buffering MPI does not guarantee to buffer arbitrary messages because memory is a finite resource on all computers. Thus, all computers will fail under sufficiently adverse communication loads. Different computers at different times are capable of providing differing amounts of buffering, so if a program relies on buffering it may fail under certain conditions, but work correctly under other conditions. This is clearly undesirable.
Given that no message passing system can guarantee that messages will be buffered as required under all circumstances, it might be asked why MPI does not guarantee a minimum amount of memory available for buffering. One major problem is that it is not obvious how to specify the amount of buffer space that is available, nor is it easy to estimate how much buffer space is consumed by a particular program.
Different buffering policies make sense in different environments. Messages can be buffered at the sending node or at the receiving node, or both. In the former case,
The choice of the right policy is strongly dependent on the hardware and software environment. For instance, in a dedicated environment, a processor with a process blocked on a send is idle and so computing resources are not wasted if this processor copies the outgoing message to a buffer. In a time shared environment, the computing resources may be used by another process. In a system where buffer space can be in paged memory, such space can be allocated from heap. If the buffer space cannot be paged, or has to be in kernel space, then a separate buffer is needed. Flow control may require that some amount of buffer space be dedicated to each pair of communicating processes.
The optimal strategy strongly depends on various performance parameters of the system: the bandwidth, the communication start-up time, scheduling and context switching overheads, the amount of potential overlap between communication and computation, etc. The choice of a buffering and scheduling policy may not be entirely under the control of the MPI implementor, as it is partially determined by the properties of the underlying communication layer. Also, experience in this arena is quite limited, and underlying technology can be expected to change rapidly: fast, user-space interprocessor communication mechanisms are an active research area .
Attempts by the MPI Forum to design mechanisms for querying or setting the amount of buffer space available to standard communication led to the conclusion that such mechanisms will either restrict allowed implementations unacceptably, or provide bounds that will be extremely pessimistic on most implementations in most cases. Another problem is that parameters such as buffer sizes work against portability. Rather then restricting the implementation strategies for standard communication, the choice was taken to provide additional communication modes for those users that do not want to trust the implementation to make the right choice for them.