Messages in this thread | | | Date | Wed, 4 Jul 2007 14:53:53 -0700 | From | Andy Isaacson <> | Subject | Re: [patch 0/4] MAP_NOZERO v2 - VM_NOZERO/MAP_NOZERO early summer madness |
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On Mon, Jul 02, 2007 at 06:55:40PM -0400, Rik van Riel wrote: > You could easily replace the cookie with a pointer to a free > page pool.
It just occurred to me that something like this is *required* to get the performance benefit from MAP_NOZERO on a busy system. With Davide's current proposal, if there are N jobs with different "nozero cookies" busy allocating and deallocating pages on a single-NUMA-node system, then there's only a 1/N chance that the page returned by __alloc_pages will have the correct cookie, so (N-1)/N percent of the time MAP_NOZERO will have no positive effect -- 90% of the time for the case of N=10.
(Of course on NUMA systems node affinity will probably make the situation a bit better.)
So I'm afraid that it sounds like additional complexity would be required so that __alloc_pages could track and preferentially return pages with the correct "nozero cookie".
In a mostly unrelated complaint, I note that having a function named "alloc_zeroed_page_vma" which returns a potentially nonzero page is, um, unintuitive. It needs a name which does not claim it returns zeroed pages. I'm no good at names, but perhaps "alloc_available_page_vma"?
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