Locality Aware Broadcast #185
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csegarragonz
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Nov 26, 2021
src/scheduler/MpiWorld.cpp
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| int MpiWorld::getMasterForHost(const std::string& host) | ||
| { | ||
| std::vector<int> ranks = getRanksForHost(host); | ||
| assert(!ranks.empty()); |
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We only call this method internally, with values from the host list that we have been broadcasted, and that necessarily must match to a rank (otherwise it wouldn't have been sent in the first place).
Thus why we assert here insted of throwing an exception.
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In this PR I change the broadcast algorithm in MPI to reduce the number of cross-host messages sent. With the new implementation, the number of cross-host messages grows linearly with the number of hosts, and not with the number of ranks.
In the process I also change some minor issues with the broadcast implementation:
reducebut only the root rank to callbroadcast(all other ranks would callrecv), see here for the implementation in faasm. Now all ranks call reduce, and the behaviour depends on the caller's rank.MPIMessage::BROADCASTin the protobuf object, soMPI_Bcastmessages were labeled asNORMALmessages.rootRankfor the rank which originates the broadcast.Regarding the algorithm itself, I introduce the notion of
localMasters in the MPI world. A local master is a selected leader for all MPI ranks living in a particular host. Then:For the moment local masters are the lowest rank in each host. Even though the load in local masters will now slightly increase, we are drastically reducing the load in rank 0 (which was always elected as the master, and therefore was doing a bunch of cross-host messaging).
To ease with testing I enhance the mocking tools in the MpiWorld.
Also note that this PR is rebased on top of #187 for faster GHA tests, so I'd recommend reviewing #187 first.