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.
