[dpdk-dev] [PATCH] keepalive: fix keepalive state alignment
Van Haaren, Harry
harry.van.haaren at intel.com
Fri Jan 19 18:31:20 CET 2018
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Andriy Berestovskyy
> Sent: Friday, January 19, 2018 2:48 PM
> To: dev at dpdk.org
> Cc: Horton, Remy <remy.horton at intel.com>
> Subject: [dpdk-dev] [PATCH] keepalive: fix keepalive state alignment
>
> The __rte_cache_aligned was applied to the whole array,
> not the array elements. This leads to a false sharing between
> the monitored cores.
>
> Fixes: e70a61ad50ab ("keepalive: export states")
> Cc: remy.horton at intel.com
> Signed-off-by: Andriy Berestovskyy <aber at semihalf.com>
> ---
> lib/librte_eal/common/rte_keepalive.c | 25 +++++++++++++++----------
> 1 file changed, 15 insertions(+), 10 deletions(-)
>
> diff --git a/lib/librte_eal/common/rte_keepalive.c
> b/lib/librte_eal/common/rte_keepalive.c
> index 7ddf201..a586e03 100644
> --- a/lib/librte_eal/common/rte_keepalive.c
> +++ b/lib/librte_eal/common/rte_keepalive.c
> @@ -13,8 +13,13 @@
>
> struct rte_keepalive {
> /** Core Liveness. */
> - enum rte_keepalive_state __rte_cache_aligned state_flags[
> - RTE_KEEPALIVE_MAXCORES];
> + struct {
> + /*
> + * Each element of the state_flags table must be cache aligned
> + * to prevent false sharing.
> + */
> + enum rte_keepalive_state s __rte_cache_aligned;
> + } state_flags[RTE_KEEPALIVE_MAXCORES];
By aligning each item in the array, we do reduce false-sharing of the cache lines for all cores, however we pay the cost of increasing the footprint of the rte_keepalive struct. Note that the code iterates the full MAX_CORES in various loops in the monitoring core.
Before
(gdb) p sizeof(struct rte_keepalive)
$1 = 1728 # 27 cache lines
After
(gdb) p sizeof(struct rte_keepalive)
$1 = 9408 # 147 cache lines
These changes do reduce false-sharing however is there actually a performance benefit? A lot of cache space will be taken up if each core requires its own cache line, which will reduce performance again.. it's a tradeoff.
Little fix for a v2: "s" is not a good variable name for the rte_keepalive_state, please use something more descriptive.
<snip>
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