Purpose of scatter operation:

to send different chunks of data to different processes

not the same as broadcast (same chunk goes to all)

Scatter requires contiguous data, uniform message size 

 

For our first example, let's compare scatter and scatterv. Recall that the purpose of a scatter is to send out different chunks of data to different tasks--that is, you're trying to split up your data among the remote processes. Recall also that a scatter is not the same as a broadcast--in a broadcast, the same chunk of data is being sent to everybody. As you can see from the figure, the basic MPI_Scatter requires that the sender's data are stored in contiguous memory addresses and that the chunks are uniform in size. Thus, the first N data items are sent to the first process in the communicator, the next N items go to the second process, and so on. However, for some applications this might be overly restrictive. What if some processes need fewer than N items, for instance? Should you just pad out the sending buffer with unnecessary data? The answer is no: you should use MPI_Scatterv instead.