Timers and timing, was: MySQL Performance 6.0rc1

[Poul-Henning Kamp]

In message <20051028183034.U3405 at fledge.watson.org>, Robert Watson writes:

>If we remove the call to nanotime() in the context switch, we may want to 
>add a callout that calls nanotime() once each tick?  Or maybe 
>automatically in the callout handler, so that any code running in a 
>callout can use getnanotime() without having to worry about accuracy 
>(much).

The timecounters already update the once every N hardclock ticks (search
for "tc_ticktock" and this happens before callouts are called.

The reason for the N divisor is to not needlessly burn CPU cycles on
systems with very high HZ.

For Hz up to and in cluding 1000, N=1 so every tick updates the timestamps
which get{bin,nano,micro}time() copies.

For Hz higher than 1000, N is set to attempt to update the timestamps
once per millisecond:

	tc_tick = (hz + 500) / 1000;

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