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.lof...@intel.com>
Signed-off-by: Georg Schmuecking <georg.schmueck...@ericsson.com>
Co-authored-by: Georg Schmuecking <georg.schmueck...@ericsson.com>
Acked-by: Kevin Traynor <ktray...@redhat.com>
---
v10:
- Fixed typo in commit message
- Only store insert_min when value has changed
- Add prints to reflect changes in the DB
v9:
- Revert back to original 1/N formula for configuring the probability
  (don't use percentiles).
- Rename option to reflect inverse probability & update documentation to
  make the wording clearer.
v8:
- Floating point precision percentiles
- Moved NEWS entry to post-2.7 section and is no longer in the DPDK
  specific section.
v7:
- Remove further code duplication
v6:
- Refactor the code to remove duplication around calls to
  emc_probabilistic_insert()
v5:
- Use percentiles for emc-insert-prob (0-100%)
- Update docs to reflect the option not exclusive to the DPDK datapath.
v4:
- Added Georg to Authors file
- Set emc-insert-prob=1 for 'PMD - stats' unit test
- Use read_relaxed on atomic var
- Correctly allow for 0 and 100% probababilites
- Cache align new element in dp_netdev struct
- Revert to default probability if invalid emc-insert-prob set
- Allow configurability for non-DPDK case
v3:
- Use new dpif other_config infrastructure to tidy up how the
  emc-insert-prob value is passed to dpif-netdev.
v2:
- Enable probability configurability via other_config:emc-insert-prob
  option.

 AUTHORS.rst                  |  1 +
 Documentation/howto/dpdk.rst | 20 ++++++++++++++++
 NEWS                         |  2 ++
 lib/dpif-netdev.c            | 57 ++++++++++++++++++++++++++++++++++++++++----
 tests/pmd.at                 |  1 +
 vswitchd/vswitch.xml         | 17 +++++++++++++
 6 files changed, 94 insertions(+), 4 deletions(-)

diff --git a/AUTHORS.rst b/AUTHORS.rst
index f247df5..9a37423 100644
--- a/AUTHORS.rst
+++ b/AUTHORS.rst
@@ -385,6 +385,7 @@ Eric Lopez                      elo...@nicira.com
 Frido Roose                     fr.ro...@gmail.com
 Gaetano Catalli                 gaetano.cata...@gmail.com
 Gavin Remaley                   gavin_rema...@selinc.com
+Georg Schmuecking               georg.schmueck...@ericsson.com
 George Shuklin                  ama...@desunote.ru
 Gerald Rogers                   gerald.rog...@intel.com
 Ghanem Bahri                    bahri.gha...@gmail.com
diff --git a/Documentation/howto/dpdk.rst b/Documentation/howto/dpdk.rst
index d1e6e89..52cb3fc 100644
--- a/Documentation/howto/dpdk.rst
+++ b/Documentation/howto/dpdk.rst
@@ -354,6 +354,26 @@ the `DPDK documentation
 
 Note: Not all DPDK virtual PMD drivers have been tested and verified to work.
 
+EMC Insertion Probability
+-------------------------
+By default 1 in every 100 flows are inserted into the Exact Match Cache (EMC).
+It is possible to change this insertion probability by setting the
+``emc-insert-inv-prob`` option::
+
+    $ ovs-vsctl --no-wait set Open_vSwitch . other_config:emc-insert-inv-prob=N
+
+where:
+
+``N``
+  is a positive integer representing the inverse probability of insertion ie.
+  on average 1 in every N packets with a unique flow will generate an EMC
+  insertion.
+
+If ``N`` is set to 1, an insertion will be performed for every flow. If set to
+0, no insertions will be performed and the EMC will effectively be disabled.
+
+For more information on the EMC refer to :doc:`/intro/install/dpdk` .
+
 .. _dpdk-ovs-in-guest:
 
 OVS with DPDK Inside VMs
diff --git a/NEWS b/NEWS
index aebd99c..b14c76d 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
 ---------------------
diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
index 0be5db5..35d3eda 100644
--- a/lib/dpif-netdev.c
+++ b/lib/dpif-netdev.c
@@ -144,6 +144,11 @@ struct netdev_flow_key {
 #define EM_FLOW_HASH_MASK (EM_FLOW_HASH_ENTRIES - 1)
 #define EM_FLOW_HASH_SEGS 2
 
+/* Default EMC insert probability is 1 / DEFAULT_EM_FLOW_INSERT_INV_PROB */
+#define DEFAULT_EM_FLOW_INSERT_INV_PROB 100
+#define DEFAULT_EM_FLOW_INSERT_MIN (UINT32_MAX /                     \
+                                    DEFAULT_EM_FLOW_INSERT_INV_PROB)
+
 struct emc_entry {
     struct dp_netdev_flow *flow;
     struct netdev_flow_key key;   /* key.hash used for emc hash value. */
@@ -254,6 +259,9 @@ struct dp_netdev {
     uint64_t last_tnl_conf_seq;
 
     struct conntrack conntrack;
+
+    /* Probability of EMC insertions is a factor of 'emc_insert_min'.*/
+    OVS_ALIGNED_VAR(CACHE_LINE_SIZE)atomic_uint32_t emc_insert_min;
 };
 
 static struct dp_netdev_port *dp_netdev_lookup_port(const struct dp_netdev *dp,
@@ -1066,6 +1074,8 @@ create_dp_netdev(const char *name, const struct 
dpif_class *class,
 
     conntrack_init(&dp->conntrack);
 
+    atomic_init(&dp->emc_insert_min, DEFAULT_EM_FLOW_INSERT_MIN);
+
     cmap_init(&dp->poll_threads);
     ovs_mutex_init_recursive(&dp->non_pmd_mutex);
     ovsthread_key_create(&dp->per_pmd_key, NULL);
@@ -1943,6 +1953,27 @@ emc_insert(struct emc_cache *cache, const struct 
netdev_flow_key *key,
     emc_change_entry(to_be_replaced, flow, key);
 }
 
+static inline void
+emc_probabilistic_insert(struct dp_netdev_pmd_thread *pmd,
+                         const struct netdev_flow_key *key,
+                         struct dp_netdev_flow *flow)
+{
+    /* Insert an entry into the EMC based on probability value 'min'. By
+     * default the value is UINT32_MAX / 100 which yields an insertion
+     * probability of 1/100 ie. 1% */
+
+    uint32_t min;
+    atomic_read_relaxed(&pmd->dp->emc_insert_min, &min);
+
+#ifdef DPDK_NETDEV
+    if (min && (key->hash ^ (uint32_t)pmd->last_cycles) <= min) {
+#else
+    if (min && (key->hash ^ random_uint32()) <= min) {
+#endif
+        emc_insert(&pmd->flow_cache, key, flow);
+    }
+}
+
 static inline struct dp_netdev_flow *
 emc_lookup(struct emc_cache *cache, const struct netdev_flow_key *key)
 {
@@ -2731,6 +2762,8 @@ dpif_netdev_set_config(struct dpif *dpif, const struct 
smap *other_config)
 {
     struct dp_netdev *dp = get_dp_netdev(dpif);
     const char *cmask = smap_get(other_config, "pmd-cpu-mask");
+    int insert_prob = smap_get_int(other_config, "emc-insert-inv-prob", -1);
+    uint32_t insert_min, cur_min;
 
     if (!nullable_string_is_equal(dp->pmd_cmask, cmask)) {
         free(dp->pmd_cmask);
@@ -2738,6 +2771,24 @@ dpif_netdev_set_config(struct dpif *dpif, const struct 
smap *other_config)
         dp_netdev_request_reconfigure(dp);
     }
 
+    atomic_read_relaxed(&dp->emc_insert_min, &cur_min);
+    if (insert_prob >= 0 && insert_prob <= UINT32_MAX) {
+        insert_min = insert_prob == 0 ? 0 : UINT32_MAX / insert_prob;
+    } else {
+        insert_min = DEFAULT_EM_FLOW_INSERT_MIN;
+        insert_prob = DEFAULT_EM_FLOW_INSERT_INV_PROB;
+    }
+
+    if (insert_min != cur_min) {
+        atomic_store_relaxed(&dp->emc_insert_min, insert_min);
+        if (insert_min == 0) {
+            VLOG_INFO("EMC has been disabled");
+        } else {
+            VLOG_INFO("EMC insertion probability changed to 1/%i (~%.2f%%)",
+                      insert_prob, (100 / (float)insert_prob));
+        }
+    }
+
     return 0;
 }
 
@@ -4193,8 +4244,7 @@ handle_packet_upcall(struct dp_netdev_pmd_thread *pmd, 
struct dp_packet *packet,
                                              add_actions->size);
         }
         ovs_mutex_unlock(&pmd->flow_mutex);
-
-        emc_insert(&pmd->flow_cache, key, netdev_flow);
+        emc_probabilistic_insert(pmd, key, netdev_flow);
     }
 }
 
@@ -4217,7 +4267,6 @@ fast_path_processing(struct dp_netdev_pmd_thread *pmd,
     struct dpcls *cls;
     struct dpcls_rule *rules[PKT_ARRAY_SIZE];
     struct dp_netdev *dp = pmd->dp;
-    struct emc_cache *flow_cache = &pmd->flow_cache;
     int miss_cnt = 0, lost_cnt = 0;
     int lookup_cnt = 0, add_lookup_cnt;
     bool any_miss;
@@ -4288,7 +4337,7 @@ fast_path_processing(struct dp_netdev_pmd_thread *pmd,
 
         flow = dp_netdev_flow_cast(rules[i]);
 
-        emc_insert(flow_cache, &keys[i], flow);
+        emc_probabilistic_insert(pmd, &keys[i], flow);
         dp_netdev_queue_batches(packet, flow, &keys[i].mf, batches, n_batches);
     }
 
diff --git a/tests/pmd.at b/tests/pmd.at
index 35dbcfe..5686bed 100644
--- a/tests/pmd.at
+++ b/tests/pmd.at
@@ -158,6 +158,7 @@ CHECK_PMD_THREADS_CREATED()
 
 AT_CHECK([ovs-appctl vlog/set dpif_netdev:dbg])
 AT_CHECK([ovs-ofctl add-flow br0 action=normal])
+AT_CHECK([ovs-vsctl set Open_vSwitch . other_config:emc-insert-inv-prob=1])
 
 sleep 1
 
diff --git a/vswitchd/vswitch.xml b/vswitchd/vswitch.xml
index 146a816..782417f 100644
--- a/vswitchd/vswitch.xml
+++ b/vswitchd/vswitch.xml
@@ -335,6 +335,23 @@
           datapaths.
         </p>
       </column>
+
+      <column name="other_config" key="emc-insert-inv-prob"
+              type='{"type": "integer", "minInteger": 0, "maxInteger": 
4294967295}'>
+        <p>
+          Specifies the inverse probability (1/emc-insert-inv-prob) of a flow
+          being inserted into the Exact Match Cache (EMC). On average one in
+          every <code>emc-insert-inv-prob</code> packets that generate a unique
+          flow will cause an insertion into the EMC.
+
+          A value of 1 will result in an insertion for every flow (1/1 = 100%)
+          whereas a value of zero will result in no insertions and essentially
+          disable the EMC.
+        </p>
+        <p>
+          Defaults to 100 ie. there is (1/100 =) 1% chance of EMC insertion.
+        </p>
+      </column>
     </group>
 
     <group title="Status">
-- 
2.4.11

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