CPU load spikes affecting performance introduced in hrev46690

[jstressman]

Up to and including hrev46689 my system would idle at almost flatline. Between 0 and 2% or so on any core. But starting with Pawel's changes in hrev46690 my CPUs will randomly spike to almost full load every few seconds, for anywhere from a fraction of a second to a couple seconds, during which time the system basically partially freezes. You can usually move the mouse, but the menus are frozen, displays like the ActivityMonitor freeze and then jump ahead when it unfreezes (which you'll see as the jagged sawtooth pattern on the screenshot).

I'd noticed this a few days ago when I finally updated my system from hrev46677, which was the last normal nightly release that worked for me. While trying to track down another boot problem that was introduced between that and the next nightly, I noticed that this issue was introduced in the same revision.

[jstressman]

performance from hrev46690 and on to the current nightlies, with hrev48579 being the most recent I've tested.