Re: [infinispan-dev] default value for virtualNodes

Hi guys I've been working on a test to search for an optimal default value here: https://github.com/danberindei/infinispan/commit/983c0328dc40be9609fcabb7... I'm measuring both the number of keys for which a node is primary owner and the number of keys for which it is one of the owners compared to the ideal distribution (K/N keys on each node). The former tells us how much more work the node could be expected to do, the latter how much memory the node is likely to need. I'm only running 10000 loops, so the max figure is not the absolute maximum. But it's certainly bigger than the 0.9999 percentile. The full results are here: http://fpaste.org/cI1r/ The uniformity of the distribution goes up with the number of virtual nodes but down with the number of physical nodes. I think we should go with a default of 48 nodes (or 50 if you prefer decimal). With 32 nodes, there's only a 0.1% chance that a node will hold more than 1.35 * K/N keys, and a 0.1% chance that the node will be primary owner for more than 1.5 * K/N keys. We could go higher, but we run against the risk of node addresses colliding on the hash wheel. According to the formula on the Birthday Paradox page (http://en.wikipedia.org/wiki/Birthday_problem), we only need 2072 addresses on our 2^31 hash wheel to get a 0.1% chance of collision. That means 21 nodes * 96 virtual nodes, 32 nodes * 64 virtual nodes or 43 nodes * 48 virtual nodes.