tap types

["Chen, Brad" <brad dot chen at intel dot com>]

Some ideas I wanted to contribute towards our thinking on the 
range of kinds of taps people might want. Some of the kinds of 
taps we are already thinking of are:
	- entry and exit taps for all kernel subroutines
	- asynchronous timer-based taps. Ability to randomize
        interval between taps is important
	- asynchronous hardware counter-based taps 
        (a generalization of the timer-based taps)
	- taps for subroutine intermediate points (not entry or exit)
I can also imagine some useful taps based (at least conceptually) 
on resources rather than control flow, such as:
	- mbuf low-water mark
	- memory low-water mark
	- I/O queue size high-water mark
	- I/O wait time high-water mark
	- ready process waiting time high water mark
We will have some of our people working on schedule and lock 
related tap ideas so there ought to be more interesting ideas 
from them soon...

Brad