Optimizing for POWER7 & AIX What's New
From Page 17:
In POWER5 and POWER6, the primary and secondary SMT threads are loaded to ~80% utilization before another Virtual Processor is unfolded
In POWER7 all of the primary threads (defined by how many VPs are available) are loaded to at least ~50% utilization before the secondary threads are used. Once the secondary threads are loaded, only then will the tertiary threads be dispatched.