With recent additions to the codebase, users here have noticed many workers are able to load input splits extremely quickly, and this has altered the behavior of Giraph during INPUT_SUPERSTEP when using the current algorithm for split reservations. A few workers process multiple splits (often overwhelming Netty and getting GC errors as they attempt to offload too much data too quick) while many (often most) of the others just sleep through the superstep, never successfully participating at all.
Essentially, the current algo is:
1. scan input split list, skipping nodes that are marked "Finsihed"
2. grab the first unfinished node in the list (reserved or not) and check its reserved status.
3. if not reserved, attempt to reserve & return it if successful.
4. if the first one you check is already taken, sleep for way too long and only wake up if another worker finishes a split, then contend with that worker for another split, while the majority of the split list might sit idle, not actually checked or claimed by anyone yet.
This does not work. By making a few simple changes (and acknowledging that ZK reads are cheap, only writes are not) this patch is able to get every worker involved, and keep them in the game, ensuring that the INPUT_SUPERSTEP passes quickly and painlessly, and without overwhelming Netty by spreading the memory load the split readers bear more evenly. If the giraph.splitmb and -w options are set correctly, behavior is now exactly as one would expect it to be.
This also results in INPUT_SUPERSTEP passing more quickly, and survive the INPUT_SUPERSTEP for a given data load on less Hadoop memory slots.