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

[Olivier Matz <olivier.matz@6wind.com>]

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.