1) Its critical for HTT. There's less foot-shooting opportunity.
2) I've seen significant improvements in interactive response to commands
over ssh sessions. I assume this is less lock contention.
3) As incentive to finish the idle cpu IPI wakeup stuff.
4) The machine on my desk was blowing hot air in my general direction
because somebody forgot to turn the hlt on, and it saves 50 watts per
cpu..
The machdep.cpu_idle_hlt sysctl is still available, but now the default
is the same as on UP kernels.
bb9cca1c2f