According to the javadoc, in Phaser class,
Phasers may be tiered (i.e., constructed in tree structures) to reduce contention. Phasers with large numbers of parties that would otherwise experience heavy synchronization contention costs may instead be set up so that groups of sub-phasers share a common parent. This may greatly increase throughput even though it incurs greater per-operation overhead.
Could anybody clarify this statement, It's given me a bit of confusion.
Balanced trees work well in recursive decomposing programs. One example of that is the Fork/Join framework in Java7. I imagine tiered was added to Phasers as another way to use this framework, but at an awful cost. When a Phase must wait for arrival, the framework creates another thread to take its place. For a large number of waiters, this can be a disaster. You can see it work yourself by downloading the example software from this article I wrote two years ago.