During INPUT_SUPERSTEP, workers wait on a barrier until the master has created a complete list of available input splits. Once the barrier is past, each worker iterates through this list of input splits, creating a znode to lay claim to the next unprocessed split the worker encounters.
For a brief moment while the master is creating the input split znodes each worker iterates through, it has access to InputSplit objects that also contain a list of hostnames on which the blocks of the file are hosted. By including that list of locations in each znode pathname we can allow each worker reading the list of available splits to sort it so that splits the worker attempts to claim first are the ones that contain a block that is local to that worker's host.
This allows the possibility for many workers to end up reading at least one split that is local to its own host. If the input split selected holds a local block, the RecordReader Hadoop supplies us with will automatically read from that block anyway. By supplying this locality data as part of the znode name rather than info inside the znode, we avoid reading the data from each znode while sorting, which is only currently done when a split is claimed and which is IO intensive. Sorting the string path data is cheap and faster, and making the final split znode's name longer doesn't seem to matter too much.
By using the BspMaster's InputSplit data to include locality information in the znode path directly, we also avoid having to access the FileSystem/BlockLocations directly from either master or workers, which could also flood the name node with queries. This is the only place I've found where some locality information is already available to Giraph free of additional cost.
Finally, by sorting each worker's split list this way, we get the contention-reduction of
GIRAPH-250 for free, since only workers on the same host will be likely to contend for a split instead of the current situation in which all workers contend for the same input splits from the same list, iterating from the same index. GIRAPH-250 has already been logged as reducing pages of contention on the first pass (when using many 100's of workers) down to 0-3 contentions before claiming a split to read.
This passes 'mvn verify' etc. I will post results of cluster testing ASAP. If anyone else could try this on an HDFS cluster where locality info is supplied to InputSplit objects, I would be really interested to see other folks' results.