Page Coloring Defines in vm_page.h

[Matthew Dillon]

:  - The page queue structures are sized according to these
:    defines at boot-time.
:
:  - If someone could fill me in on how the primes are arrived at that
:    would be very helpful.
:
:Comments/discussion/correction welcomed, it would be good to get some
:feedback on this before I start patching my tree.
:
:BMS

    The primes are designed such that the page allocation code covers
    *ALL* the free lists in the array, so it will still be able to find
    any available free pages if its first choice(s) are empty.

    For example, prime number 3 an array size 8 will scan the array in
    the following order  N = (N + PRIME) & (ARRAY_SIZE_MASK).
    N = (N + 3) & 7:

    0 3 6 1 4 7 2 5 ... 0

    As you can see, all the array entries are covered before the sequence
    repeats.  So if we want a free page in array slot 0 but the only
    free pages available happen to be in array slot 5, the above algorithm
    is guarenteed to find it.

    Only certain prime number / power-of-2-array size combinations have
    this effect, but it is very easy to write a little program to test
    combinations and find the numbers best suited to your goals.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>