The max-spare-threads Element
It specifies the maximum allowed number of the thread pool for queuing the idle event processing threads. ZK will reuse the idle event processing threads by keeping them in a thread pool. The number specified here then controls the maximum size of the pool.
A negative value indicates that there is no limit. Zero means no pool at all.
Notice this option is applicable only if the event processing thread is enabled (it is disabled by default). Please refer to the disable-event-thread element for details.