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    SubjectRe: [patch 0/4] mm: de-skew page refcount
    On Wed, Jan 18, 2006 at 08:38:44AM -0800, Linus Torvalds wrote:
    >
    >
    > On Wed, 18 Jan 2006, Nick Piggin wrote:
    > >
    > > The following patchset (against 2.6.16-rc1 + migrate race fixes) uses the new
    > > atomic ops to do away with the offset page refcounting, and simplify the race
    > > that it was designed to cover.
    > >
    > > This allows some nice optimisations
    >
    > Why?
    >
    > The real downside is that "atomic_inc_nonzero()" is a lot more expensive
    > than checking for zero on x86 (and x86-64).
    >
    > The reason it's offset is that on architectures that automatically test
    > the _result_ of an atomic op (ie x86[-64]), it's easy to see when
    > something _becomes_ negative or _becomes_ zero, and that's what
    >
    > atomic_add_negative
    > atomic_inc_and_test
    >
    > are optimized for (there's also "atomic_dec_and_test()" which reacts on
    > the count becoming zero, but that doesn't have a pairing: there's no way
    > to react to the count becoming one for the increment operation, so the
    > "atomic_dec_and_test()" is used for things where zero means "free it").
    >
    > Nothing else can be done that fast on x86. Everything else requires an
    > insane "load, update, cmpxchg" sequence.
    >

    Yes, I realise inc_not_zero isn't as fast as dec_and_test on x86(-64).
    In this case when the cacheline will already be exclusive I bet it isn't
    that much of a difference (in the noise from my testing).

    > So I disagree with this patch series. It has real downsides. There's a
    > reason we have the offset.
    >

    Yes, there is a reason, I detailed it in the changelog and got rid of it.

    > "nice optimizations"

    They're in this patchset.

    Nick
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