MPI: Difference between revisions

A process sends data by calling the <tt>MPI_Send</tt> function. Referring to the following function prototypes, <tt>MPI_Send</tt> can be summarized as sending '''count''' contiguous instances of '''datatype''' to the process with the specified '''rank''', and the data is in the buffer pointed to by '''message'''.  '''Tag''' is a programmer-specified identifier that becomes associated with the message, and can be used to organize the communication process (for example, to distinguish two distinct streams of interleaved data). Our examples do not require this, so we will pass in the value 0 for the '''tag'''. '''Comm''' is the communicator described above, and we will continue to use <tt>MPI_COMM_WORLD</tt>.

Note that the '''datatype''' argument, specifying the type of data contained in the '''message''' buffer, is a variable. This is intended to provide a layer of compatibility between processes that could be running on architectures for which the native format for these types differs. It is possible to register new data types, but for this tutorial we will only use the pre-defined types provided by MPI. There is an MPI type pre-defined for all atomic data types in the source language (for C: <tt>MPI_CHAR</tt>, <tt>MPI_FLOAT</tt>, <tt>MPI_SHORT</tt>, <tt>MPI_INT</tt>, etc. and for Fortran: <tt>MPI_CHARACTER</tt>, <tt>MPI_INTEGER</tt>, <tt>MPI_REAL</tt>, etc.). YOu can find a full list of these types in the references provided below.

<tt>MPI_Recv</tt> works in much the same way as <tt>MPI_Send</tt>. Referring to the function prototypes below, '''message''' is now a pointer to an allocated buffer of sufficient size to receive '''count''' contiguous instances of '''datatype''', which is received from process '''rank'''. <tt>MPI_Recv</tt> takes one additional argument, '''status''', which in C should be a reference to an allocated <tt>MPI_Status</tt> structure, and in Fortran an array of <tt>MPI_STATUS_SIZE</tt> integers. Upon return it will be filled in with information related to the received message. We will not make use of this argument in this tutorial, but it must be present.

The sending process must know the rank of the receiving process, and the receiving process must know the rank of the sending process. In our example, we need to do some arithmetic such that, given its rank, each process knows both where to send its data, <tt>[(rank + 1) % size]</tt>, and where to receive from <tt>[(rank + size) - 1) % size]</tt>. We'll now make the required modifications to our parallel "Hello, world!" program. As a first cut, we'll simply have each process first send its message, then receive what is being sent to it.

     recvfrom = ((rank + size) - 1) % size;

     recvfrom = mod(((rank + num_procs) - 1), num_procs)

Compile and run this program on 2, 4 and 8 processors. While it certainly seems to be working as intended, there is something we are overlooking. The MPI standard says nothing about whether sends are buffered---that is to say, we should not assume that <tt>MPI_Send</tt> returns immediately buffering our message. If the send isn't buffered, the code as written would deadlock with all processes performing its send and blocking as there are no receives to consume the message until after the send returns. Clearly there is buffering in the libraries on our systems as the code did not deadlock, however it is poor design to rely on this. You may find your code fails if used on a system in which there is no buffering provided by the library, and even where buffering is provided, the call will still block if the buffer fills up.