TAILIEUCHUNG - Parallel Programming: for Multicore and Cluster Systems- P22

Parallel Programming: for Multicore and Cluster Systems- P22: Innovations in hardware architecture, like hyper-threading or multicore processors, mean that parallel computing resources are available for inexpensive desktop computers. In only a few years, many standard software products will be based on concepts of parallel programming implemented on such hardware, and the range of applications will be much broader than that of scientific computing, up to now the main application area for parallel computing | 202 5 Message-Passing Programming int MPLGeLcount MPIJStatus status MPI_Datatype datatype int counLptr where status is a pointer to the data structure status returned by MPI_Recv . The function returns the number of elements received in the variable pointed to by counLptr Internally a message transfer in MPI is usually performed in three steps 1. The data elements to be sent are copied from the send buffer smessage specified as parameter into a system buffer of the MPI runtime system. The message is assembled by adding a header with information on the sending process the receiving process the tag and the communicator used. 2. The message is sent via the network from the sending process to the receiving process. 3. At the receiving side the data entries of the message are copied from the system buffer into the receive buffer rmessage specified by MPI_Recv . Both MPI_Send and MPI_Recv are blocking asynchronous operations. This means that an MPI_Recv operation can also be started when the corresponding MPI_Send operation has not yet been started. The process executing the MPI_Recv operation is blocked until the specified receive buffer contains the data elements sent. Similarly an MPI_Send operation can also be started when the corresponding MPI_Recv operation has not yet been started. The process executing the MPI_Send operation is blocked until the specified send buffer can be reused. The exact behavior depends on the specific MPI library used. The following two behaviors can often be observed If the message is sent directly from the send buffer specified without using an internal system buffer then the MPI_Send operation is blocked until the entire message has been copied into a receive buffer at the receiving side. In particular this requires that the receiving process has started the corresponding MPI_Recv operation. If the message is first copied into an internal system buffer of the runtime system the sender can continue its operations as soon as the copy .

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