Galder Zamarreño updated ISPN-1801:
Status: Resolved (was: Pull Request Sent)
Virtual nodes should be enabled by default
Issue Type: Feature Request
Components: Distributed Cache
Affects Versions: 5.1.0.FINAL
Reporter: Mircea Markus
Assignee: Dan Berindei
Fix For: 5.1.1.CR1, 5.1.1.FINAL
ATM the default value for virtualNodes is 1. This means that the wheel-share each node
has can be very uneven for small(up to 15 nodes) clusters.
Increasing this value even to a small number(10-30) would significantly improve each
node's share of wheel and the chance for a well balanced data distribution over the
Here are some suggestions from an email from Dan:
I've been working on a test to search for an optimal default value here:
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:
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.
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