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    SubjectRe: [patch 2/3] MAP_NOZERO - implement sys_brk2()
    On Wed, 27 Jun 2007, Davide Libenzi wrote:
    > On Wed, 27 Jun 2007, Hugh Dickins wrote:
    >
    > > In honesty, I should add that I dislike and distrust Davide's
    > > MAP_NOZERO very much indeed! Would much rather leave my cpus
    > > spending a little time in clear_page(). A uid in struct page
    > > (though I'm sure we could find somewhere to tuck it away) -
    > > the horror, the horror! But I've so far failed to find a killer
    > > argument against it, and am hoping for someone else to do so.
    >
    > Little time? Please, do not trust me. Start oprofile and run a kernel
    > build. Look, I'm not even talking about som micro benchmark explicitly
    > built to exploit the thing. A kernel build.
    > You will find clear_page to be the *1st* kernel entry after cc1 and as.
    > That is bad for two reasons. The time it spends in there, and the cache it
    > blows.

    I don't doubt that it shows real benefits; but dangerously cutting
    corners usually shows benefits too. Relying on a uid at this level
    feels very wrong to me - but as I said, I've not found a killer
    argument against it.

    And we both know that clear_page features so high in part because
    it's bringing cachelines in from the cold, which are about to be
    accessed again by userspace; so it's often not so bad as it appears.

    Though I probably wouldn't be citing that argument if we were
    talking about offloading clear_page to another engine.

    Hugh
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