Re: [RFC] non-refcounted pages, application to slab?

[Nick Piggin <npiggin@suse.de>]

On Wed, Jan 25, 2006 at 12:10:48PM +0100, Eric Dumazet wrote:
> 
> So we cannot change atomic_dec_and_test(atomic_t *v) but introduce a new 
> function like :
> 
> int atomic_dec_refcount(atomic_t *v)
> {
> #ifdef CONFIG_SMP
>        /* avoid an atomic op if we are the last user of this refcount */
>        if (atomic_read(v) == 1) {
>                atomic_set(v, 0); /* not a real atomic op on most machines */
>                return 1;
>        }
> #endif
> 	return atomic_dec_and_test(v);
> }
> 
> The cost of the extra conditional branch is worth, if it can avoid an 
> atomic op.
> 

If it can always avoid an atomic op then the conditional branch is
useless, and if it can avoid the atomic op in 20% of cases then it
might still be useless (especially considering the extra icache).

Actual measurements would be required I think.