CONTEXT

In the BLACS, each process grid is enclosed in a context. A context may be thought of as a message passing universe. This means that a grid can safely communicate even if other (possibly overlapping) grids are also communicating.

In most respects, we can use the terms "grid" and "context" interchangeably. I.e., we may say "perform operation in context X" or "in grid X". The slight difference here is that the user may define two exactly identical grids (say, two 1x3 process grids, both of which use processes 0, 1, and 2), but each will be wrapped in its own context, so that they are distinct in operation, even though they are indistinguishable from a process grid standpoint.

Contexts are used so that individual routines using the BLACS can, when required, safely operate without worrying if other distributed codes are being executed on the same machine.

Another example of the use of context might be to define a normal 2D process grid about which most computation takes place. However, at certain sections it may be more convenient to access the processes as a 1D grid, and at certain others we may wish, for instance, to share information among nearest neighbors. We will therefore want each process to have access to three contexts: the 2D grid, the 1D grid, and a small grid which contains the process and its nearest neighbors.

Therefore, we see that context allows us to:

Create arbitrary groups of processes upon which to execute
Create an indeterminate number of overlapping or disjoint grids
Isolate each grid so that grids do not interfere with each other

In the BLACS, there are two grid creation routines (BLACS_GRIDINIT and BLACS_GRIDMAP ) which create a process grid and its enclosing context. These routines return context handles, which are simple integers. Subsequent BLACS routines will be passed these handles, which allow the BLACS to determine what context/grid a routine is being called from. The user should never actually manipulate these handles: they are opaque data objects which are only meaningful for the BLACS routines.

Contexts consume resources. It is therefore advisable to release contexts when they are no longer needed. This is done via the routine BLACS_GRIDEXIT. When the entire BLACS system is shut down (via a call to BLACS_EXIT), all outstanding contexts are automatically freed.