140. Caching

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MPI provides a ``caching'' facility that allows an application to attach arbitrary pieces of information, called attributes, to three kinds of MPI objects, communicators, windows and datatypes. More precisely, the caching facility allows a portable library to do the following:

pass information between calls by associating it with an MPI intra- or inter-communicator, window or datatype,

quickly retrieve that information, and

be guaranteed that out-of-date information is never retrieved, even if the object is freed and its handle subsequently reused by MPI.

The caching capabilities, in some form, are required by built-in MPI routines such as collective communication and application topology. Defining an interface to these capabilities as part of the MPI standard is valuable because it permits routines like collective communication and application topologies to be implemented as portable code, and also because it makes MPI more extensible by allowing user-written routines to use standard MPI calling sequences.

Advice to users.

The communicator MPI_COMM_SELF is a suitable choice for posting process-local attributes, via this attributing-caching mechanism. ( End of advice to users.)

Rationale.

In one extreme one can allow caching on all opaque handles. The other extreme is to only allow it on communicators. Caching has a cost associated with it and should only be allowed when it is clearly needed and the increased cost is modest. This is the reason that windows and datatypes were added but not other handles. ( End of rationale.)
One difficulty is the potential for size differences between Fortran integers and C pointers. To overcome this problem with attribute caching on communicators, functions are also given for this case. The functions to cache on datatypes and windows also address this issue. For a general discussion of the address size problem, see Section Addresses .
Advice to implementors.

High-quality implementations should raise an error when a keyval that was created by a call to MPI_XXX_CREATE_KEYVAL is used with an object of the wrong type with a call to MPI_YYY_GET_ATTR, MPI_YYY_SET_ATTR, MPI_YYY_DELETE_ATTR, or MPI_YYY_FREE_KEYVAL. To do so, it is necessary to maintain, with each keyval, information on the type of the associated user function. ( End of advice to implementors.)

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