Date: Tue, 24 Jun 2003 09:30:25 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Bruce M Simpson <bms@spc.org> Cc: hackers@FreeBSD.ORG Subject: Re: Page Coloring Defines in vm_page.h Message-ID: <200306241630.h5OGUPU6094228@apollo.backplane.com> References: <20030624111942.GO31354@spc.org>
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: - 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@backplane.com>
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