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