Distributed representation and estimation of WFST-based n-gram models

We present methods for partitioning a weighted finite-state transducer (WFST) representation of an n-gram language model into multiple blocks or shards, each of which is a stand-alone WFST n-gram model in its own right, allowing processing with existing algorithms. After independent estimation, including normalization, smoothing and pruning on each shard, the shards can be reassembled into a single WFST that is identical to the model that would have resulted from estimation without sharding. We then present an approach that uses data partitions in conjunction with WFST sharding to estimate models on orders-of-magnitude more data than would have otherwise been feasible with a single process. We present some numbers on shard characteristics when large models are trained from a very large data set. Functionality to support distributed n-gram modeling has been added to the open-source OpenGrm library.