[Mpi3-ft] Problem with reusing rendezvous memory buffers

[Wesley Bland]

Pavan pointed out a problem to me yesterday related to memory buffers used with rendezvous protocols. If a process passes a piece of memory to the library in an MPI_RECV and the library gives that memory to the hardware, where it is pinned, we can get into trouble if one of the processes that could write into that memory fails. The problem comes from a process sending a slow message and then dying. It is possible that the other processes could detect and handle the failure before the slow message arrives. Then when the message does arrive, it could corrupt the memory without the application having a way to handle this. My whiteboard example is attached as an image.
  

We can't just unmap memory from the NIC when a failure occurs because that memory is still being used by another process's message. Some hardware supports unmapping memory for specific senders which would solve this issue, but some don't, such as InfiniBand, where the memory region just has a key and unmapping it removes it for all senders.

This problem doesn't have a good solution (that I've come up with), but I did come up with a solution. We would need to introduce another error code (something like MPI_ERR_BUFFER_UNUSABLE) that would be able to tell the application that the buffer that the library was using is no longer usable because it might be corrupted. For some hardware, this wouldn't have to be returned, but for hardware where this isn't possible, the library could pass this error to the implementation to say that I need a new buffer in order to complete this operation. On the sender side, the operation would probably complete successfully since to it, the memory was still available. That means that there will be some rollback necessary, but that's up to the application to figure out.

I know this is an expensive and painful solution, but this is all I've come up with so far. Thoughts from the group?

Thanks,
Wesley

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