On Fri, Nov 02, 2018 at 07:21:27PM +0800, Gavin Hu wrote:
> 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: [email protected]
>
> Signed-off-by: Gavin Hu <[email protected]>
> Reviewed-by: Honnappa Nagarahalli <[email protected]>
> Reviewed-by: Steve Capper <[email protected]>
> Reviewed-by: Ola Liljedahl <[email protected]>
> Reviewed-by: Jia He <[email protected]>
> Acked-by: Jerin Jacob <[email protected]>
> Tested-by: Jerin Jacob <[email protected]>
Acked-by: Olivier Matz <[email protected]>
