[hackers] Re: Page Coloring Defines in vm_page.h

[David Schultz]

On Wed, Jun 25, 2003, David Gilbert wrote:
> >>>>> "Matthew" == Matthew Dillon <dillon at apollo.backplane.com> writes:
> 
> Matthew>     The primes are designed such that the page allocation
> Matthew> code covers *ALL* the free lists in the array, so it will
> Matthew> still be able to find any available free pages if its first
> Matthew> choice(s) are empty.
> 
> Matthew>     For example, prime number 3 an array size 8 will scan the
> Matthew> array in the following order N = (N + PRIME) &
> Matthew> (ARRAY_SIZE_MASK).  N = (N + 3) & 7:
> 
> Matthew>     0 3 6 1 4 7 2 5 ... 0
> 
> Matthew>     As you can see, all the array entries are covered before
> Matthew> the sequence repeats.  So if we want a free page in array
> Matthew> slot 0 but the only free pages available happen to be in
> Matthew> array slot 5, the above algorithm is guarenteed to find it.
> 
> Matthew>     Only certain prime number / power-of-2-array size
> Matthew> combinations have this effect, but it is very easy to write a
> Matthew> little program to test combinations and find the numbers best
> Matthew> suited to your goals.
> 
> For the mathematically inclined, 3 would be a 'generator' of the
> group.

That's the part I already know.  I want to know why 4 MB and 2 MB
caches use primes less than 32, 1 MB caches use primes less than
16, 512K caches use a non-prime, and 256K caches use primes
smaller than 8.  The code refers to PQ_HASH_SIZE, which has never
existed as far as I can tell...