On the importance of not reinventing the wheel in distributed applications

Sometimes, it’s so easy to rewrite some existing code because it doesn’t fit exactly your bill.

I just so the example with an All To All communication that was written by hand. The goal was to share how many elements would be sent from one MPI process to another, and these elements were stored on one process in different structure instances, one for each MPI process. So in the end, you had n structures on each of the n MPI processes.

The MPI_Alltoall cannot map directly to this scattered structure, so it sounds fair to assume that using MPI_Isend and MPI_Irecv would be simpler to implement. The issue is that this pattern uses buffers on each process for each other process it will send values to or receive values from. A lot of MPI library allocate their buffer when needed, but will never let go of the memory until the end. So you end up with a memory consumption that doesn’t scale. In my case, when using more than 1000 cores, the MPI library uses more than 1GB per MPI process when it hits these calls, just for these additional hidden buffers. This is just no manageable.

Now, if you use MPI_Alltoall, two things happen:

  • there are no additional buffer allocated, so this scales nicely when you increase the number of cores
  • it is actually faster than your custom implementation

Now with MPI 3 standard having non-blocking collective operations, there is absolutely no reason to try to outsmart the library when you need a collective operation. It has heuristics when it knows that it is doing a collective call, so let them work. You won’t be smarter if you try, but you will if you use them.

In my case, the code to retrieve all values and store them in an intermediate buffer was smaller that the one with the Isend/Irecv.

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