The following experimental patch implements token based thrashing
protection, using the algorithm described in:

	http://www.cs.wm.edu/~sjiang/token.htm

When there are pageins going on, a task can grab a token, that protects the
task from pageout (except by itself) until it is no longer doing heavy
pageins, or until the maximum hold time of the token is over.

If the maximum hold time is exceeded, the task isn't eligable to hold the
token for a while more, since it wasn't doing it much good anyway.

I have run a very unscientific benchmark on my system to test the
effectiveness of the patch, timing how a 230MB two-process qsbench run
takes, with and without the token thrashing protection present.

normal 2.6.8-rc6:	6m45s
2.6.8-rc6 + token:	4m24s

This is a quick hack, implemented without having talked to the inventor of
the algorithm.  He's copied on the mail and I suspect we'll be able to do
better than my quick implementation ...
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>