Re: [dpdk-dev] [RFC] ring: count and empty optimizations

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> 
> Hi Morten,
> 
> On Tue, Apr 28, 2020 at 03:53:15PM +0200, Morten Brørup wrote:
> > Olivier (maintainer of the Ring),
> 
> I'm not anymore, CC'ing Konstantin and Honnappa.
> 
> > I would like to suggest a couple of minor optimizations to the ring library.
> >
> >
> > 1. Testing if the ring is empty is as simple as comparing the producer and
> consumer pointers:
> >
> > static inline int
> > rte_ring_empty(const struct rte_ring *r) {
> > -	return rte_ring_count(r) == 0;
> > +	uint32_t prod_tail = r->prod.tail;
> > +	uint32_t cons_tail = r->cons.tail;
> > +	return cons_tail == prod_tail;
> > }
> >
> > In theory, this optimization reduces the number of potential cache misses
> from 3 to 2 by not having to read r->mask in rte_ring_count().
> 
> This one looks correct to me.
> 
> 
> > 2. It is not possible to enqueue more elements than the capacity of a ring,
> so the count function does not need to test if the capacity is exceeded:
> >
> > static inline unsigned
> > rte_ring_count(const struct rte_ring *r) {
> > 	uint32_t prod_tail = r->prod.tail;
> > 	uint32_t cons_tail = r->cons.tail;
> > 	uint32_t count = (prod_tail - cons_tail) & r->mask;
> > -	return (count > r->capacity) ? r->capacity : count;
> > + 	return count;
> > }
> >
> > I cannot even come up with a race condition in this function where the
> count would exceed the capacity. Maybe I missed something?
> 
> Since there is no memory barrier, the order in which prod_tail and cons_tail
> are fetched is not guaranteed. Or the thread could be interrupted by the
> kernel in between.
The '__rte_ring_move_prod_head' function ensures that the distance between prod.head and cons.tail is always within the capacity irrespective of whether the consumers/producers are sleeping.

> 
> This function may probably return invalid results in MC/MP cases.
> We just ensure that the result is between 0 and r->capacity.