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 $\mu$s,
perform a broadcast with an aggregate bandwidth of more than 100
GB/s and sustain heavy hot-spot traffic with a limited performance
degradation.