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

[Nick Piggin <npiggin@suse.de>]

On Wed, Jan 25, 2006 at 11:26:01AM +0100, Eric Dumazet wrote:
> Nick Piggin a ecrit :
> >If an allocator knows exactly the lifetime of its page, then there is no
> >need to do refcounting or the final put_page_zestzero (atomic op + mem
> >barriers).
> >
> >This is probably not worthwhile for most cases, but slab did strike me
> >as a potential candidate (however the complication here is that some
> >code I think uses the refcount of underlying pages of slab allocations
> >eg nommu code). So it is not a complete patch, but I wonder if anyone
> >thinks the savings might be worth the complexity?
> >
> >Is there any particular code that is really heavy on slab allocations?
> >That isn't mostly handled by the slab's internal freelists?
> 
> Hi Nick
> 
> After reading your patch, I have some crazy idea.
> 
> The atomic op + mem barrier you want to avoid could be avoided more 
> generally just by changing atomic_dec_and_test(atomic_t *v).
> 
> If the current thread is the last referer (refcnt = 1), then it can safely 
> set the value to 0 because no other CPU can be touching the value (or else 
> there must be a bug somewhere, as the 'other cpu' could touch the value 
> just after us and we could free an object still in use by 'other cpu'
> 

I think that would work for this case, but you change the semantics
of the function for all users which is bad.

Such a test could be open coded in __free_page, although that does
add a branch + some icache, but that might also be an option. (and
my patch does also add to total icache footprint and is much uglier ;))

Thanks,
Nick

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