The given node may own the target entries in the linear algebra object. In this case, a local operation suffices. Another node owns the target entries in the linear algebra object. Now, communication is required before the data can be added to the linear algebra object. Moreover, if conventional MPI communication calls are used, the target node must be aware that the data is about to arrive.
Many nodes own some of the the target entries in the linear algebra object.
Now a number of communications are required, and all the target nodes must be aware of the data that is about to arrive. Clearly, considerable complexity results from managing the required communication. The latter call will retrieving information from global objects. The displacement, in displ , gives the starting index in the target object where the data in the local vector is to be placed. The scaling parameter, in alpha , allows a multiple of the local data to be added.