NDB cluster is a strange beast. Usually, performance wise, it is a good idea to limit the number of threads inside MySQL, that’s why there are parameters like thread_concurrency and innodb_thread_concurrency. MySQL is known to show mutexes contention with a number of active threads greater than a hundred (actually even less) but with NDB the situation is quite different since threads have to wait for the network latency. With NDB Cluster, be prepared to use unusually high numbers of connections and be prepared to crank up the number of active workers if you want to push NDB to its limit. The following figure shows some tests I made recently during one my engagements. As one can see, the number of active connections has an important impact on the overall throughput and it peaks at approximately 800 connections!!!
We did by mistake one run of our test with InnoDB with 800 clients… we had to kill the server after a while, it just frozed. The morale of this post, tune max_connections with NDB. The actual best value is dependent on the type of queries you are running.