Re: [patch 0/4] mm: de-skew page refcount

[Nick Piggin]

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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