Request for information - timers, hz, interrupts

[Ivan Voras]

For a long time, at least in the 6-stable timeframe, I was used to 
seeing timer interrupts going at the frequency of 2*HZ, e.g. this is 
from 6.4-RELEASE:

kern.clockrate: { hz = 250, tick = 4000, profhz = 166, stathz = 33 }
debug.psm.hz: 20

cpu0: timer                   6789885563        499
cpu2: timer                   6789885538        499
cpu1: timer                   6789885538        499
cpu3: timer                   6789885537        499

Then sometime in 7.x this changed to 4*HZ, which continues in 8.x, e.g. 
from 7.2-RELEASE:

kern.clockrate: { hz = 250, tick = 4000, profhz = 1000, stathz = 142 }
kern.hz: 250

cpu0: timer                   1368329715        988
cpu1: timer                   1368324640        988
cpu2: timer                   1367642854        988
cpu3: timer                   1367642874        988

I'm not very worried about it (though maybe laptop users might be 
because of potential power drainage) but would like to know the 
explanation behind it.

Presumably it has something to do with profhz but what and why? There 
isn't an obvious correlation between profhz frequency in 6.x and HZ and 
in 7.x. and HZ.