ObjectStore runs out of memory for large batches of addPartitions().

For large batches (e.g. hundreds) of addPartitions(), the ObjectStore runs out of memory. Flushing the PersistenceManager alleviates the problem.

Note: The problem being addressed here isn't so much with the size of the hundreds of Partition objects, but the cruft that builds with the PersistenceManager, in the JDO layer, as confirmed through memory-profiling.

(Raising this on behalf of cdrome and thiruvel.)