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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Morten,

> Testing if the ring is empty is as simple as comparing the producer and
> consumer pointers.
> 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().
> 
> It is not possible to enqueue more elements than the capacity of a ring,
> so the capacity comparison is a safeguard for observer threads only.
> Instead of completely removing the comparison, I have reorganized it to
> resemble the other trigrahps in the ring library and added a likely().
> 
> The modification of these two functions were discussed in the RFC here:
> https://mails.dpdk.org/archives/dev/2020-April/165752.html
> 
> Also fixed some existing code not passing checkpatch.
> 
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> ---
>  lib/librte_ring/rte_ring.h | 36 +++++++++++++++++++-----------------
>  1 file changed, 19 insertions(+), 17 deletions(-)
> 
> diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
> index 86faede81..36438d9cd 100644
> --- a/lib/librte_ring/rte_ring.h
> +++ b/lib/librte_ring/rte_ring.h
> @@ -55,7 +55,7 @@ extern "C" {
>   *   - The memory size needed for the ring on success.
>   *   - -EINVAL if count is not a power of 2.
>   */
> -ssize_t rte_ring_get_memsize(unsigned count);
> +ssize_t rte_ring_get_memsize(unsigned int count);

All these changes to replace 'unsigned' with insigned int' -
seems to be irrelevant to the patch subject, so can you
put them to a separate patch in the series. 
 
>  /**
>   * Initialize a ring structure.
> @@ -109,8 +109,8 @@ ssize_t rte_ring_get_memsize(unsigned count);
>   * @return
>   *   0 on success, or a negative value on error.
>   */
> -int rte_ring_init(struct rte_ring *r, const char *name, unsigned count,
> -	unsigned flags);
> +int rte_ring_init(struct rte_ring *r, const char *name, unsigned int count,
> +	unsigned int flags);
> 
>  /**
>   * Create a new ring named *name* in memory.
> @@ -169,8 +169,8 @@ int rte_ring_init(struct rte_ring *r, const char *name, unsigned count,
>   *    - EEXIST - a memzone with the same name already exists
>   *    - ENOMEM - no appropriate memory area found in which to create memzone
>   */
> -struct rte_ring *rte_ring_create(const char *name, unsigned count,
> -				 int socket_id, unsigned flags);
> +struct rte_ring *rte_ring_create(const char *name, unsigned int count,
> +				 int socket_id, unsigned int flags);
> 
>  /**
>   * De-allocate all memory used by the ring.
> @@ -199,7 +199,7 @@ void rte_ring_dump(FILE *f, const struct rte_ring *r);
>  	uint32_t idx = prod_head & (r)->mask; \
>  	obj_type *ring = (obj_type *)ring_start; \
>  	if (likely(idx + n < size)) { \
> -		for (i = 0; i < (n & ((~(unsigned)0x3))); i+=4, idx+=4) { \
> +		for (i = 0; i < (n & ~0x3); i += 4, idx += 4) { \
>  			ring[idx] = obj_table[i]; \
>  			ring[idx+1] = obj_table[i+1]; \
>  			ring[idx+2] = obj_table[i+2]; \
> @@ -230,7 +230,7 @@ void rte_ring_dump(FILE *f, const struct rte_ring *r);
>  	const uint32_t size = (r)->size; \
>  	obj_type *ring = (obj_type *)ring_start; \
>  	if (likely(idx + n < size)) { \
> -		for (i = 0; i < (n & (~(unsigned)0x3)); i+=4, idx+=4) {\
> +		for (i = 0; i < (n & ~0x3); i += 4, idx += 4) {\
>  			obj_table[i] = ring[idx]; \
>  			obj_table[i+1] = ring[idx+1]; \
>  			obj_table[i+2] = ring[idx+2]; \
> @@ -683,13 +683,13 @@ rte_ring_reset(struct rte_ring *r);
>   * @return
>   *   The number of entries in the ring.
>   */
> -static inline unsigned
> +static inline unsigned int
>  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 likely(count <= r->capacity) ? count : r->capacity;

Honestly, I don't see there is any point of that change:
I think it wouldn't change anything in terms of functionality
or performance. 

>  }
> 
>  /**
> @@ -700,7 +700,7 @@ rte_ring_count(const struct rte_ring *r)
>   * @return
>   *   The number of free entries in the ring.
>   */
> -static inline unsigned
> +static inline unsigned int
>  rte_ring_free_count(const struct rte_ring *r)
>  {
>  	return r->capacity - rte_ring_count(r);
> @@ -733,7 +733,9 @@ rte_ring_full(const struct rte_ring *r)
>  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;
>  }
> 
>  /**
> @@ -860,7 +862,7 @@ struct rte_ring *rte_ring_lookup(const char *name);
>   * @return
>   *   - n: Actual number of objects enqueued.
>   */
> -static __rte_always_inline unsigned
> +static __rte_always_inline unsigned int
>  rte_ring_mp_enqueue_burst(struct rte_ring *r, void * const *obj_table,
>  			 unsigned int n, unsigned int *free_space)
>  {
> @@ -883,7 +885,7 @@ rte_ring_mp_enqueue_burst(struct rte_ring *r, void * const *obj_table,
>   * @return
>   *   - n: Actual number of objects enqueued.
>   */
> -static __rte_always_inline unsigned
> +static __rte_always_inline unsigned int
>  rte_ring_sp_enqueue_burst(struct rte_ring *r, void * const *obj_table,
>  			 unsigned int n, unsigned int *free_space)
>  {
> @@ -910,7 +912,7 @@ rte_ring_sp_enqueue_burst(struct rte_ring *r, void * const *obj_table,
>   * @return
>   *   - n: Actual number of objects enqueued.
>   */
> -static __rte_always_inline unsigned
> +static __rte_always_inline unsigned int
>  rte_ring_enqueue_burst(struct rte_ring *r, void * const *obj_table,
>  		      unsigned int n, unsigned int *free_space)
>  {
> @@ -954,7 +956,7 @@ rte_ring_enqueue_burst(struct rte_ring *r, void * const *obj_table,
>   * @return
>   *   - n: Actual number of objects dequeued, 0 if ring is empty
>   */
> -static __rte_always_inline unsigned
> +static __rte_always_inline unsigned int
>  rte_ring_mc_dequeue_burst(struct rte_ring *r, void **obj_table,
>  		unsigned int n, unsigned int *available)
>  {
> @@ -979,7 +981,7 @@ rte_ring_mc_dequeue_burst(struct rte_ring *r, void **obj_table,
>   * @return
>   *   - n: Actual number of objects dequeued, 0 if ring is empty
>   */
> -static __rte_always_inline unsigned
> +static __rte_always_inline unsigned int
>  rte_ring_sc_dequeue_burst(struct rte_ring *r, void **obj_table,
>  		unsigned int n, unsigned int *available)
>  {
> @@ -1006,7 +1008,7 @@ rte_ring_sc_dequeue_burst(struct rte_ring *r, void **obj_table,
>   * @return
>   *   - Number of objects dequeued
>   */
> -static __rte_always_inline unsigned
> +static __rte_always_inline unsigned int
>  rte_ring_dequeue_burst(struct rte_ring *r, void **obj_table,
>  		unsigned int n, unsigned int *available)
>  {
> --
> 2.17.1