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Communicator splitting

As you might remember from the introduction, it is possible to create your own communicators in MPI and use them instead of MPI_COMM_WORLD. That's what we are going to see in this lesson. Communicators are a complex subject in MPI and the standard documentation of MPI dedicates a full chapter to them and to related concepts so bear in mind that we will only be scratching the surface of a very broad and advanced topic. Here, we will limit ourselves at the most straightforward way of creating a new communicator : splitting.

Definitions

There are a lot of levels of subtlety in communicators in MPI and if you are interested in the full detail, I would advise you to read the specifications in the standard. Without entering too much into details, here are a few terms that are needed to understand what communicators are :

  • Group : A group is an ordered collection of process. These processes are ordered via their rank.
  • Context : A context can be seen as a tag that provides a safe-space for communication. This "tag" labels the communication and links it to its context, providing additional semantics.
  • Intra-communicator : An intra-communicator is the reunion of a group and a context. Until now, we have been using MPI_COMM_WORLD as a communicator : it actually is an intra-communicator.

Now you understand that a communicator limits its communications to a specific group of processes but not only : if you have two communicators on the same group, their context will be different and the messages that will be sent will be virtually separated from each other.

Aside from intra-communicators we can also define the notion of inter-communicators that allow to pass message between two non-overlapping groups. An inter-communicator is the reunion of TWO groups and a context.

Finally, there is one last concept that we need to define although we won't really need it here : a topology is a way to provide a mapping between a specified disposition of processes and their actual ranks. The most classical example is the cartesian topology where instead of addressing a process by its rank we will address it by a coordinate in a grid. Thge topology can be virtual (mapping the processes ranks in some custom way) or physical (mapping the processes ranks so they correspond to the actual core organization on the machine).

What's useful in all of this ?

In the following examples, we won't be talking anymore about inter-communicators. These communicators can be important, but they don't belong in an introduction course. If you are really interested in reading about them, you will find more information in the MPI standard, as well as a brief example here.

As for the rest, we will now be vague and get back to calling intra-communicators by simply communicators. Please bear in mind though that everytime there is a mention to a communicator in MPI, it can be either intra or inter communicators.

But why would I want to use other communicators?

That a totally legitimate question to ask and there are multiple answers to that but the most straightforward one has to do with what is called Flynn's taxonomy. Under this very simple taxonomy, you can divide any program in four categories.

Sequential (non-parallel) programs are usually SISD programs : Single Instruction, Single Data. Note that, SISD is never used anywhere, the term sequential is preferred.

Until now most of what we have been aiming for in the parallel course is what we call SIMD : Single Instruction Multiple Data. Basically, we have a bunch of data and we want to apply the same treatment to all of them.

MISD means Multiple Instruction, Single Data. It basically means that you have different programs exploiting the same set of data. These programs can be parallelised in which case you will have a dedicated communicator for each program, and then a communicator between the two processes.

The final paradigm : MIMD for Multiple Instruction, Multiple Data is exactly the same as MISD but on top of that, the different programs will be working on different sets of data.

Communicators creation

There are various ways of creating new communicators in MPI. A new communicator is always created from another so every communicator derives from MPI_COMM_WORLD. There are two easy ways of doing things : either you create a new group and derive a communicator from it, or you split an existing communicators into sub-communicators. In this lesson, we are going to explore the latest.

Communicator splitting

To explain splitting, let's take an example. Consider we have a program with 8 processes. At the beginning, all of these processes are grouped inside MPI_COMM_WORLD.

Comm splitting, part 1

Now, let's imagine that we want to make two new communicators. One will group processes 0, 1, 2 and 3. The other will group processes 4, 5 and 6. Process 7 will stay only be in MPI_COMM_WORLD. Now assigning a process to another communicator does not mean that the process leaves the original communicator. So all these processes will stay in MPI_COMM_WORLD.

Splitting is the action of taking an existing communicator and distributing all of its processes to new communicators. The way we do it is by assigning every process a number that will tell in which communicator it will end after the splitting. That number is called a colour.

Comm splitting, part2

As you can see, we assign processes 0, 1, 2 and 3 the color 0, and the color 1 to processes 4, 5 and 6. Process 7 has no colour. There is a way in MPI to indicate that you don't want the process to be placed in any new communicator. Then, we act the splitting, and create the new communicators.

Comm splitting, part3

Now the processes 0 to 6 are belonging to two communicators. Processes 0, 1, 2 and 3 are in MPI_COMM_WORLD and in CUSTOM_COMM1 while processes 4, 5 and 6 in MPI_COMM_WORLD and CUSTOM_COMM2. A very important thing to understand is that every communicator has its own size. Thus the size of MPI_COMM_WORLD is 8, the size of CUSTOM_COMM1 is 4 and the one of CUSTOM_COMM2 is 3.

Also note that, as stated in the introduction, the rank of a process in a communicator will always start from 0. Thus, process 0 in MPI_COMM_WORLD is still process 0 in COMM_CUSTOM1, but also process 4 in MPI_COMM_WORLD is process 0 in COMM_CUSTOM2. Bear in mind that the rank of a process only has a meaning with respect to a specific communicator.

The ordering of the processes in the communicator can also be defined when we split, but we will see this during the next exercise.

One last thing before practicing : nothing prevents you to create subsequent communicators. After doing this first split, you can do another split, either on MPI_COMM_WORLD, on CUSTOM_COMM1 or on CUSTOM_COMM2. Let's imagine that now we want to create a third custom communicator CUSTOM_COMM3 grouping processes 0 and 4 of MPI_COMM_WORLD. Then we can split MPI_COMM_WORLD again, and create that new communicator.

Comm splitting, part4

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