On 11.03.2019 19:31, Ilya Maximets wrote:
> This partially reverts commit bde94613e6276d48a6e0be7a592ebcf9836b4aaf.
>
> Commit bde94613e627 was aimed to slightly ( < 1%) increase performance
> in the case where EMC disabled, but it avoids RSS hash calculation and
> OVS has to calculate it while executing OVS_ACTION_ATTR_HASH in order
> to handle balanced-tcp bonding. At the time of executing that action
> there is no parsed flow, and OVS parses the packet for the second time
> to calculate the hash. This happens for all packets received from the
> virtual interfaces because they have no HW RSS.
>
> Here is the example of 'perf' output for VM --> (bonded PHY) traffic:
>
> Samples: 401K of event 'cycles', Event count (approx.): 50964771478
> Overhead Shared Object Symbol
> 27.50% ovs-vswitchd [.] dpcls_lookup.370382
> 16.30% ovs-vswitchd [.] rte_vhost_dequeue_burst.9267
> 14.95% ovs-vswitchd [.] miniflow_extract
> 7.22% ovs-vswitchd [.] flow_extract
> 7.10% ovs-vswitchd [.] dp_netdev_input__.371002.4826
> 4.01% ovs-vswitchd [.] fast_path_processing.370987.4893
>
> We can see that packet parsed twice. First time by 'miniflow_extract'
> right after receiving and the second time by 'flow_extract' while
> executing actions.
>
> In this particular case calculating RSS on receive saves > 7% of the
> total CPU processing time. It varies from ~7 to ~10 % depending on
> scenario/traffic types.
>
> It's better to calculate hash each time because performance
> improvements of avoiding are negligible in compare with performance
> drop in case of sending packets to bonded interface.
>
> Another solution could be to pass the parsed flow explicitly through
> the datapath, but this will require big code changes and will have
> additional overhead for metadata updating on packet changes.
>
> Also, this change should have small impact since SMC works well in most
> cases and will be enabled/recommended by default in the future.
>
> CC: Antonio Fischetti <[email protected]>
> Fixes: bde94613e627 ("dpif-netdev: Avoid reading RSS hash when EMC is
> disabled.")
> Signed-off-by: Ilya Maximets <[email protected]>
> ---
Hi.
Any thoughts about this patch?
This is a big performance issue on setups with OVS bonding.
Best regards, Ilya Maximets.
> lib/dpif-netdev.c | 21 +++++++--------------
> 1 file changed, 7 insertions(+), 14 deletions(-)
>
> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> index 4d6d0c372..35cd00712 100644
> --- a/lib/dpif-netdev.c
> +++ b/lib/dpif-netdev.c
> @@ -6438,20 +6438,13 @@ dfc_processing(struct dp_netdev_pmd_thread *pmd,
>
> miniflow_extract(packet, &key->mf);
> key->len = 0; /* Not computed yet. */
> - /* If EMC and SMC disabled skip hash computation */
> - if (smc_enable_db == true || cur_min != 0) {
> - if (!md_is_valid) {
> - key->hash = dpif_netdev_packet_get_rss_hash_orig_pkt(packet,
> - &key->mf);
> - } else {
> - key->hash = dpif_netdev_packet_get_rss_hash(packet,
> &key->mf);
> - }
> - }
> - if (cur_min) {
> - flow = emc_lookup(&cache->emc_cache, key);
> - } else {
> - flow = NULL;
> - }
> + key->hash =
> + (md_is_valid == false)
> + ? dpif_netdev_packet_get_rss_hash_orig_pkt(packet, &key->mf)
> + : dpif_netdev_packet_get_rss_hash(packet, &key->mf);
> +
> + /* If EMC is disabled skip emc_lookup */
> + flow = (cur_min != 0) ? emc_lookup(&cache->emc_cache, key) : NULL;
> if (OVS_LIKELY(flow)) {
> tcp_flags = miniflow_get_tcp_flags(&key->mf);
> n_emc_hit++;
>
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