Re: [dpdk-dev] [PATCH v3 2/3] ring: synchronize the load and store of the tail

[Gavin Hu (Arm Technology China)]

+Justin He for review.

> -----Original Message-----
> From: Gavin Hu <gavin...@arm.com>
> Sent: Monday, September 17, 2018 4:17 PM
> To: dev@dpdk.org
> Cc: Gavin Hu (Arm Technology China) <gavin...@arm.com>; Honnappa
> Nagarahalli <honnappa.nagaraha...@arm.com>; Steve Capper
> <steve.cap...@arm.com>; Ola Liljedahl <ola.liljed...@arm.com>;
> jerin.ja...@caviumnetworks.com; nd <n...@arm.com>; sta...@dpdk.org
> Subject: [PATCH v3 2/3] ring: synchronize the load and store of the tail
> 
> Synchronize the load-acquire of the tail and the store-release within
> update_tail, the store release ensures all the ring operations, enqueue or
> dequeue, are seen by the observers on the other side as soon as they see
> the updated tail. The load-acquire is needed here as the data dependency is
> not a reliable way for ordering as the compiler might break it by saving to
> temporary values to boost performance.
> When computing the free_entries and avail_entries, use atomic semantics to
> load the heads and tails instead.
> 
> The patch was benchmarked with test/ring_perf_autotest and it decreases
> the enqueue/dequeue latency by 5% ~ 27.6% with two lcores, the real gains
> are dependent on the number of lcores, depth of the ring, SPSC or MPMC.
> For 1 lcore, it also improves a little, about 3 ~ 4%.
> It is a big improvement, in case of MPMC, with two lcores and ring size of 32,
> it saves latency up to (3.26-2.36)/3.26 = 27.6%.
> 
> This patch is a bug fix, while the improvement is a bonus. In our analysis the
> improvement comes from the cacheline pre-filling after hoisting load-
> acquire from _atomic_compare_exchange_n up above.
> 
> The test command:
> $sudo ./test/test/test -l 16-19,44-47,72-75,100-103 -n 4 --socket-mem=\
> 1024 -- -i
> 
> Test result with this patch(two cores):
>  SP/SC bulk enq/dequeue (size: 8): 5.86
>  MP/MC bulk enq/dequeue (size: 8): 10.15  SP/SC bulk enq/dequeue (size:
> 32): 1.94  MP/MC bulk enq/dequeue (size: 32): 2.36
> 
> In comparison of the test result without this patch:
>  SP/SC bulk enq/dequeue (size: 8): 6.67
>  MP/MC bulk enq/dequeue (size: 8): 13.12  SP/SC bulk enq/dequeue (size:
> 32): 2.04  MP/MC bulk enq/dequeue (size: 32): 3.26
> 
> Fixes: 39368ebfc6 ("ring: introduce C11 memory model barrier option")
> Cc: sta...@dpdk.org
> 
> Signed-off-by: Gavin Hu <gavin...@arm.com>
> Reviewed-by: Honnappa Nagarahalli <honnappa.nagaraha...@arm.com>
> Reviewed-by: Steve Capper <steve.cap...@arm.com>
> Reviewed-by: Ola Liljedahl <ola.liljed...@arm.com>
> ---
>  lib/librte_ring/rte_ring_c11_mem.h | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/librte_ring/rte_ring_c11_mem.h
> b/lib/librte_ring/rte_ring_c11_mem.h
> index 234fea0..0eae3b3 100644
> --- a/lib/librte_ring/rte_ring_c11_mem.h
> +++ b/lib/librte_ring/rte_ring_c11_mem.h
> @@ -68,13 +68,18 @@ __rte_ring_move_prod_head(struct rte_ring *r,
> unsigned int is_sp,
>               *old_head = __atomic_load_n(&r->prod.head,
>                                       __ATOMIC_ACQUIRE);
> 
> -             /*
> -              *  The subtraction is done between two unsigned 32bits
> value
> +             /* load-acquire synchronize with store-release of ht->tail
> +              * in update_tail.
> +              */
> +             const uint32_t cons_tail = __atomic_load_n(&r->cons.tail,
> +
>       __ATOMIC_ACQUIRE);
> +
> +             /* The subtraction is done between two unsigned 32bits
> value
>                * (the result is always modulo 32 bits even if we have
>                * *old_head > cons_tail). So 'free_entries' is always
> between 0
>                * and capacity (which is < size).
>                */
> -             *free_entries = (capacity + r->cons.tail - *old_head);
> +             *free_entries = (capacity + cons_tail - *old_head);
> 
>               /* check that we have enough room in ring */
>               if (unlikely(n > *free_entries))
> @@ -132,15 +137,22 @@ __rte_ring_move_cons_head(struct rte_ring *r, int
> is_sc,
>       do {
>               /* Restore n as it may change every loop */
>               n = max;
> +
>               *old_head = __atomic_load_n(&r->cons.head,
>                                       __ATOMIC_ACQUIRE);
> 
> +             /* this load-acquire synchronize with store-release of ht->tail
> +              * in update_tail.
> +              */
> +             const uint32_t prod_tail = __atomic_load_n(&r->prod.tail,
> +                                     __ATOMIC_ACQUIRE);
> +
>               /* The subtraction is done between two unsigned 32bits
> value
>                * (the result is always modulo 32 bits even if we have
>                * cons_head > prod_tail). So 'entries' is always between 0
>                * and size(ring)-1.
>                */
> -             *entries = (r->prod.tail - *old_head);
> +             *entries = (prod_tail - *old_head);
> 
>               /* Set the actual entries for dequeue */
>               if (n > *entries)
> --
> 2.7.4