[ovs-dev] [PATCH v5] dpif-netdev: Conditional EMC insert

Ciara Loftus ciara.loftus at intel.com
Thu Feb 9 11:41:50 UTC 2017


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-prob option. For example the following command
increases the insertion probability to 10%.

ovs-vsctl set Open_vSwitch . other_config:emc-insert-prob=10

Signed-off-by: Ciara Loftus <ciara.loftus at intel.com>
Signed-off-by: Georg Schmuecking <georg.schmuecking at ericsson.com>
Co-authored-by: Georg Schmuecking <georg.schmuecking at ericsson.com>
---
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 | 19 +++++++++++++++
 NEWS                         |  2 ++
 lib/dpif-netdev.c            | 57 ++++++++++++++++++++++++++++++++++++++++++--
 tests/pmd.at                 |  1 +
 vswitchd/vswitch.xml         | 13 ++++++++++
 6 files changed, 91 insertions(+), 2 deletions(-)

diff --git a/AUTHORS.rst b/AUTHORS.rst
index b567fcc..3382ad8 100644
--- a/AUTHORS.rst
+++ b/AUTHORS.rst
@@ -382,6 +382,7 @@ Eric Lopez                      elopez at nicira.com
 Frido Roose                     fr.roose at gmail.com
 Gaetano Catalli                 gaetano.catalli at gmail.com
 Gavin Remaley                   gavin_remaley at selinc.com
+Georg Schmuecking               georg.schmuecking at ericsson.com
 George Shuklin                  amarao at desunote.ru
 Gerald Rogers                   gerald.rogers at intel.com
 Ghanem Bahri                    bahri.ghanem at gmail.com
diff --git a/Documentation/howto/dpdk.rst b/Documentation/howto/dpdk.rst
index d1e6e89..29b47b6 100644
--- a/Documentation/howto/dpdk.rst
+++ b/Documentation/howto/dpdk.rst
@@ -354,6 +354,25 @@ 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-prob`` option::
+
+    $ ovs-vsctl --no-wait set Open_vSwitch . other_config:emc-insert-prob=N
+
+where:
+
+``N``
+  is a positive integer between 0 and 100 representing the percentage
+  probability of EMC insertion.
+
+If ``N`` is set to 100, 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..6fc586a 100644
--- a/NEWS
+++ b/NEWS
@@ -70,6 +70,8 @@ v2.7.0 - xx xxx xxxx
        specified.
      * Virtual DPDK Poll Mode Driver (vdev PMD) support.
      * Removed experimental tag.
+     * EMC insertion probability is reduced to 1% and is configurable via the
+       new 'other_config:emc-insert-prob' option.
    - Fedora packaging:
      * A package upgrade does not automatically restart OVS service.
    - ovs-vswitchd/ovs-vsctl:
diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
index 0be5db5..488753d 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
 
+/* Percentage probability of calling emc_insert */
+#define DEFAULT_EM_FLOW_INSERT_PROB 1
+#define DEFAULT_EM_FLOW_INSERT_MIN (UINT32_MAX /                        \
+                                    (100 / DEFAULT_EM_FLOW_INSERT_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 +1953,20 @@ 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 emc_cache *cache,
+                         const struct netdev_flow_key *key,
+                         struct dp_netdev_flow *flow,
+                         uint32_t random_val, uint32_t min)
+{
+    /* Insert an entry into the EMC based on probability value 'min'. By
+     * default the value is UINT32_MAX / 100/1 which yields an insertion
+     * probability of 1/100. */
+    if (min && (key->hash ^ random_val) <= min) {
+        emc_insert(cache, key, flow);
+    }
+}
+
 static inline struct dp_netdev_flow *
 emc_lookup(struct emc_cache *cache, const struct netdev_flow_key *key)
 {
@@ -2731,6 +2755,7 @@ 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-prob", -1);
 
     if (!nullable_string_is_equal(dp->pmd_cmask, cmask)) {
         free(dp->pmd_cmask);
@@ -2738,6 +2763,18 @@ dpif_netdev_set_config(struct dpif *dpif, const struct smap *other_config)
         dp_netdev_request_reconfigure(dp);
     }
 
+    if (insert_prob >= 0 && insert_prob <= 100) {
+        uint32_t insert_min = insert_prob == 0 ? 0 :
+                                  UINT32_MAX / (100 / insert_prob);
+        uint32_t cur_min;
+        atomic_read_relaxed(&dp->emc_insert_min, &cur_min);
+        if (insert_min != cur_min) {
+            atomic_store_relaxed(&dp->emc_insert_min, insert_min);
+        }
+    } else {
+        atomic_store_relaxed(&dp->emc_insert_min, DEFAULT_EM_FLOW_INSERT_MIN);
+    }
+
     return 0;
 }
 
@@ -4194,7 +4231,15 @@ handle_packet_upcall(struct dp_netdev_pmd_thread *pmd, struct dp_packet *packet,
         }
         ovs_mutex_unlock(&pmd->flow_mutex);
 
-        emc_insert(&pmd->flow_cache, key, netdev_flow);
+        uint32_t min;
+        atomic_read_relaxed(&pmd->dp->emc_insert_min, &min);
+#ifdef DPDK_NETDEV
+        emc_probabilistic_insert(&pmd->flow_cache, key, netdev_flow,
+                                 pmd->last_cycles, min);
+#else
+        emc_probabilistic_insert(&pmd->flow_cache, key, netdev_flow,
+                                 random_uint32(), min);
+#endif
     }
 }
 
@@ -4288,7 +4333,15 @@ fast_path_processing(struct dp_netdev_pmd_thread *pmd,
 
         flow = dp_netdev_flow_cast(rules[i]);
 
-        emc_insert(flow_cache, &keys[i], flow);
+        uint32_t min;
+        atomic_read_relaxed(&pmd->dp->emc_insert_min, &min);
+#ifdef DPDK_NETDEV
+        emc_probabilistic_insert(flow_cache, &keys[i], flow,
+                                 pmd->last_cycles, min);
+#else
+        emc_probabilistic_insert(flow_cache, &keys[i], flow,
+                                 random_uint32(), min);
+#endif
         dp_netdev_queue_batches(packet, flow, &keys[i].mf, batches, n_batches);
     }
 
diff --git a/tests/pmd.at b/tests/pmd.at
index 35dbcfe..b3132ff 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-prob=100])
 
 sleep 1
 
diff --git a/vswitchd/vswitch.xml b/vswitchd/vswitch.xml
index 146a816..e024951 100644
--- a/vswitchd/vswitch.xml
+++ b/vswitchd/vswitch.xml
@@ -335,6 +335,19 @@
           datapaths.
         </p>
       </column>
+
+      <column name="other_config" key="emc-insert-prob"
+              type='{"type": "integer", "minInteger": 0, "maxInteger": 100}'>
+        <p>
+          Specifies the percentage probability (0-100%) of a flow being
+          inserted into the Exact Match Cache (EMC). A value of 100 will result
+          in an insertion for every flow whereas a value of zero will result in
+          no insertions and essentially disable the EMC.
+        </p>
+        <p>
+          Defaults to 1 ie. there is 1% chance of EMC insertion.
+        </p>
+      </column>
     </group>
 
     <group title="Status">
-- 
2.4.11



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