Date: Thu, 26 Jun 2003 03:33:07 -0700 From: David Schultz <das@FreeBSD.ORG> To: David Gilbert <dgilbert@velocet.ca> Cc: davidg@FreeBSD.ORG Subject: Re: [hackers] Re: Page Coloring Defines in vm_page.h Message-ID: <20030626103307.GC94891@HAL9000.homeunix.com> In-Reply-To: <16121.47185.522249.637280@canoe.velocet.net> References: <20030624111942.GO31354@spc.org> <200306241630.h5OGUPU6094228@apollo.backplane.com> <16121.47185.522249.637280@canoe.velocet.net>
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On Wed, Jun 25, 2003, David Gilbert wrote: > >>>>> "Matthew" == Matthew Dillon <dillon@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...
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