香港浸会大学：点对点通信（PPT讲稿）Point-to-Point Communication

Point - to-Point Communication Reference:http://foxtrot.ncsa.uiuc.edu.8900/publicmmpl

Reference: http://foxtrot.ncsa.uiuc.edu:8900/public/MPI/ Point-to-Point Communication

Introduction Point-to-point communication is the fundamental communication facility provided by the mPl library Point-to-point communication is conceptually simple: one process sends a message and another process receives it MP/ Send and MP/ Recv work together to complete a transter of data from one process to another

Introduction • Point-to-point communication is the fundamental communication facility provided by the MPI library. • Point-to-point communication is conceptually simple: one process sends a message and another process receives it. • MPI_Send and MPI_Recv work together to complete a transfer of data from one process to another

o Point-to-point communication However, it is less simple in practice. For example, a process may have many messages waiting to be received. In that case a crucial issue is how MP and the receiving process determine what message to receive Another issue is whether send and receive routines initiate communication operations and return immediately, or wait for the initiated communication operation to complete before returning The underlying communication operations are the same in both cases, but the programming interface is very different

Point-to-point communication • However, it is less simple in practice. For example, a process may have many messages waiting to be received. In that case, a crucial issue is how MPI and the receiving process determine what message to receive. • Another issue is whether send and receive routines initiate communication operations and return immediately, or wait for the initiated communication operation to complete before returning. The underlying communication operations are the same in both cases, but the programming interface is very different

Topics Fundamentals of point-to-point communication Blocking send and receive Nonblocking send and receive Send modes

Topics • Fundamentals of point-to-point communication • Blocking send and receive • Nonblocking send and receive • Send modes

Fundamentals Reference:http://foxtrot.ncsa.uiuc.edu.8900/publicmmpl

Reference: http://foxtrot.ncsa.uiuc.edu:8900/public/MPI/ Fundamentals

Fundamentals The following issues are fundamental to point-to point communication in MPl. These apply to all versions of send and receive both blocking and nonblocking, and to all send modes Source and destination Messages Sending and Receiving Messages

Fundamentals • The following issues are fundamental to point-to￾point communication in MPI. These apply to all versions of send and receive, both blocking and nonblocking, and to all send modes. – Source and Destination – Messages – Sending and Receiving Messages

Source and Destination The point-to-point communication discussed here are two-sided and require active participation from the processes on both sides. One process(the source) sends and another process( the destination) receives In general, the source and destination processes operate asynchronously. Even the sending and receiving of a single message is typically not synchronized. The source process may complete sending a message long before the destination process gets around to receiving it, and the destination process may initiate receiving a message that has not yet been sent

Source and Destination • The point-to-point communication discussed here are two-sided and require active participation from the processes on both sides. One process (the source) sends, and another process (the destination) receives. • In general, the source and destination processes operate asynchronously. – Even the sending and receiving of a single message is typically not synchronized. The source process may complete sending a message long before the destination process gets around to receiving it, and the destination process may initiate receiving a message that has not yet been sent

Source and Destination Because sending and receiving are typically not synchronized, processes often have one or more messages that have been sent but not yet received These sent, but not yet received messages are called pending messages It is an important feature of MPI that pending messages are not maintained in a simple FIFo queue. Instead, each pending message has several attributes and the destination process( the receiving process)can use the attributes to determine which message to receive

Source and Destination • Because sending and receiving are typically not synchronized, processes often have one or more messages that have been sent but not yet received. • These sent, but not yet received messages are called pending messages. • It is an important feature of MPI that pending messages are not maintained in a simple FIFO queue. Instead, each pending message has several attributes and the destination process (the receiving process) can use the attributes to determine which message to receive

Messages Messages consist of 2 parts: the envelope and the message bod MPI Send (sendbuf, cnt, datatype, dest, tag, comm) message body envelope MPI Recv(recvbuf, cnt, datatype, source, tag, comm, status) MPI Message Letter Send/receive buffer letter content Count/size etter weight Source(receive) Return address Destination(send) Destination address Communicator Country

Messages • Messages consist of 2 parts: the envelope and the message body. MPI_Send(sendbuf,cnt,datatype,dest,tag,comm); MPI_Recv(recvbuf,cnt,datatype,source,tag,comm,status); MPI Message Letter Send/receive buffer letter content Count/size Letter weight Source (receive) Return address Destination (send) Destination address Communicator Country message body envelope

)Message body The message body has 3 parts Buffer It is the space in the computer's memory where the MPl message data are to be sent from or stored to Dataty pe The type of the message data to be transmitted(e.g. floating point). The datatype should be the same for the send and receive call count The number of items of data in the message. The count specified by the receive call should be equal to or greater than the count specified by the send

Message body • The message body has 3 parts: – Buffer It is the space in the computer's memory where the MPI message data are to be sent from or stored to – Datatype The type of the message data to be transmitted(e.g.floating point). The datatype should be the same for the send and receive call. – count The number of items of data in the message. The count specified by the receive call should be equal to or greater than the count specified by the send