Messages in this thread |  | | | Date | Wed, 18 Jan 2006 18:05:58 +0100 | | From | Nick Piggin <> | | Subject | Re: [patch 0/4] mm: de-skew page refcount |
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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.
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