From: Matt Mackall <mpm@selenic.com>

The new locking in the random driver is consuming 60% of CPU resources in
Anton's monster power5 boxes.


Basically, when the primary pool is 7/8th full, we shut off the firehose
and go into a trickle mode to keep the pool fresh.  Saves CPU for everyone
and should make the contention drop off the charts too (though the trickle
factor might need adjusting again for Origin-class machines).