dpif-netdev: Conditional EMC insert

Unconditional insertion of EMC entries results in EMC thrashing at high
numbers of parallel flows. When this occurs, the performance of the EMC
often falls below that of the dpcls classifier, rendering the EMC
practically useless.

Instead of unconditionally inserting entries into the EMC when a miss
occurs, use a 1% probability of insertion. This ensures that the most
frequent flows have the highest chance of creating an entry in the EMC,
and the probability of thrashing the EMC is also greatly reduced.

The probability of insertion is configurable, via the
other_config:emc-insert-inv-prob option. This value sets the average
probability of insertion to 1/emc-insert-inv-prob.

For example the following command changes the insertion probability to
(on average) 1 in every 20 packets ie. 1/20 ie. 5%.

ovs-vsctl set Open_vSwitch . other_config:emc-insert-inv-prob=20

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Georg Schmuecking <georg.schmuecking@ericsson.com>
Co-authored-by: Georg Schmuecking <georg.schmuecking@ericsson.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 68f049fac..ce9fe8803 100644
--- a/NEWS
+++ b/NEWS
@@ -3,6 +3,8 @@ Post-v2.7.0
    - Tunnels:
      * Added support to set packet mark for tunnel endpoint using
        `egress_pkt_mark` OVSDB option.
+   - EMC insertion probability is reduced to 1% and is configurable via
+     the new 'other_config:emc-insert-inv-prob' option.
 
 v2.7.0 - xx xxx xxxx
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