG) break; IsComposite = 0; for (I = 7; I*I <= ToCheck; I += 2) if (ToCheck % I == 0) { IsComposite = 1; break; } if (!IsComposite) PrimeCount++; } /* check the time again, and subtract to find run time */ T2 = MPI_Wtime(); printf("elapsed time = %f\n",(float)(T2-T1)); /* print results */ printf("number of primes = %d\n",PrimeCount); } main(argc,argv) int argc; char **argv; { Init(argc,argv); /* note: instead of having a switch statement, we could write three different programs, each running on a different node */ switch (Me) { case 0: Node0(); break; case 1: Node1(); break; case 2: Node2(); }; /* mandatory for all MPI programs */ MPI_Finalize(); } /* explanation of "number of items" and "status" arguments at the end of MPI_Recv(): when receiving a message you must anticipate the longest possible message, but the actual received message may be much shorter than this; you can call the MPI_Get_count() function on the status argument to find out how many items were actually received the status argument will be a pointer to a struct, containing the node number, message type and error status of the received message say our last parameter is Status; then Status.MPI_SOURCE will contain the number of the sending node, and Status.MPI_TAG will contain the message type; these are important if used MPI_ANY_SOURCE or MPI_ANY_TAG in our node or tag fields but still have to know who sent the message or what kind it is */