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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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