[$] The multi-generational LRU

One of the key tasks assigned to the memory-management subsystem is tooptimize the system's use of the available memory; that means pushing outpages containing unused data so that they can be put to better useelsewhere. Predicting which pages will be accessed in the near future is atricky task, and the kernel has evolved a number of mechanisms designed toimprove its chances of guessing right. But the kernel not only often getsit wrong, it also can expend a lot of CPU time to make the incorrectchoice. The multi-generationalLRU patch set posted by Yu Zhao is an attempt to improve thatsituation.