Scientific codes spend a considerable part of their run time executing
collective communication operations. Such operations can also be
critical for efficient resource management in large-scale machines.
Therefore, scalable collective communication is a key factor to
achieve good performance in large-scale parallel computers.

In this paper we describe the performance and scalability of some
common collective communication patterns on the ASCI Q machine.
Experimental results conducted on a 1024-node/4096-processor segment
show that the network is fast and scalable.The network is able to
barrier-synchronize in a few tens of microseconds, perform a broadcast
with an aggregate bandwidth of more than 100 GB/second and sustain
heavy hot-spot traffic with a limited performance degradation.