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It is a while ago I did this myself. I interpret ids.num-partitions as a stock of reserved id blocks that can be delegated to a janugraph instance. It does not have a large value to not waste ids space.
Actually, parallel tasks is not the number we want. We want the ids.num-partitions to be equal to the number of janusgraph instances, because initially all janusgraph instances ask for an ids block at the same time. Note that the cores in a spark executor can share the same janusgraph instance if you use a singleton object for that.
So, if you have 50 executors with 5 cores each (and using a singleton janusgraph instance), I would try ids.num-partitions =50
Best wishes, Marc