[ovs-dev] [PATCH v2 2/9] doc: Add "PMD" topic document

Stephen Finucane stephen at that.guru
Mon Apr 16 14:30:19 UTC 2018


This continues the breakup of the huge DPDK "howto" into smaller
components. There are a couple of related changes included, such as
using "Rx queue" instead of "rxq" and noting how Tx queues cannot be
configured.

Signed-off-by: Stephen Finucane <stephen at that.guru>
---
v2:
- Add cross-references from 'pmd' doc to 'vhost-user' and 'phy' docs
- Add 'versionchanged' warning about automatic assignment of Rx queues
- Add a 'todo' to describe Tx queue behavior
---
 Documentation/howto/dpdk.rst             |  86 -----------------
 Documentation/topics/dpdk/index.rst      |   1 +
 Documentation/topics/dpdk/phy.rst        |  12 +++
 Documentation/topics/dpdk/pmd.rst        | 156 +++++++++++++++++++++++++++++++
 Documentation/topics/dpdk/vhost-user.rst |  17 ++--
 5 files changed, 177 insertions(+), 95 deletions(-)
 create mode 100644 Documentation/topics/dpdk/pmd.rst

diff --git a/Documentation/howto/dpdk.rst b/Documentation/howto/dpdk.rst
index 79b626c76..388728363 100644
--- a/Documentation/howto/dpdk.rst
+++ b/Documentation/howto/dpdk.rst
@@ -81,92 +81,6 @@ To stop ovs-vswitchd & delete bridge, run::
     $ ovs-appctl -t ovsdb-server exit
     $ ovs-vsctl del-br br0
 
-PMD Thread Statistics
----------------------
-
-To show current stats::
-
-    $ ovs-appctl dpif-netdev/pmd-stats-show
-
-To clear previous stats::
-
-    $ ovs-appctl dpif-netdev/pmd-stats-clear
-
-Port/RXQ Assigment to PMD Threads
----------------------------------
-
-To show port/rxq assignment::
-
-    $ ovs-appctl dpif-netdev/pmd-rxq-show
-
-To change default rxq assignment to pmd threads, rxqs may be manually pinned to
-desired cores using::
-
-    $ ovs-vsctl set Interface <iface> \
-        other_config:pmd-rxq-affinity=<rxq-affinity-list>
-
-where:
-
-- ``<rxq-affinity-list>`` is a CSV list of ``<queue-id>:<core-id>`` values
-
-For example::
-
-    $ ovs-vsctl set interface dpdk-p0 options:n_rxq=4 \
-        other_config:pmd-rxq-affinity="0:3,1:7,3:8"
-
-This will ensure:
-
-- Queue #0 pinned to core 3
-- Queue #1 pinned to core 7
-- Queue #2 not pinned
-- Queue #3 pinned to core 8
-
-After that PMD threads on cores where RX queues was pinned will become
-``isolated``. This means that this thread will poll only pinned RX queues.
-
-.. warning::
-  If there are no ``non-isolated`` PMD threads, ``non-pinned`` RX queues will
-  not be polled. Also, if provided ``core_id`` is not available (ex. this
-  ``core_id`` not in ``pmd-cpu-mask``), RX queue will not be polled by any PMD
-  thread.
-
-If pmd-rxq-affinity is not set for rxqs, they will be assigned to pmds (cores)
-automatically. The processing cycles that have been stored for each rxq
-will be used where known to assign rxqs to pmd based on a round robin of the
-sorted rxqs.
-
-For example, in the case where here there are 5 rxqs and 3 cores (e.g. 3,7,8)
-available, and the measured usage of core cycles per rxq over the last
-interval is seen to be:
-
-- Queue #0: 30%
-- Queue #1: 80%
-- Queue #3: 60%
-- Queue #4: 70%
-- Queue #5: 10%
-
-The rxqs will be assigned to cores 3,7,8 in the following order:
-
-Core 3: Q1 (80%) |
-Core 7: Q4 (70%) | Q5 (10%)
-core 8: Q3 (60%) | Q0 (30%)
-
-To see the current measured usage history of pmd core cycles for each rxq::
-
-    $ ovs-appctl dpif-netdev/pmd-rxq-show
-
-.. note::
-
-  A history of one minute is recorded and shown for each rxq to allow for
-  traffic pattern spikes. An rxq's pmd core cycles usage changes due to traffic
-  pattern or reconfig changes will take one minute before they are fully
-  reflected in the stats.
-
-Rxq to pmds assignment takes place whenever there are configuration changes
-or can be triggered by using::
-
-    $ ovs-appctl dpif-netdev/pmd-rxq-rebalance
-
 QoS
 ---
 
diff --git a/Documentation/topics/dpdk/index.rst b/Documentation/topics/dpdk/index.rst
index 5f836a6e9..dfde88377 100644
--- a/Documentation/topics/dpdk/index.rst
+++ b/Documentation/topics/dpdk/index.rst
@@ -31,3 +31,4 @@ The DPDK Datapath
    phy
    vhost-user
    ring
+   pmd
diff --git a/Documentation/topics/dpdk/phy.rst b/Documentation/topics/dpdk/phy.rst
index a3f8b475c..ad191dad0 100644
--- a/Documentation/topics/dpdk/phy.rst
+++ b/Documentation/topics/dpdk/phy.rst
@@ -113,3 +113,15 @@ tool::
 For more information, refer to the `DPDK documentation <dpdk-drivers>`__.
 
 .. _dpdk-drivers: http://dpdk.org/doc/guides/linux_gsg/linux_drivers.html
+
+.. _dpdk-phy-multiqueue:
+
+Multiqueue
+----------
+
+Poll Mode Driver (PMD) threads are the threads that do the heavy lifting for
+the DPDK datapath. Correct configuration of PMD threads and the Rx queues they
+utilize is a requirement in order to deliver the high-performance possible with
+DPDK acceleration. It is possible to configure multiple Rx queues for ``dpdk``
+ports, thus ensuring this is not a bottleneck for performance. For information
+on configuring PMD threads, refer to :doc:`pmd`.
diff --git a/Documentation/topics/dpdk/pmd.rst b/Documentation/topics/dpdk/pmd.rst
new file mode 100644
index 000000000..1be25ade0
--- /dev/null
+++ b/Documentation/topics/dpdk/pmd.rst
@@ -0,0 +1,156 @@
+..
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+      Convention for heading levels in Open vSwitch documentation:
+
+      =======  Heading 0 (reserved for the title in a document)
+      -------  Heading 1
+      ~~~~~~~  Heading 2
+      +++++++  Heading 3
+      '''''''  Heading 4
+
+      Avoid deeper levels because they do not render well.
+
+===========
+PMD Threads
+===========
+
+Poll Mode Driver (PMD) threads are the threads that do the heavy lifting for
+the DPDK datapath and perform tasks such as continuous polling of input ports
+for packets, classifying packets once received, and executing actions on the
+packets once they are classified.
+
+PMD threads utilize Receive (Rx) and Transmit (Tx) queues, commonly known as
+*rxq*\s and *txq*\s. While Tx queue configuration happens automatically, Rx
+queues can be configured by the user. This can happen in one of two ways:
+
+- For physical interfaces, configuration is done using the
+  :program:`ovs-appctl` utility.
+
+- For virtual interfaces, configuration is done using the :program:`ovs-appctl`
+  utility, but this configuration must be reflected in the guest configuration
+  (e.g. QEMU command line arguments).
+
+The :program:`ovs-appctl` utility also provides a number of commands for
+querying PMD threads and their respective queues. This, and all of the above,
+is discussed here.
+
+.. todo::
+
+   Add an overview of Tx queues including numbers created, how they relate to
+   PMD threads, etc.
+
+PMD Thread Statistics
+---------------------
+
+To show current stats::
+
+    $ ovs-appctl dpif-netdev/pmd-stats-show
+
+To clear previous stats::
+
+    $ ovs-appctl dpif-netdev/pmd-stats-clear
+
+Port/Rx Queue Assigment to PMD Threads
+--------------------------------------
+
+.. todo::
+
+   This needs a more detailed overview of *why* this should be done, along with
+   the impact on things like NUMA affinity.
+
+Correct configuration of PMD threads and the Rx queues they utilize is a
+requirement in order to achieve maximum performance. This is particularly true
+for enabling things like multiqueue for :ref:`physical <dpdk-phy-multiqueue>`
+and :ref:`vhost-user <dpdk-vhost-user>` interfaces.
+
+To show port/Rx queue assignment::
+
+    $ ovs-appctl dpif-netdev/pmd-rxq-show
+
+Rx queues may be manually pinned to cores. This will change the default Rx
+queue assignment to PMD threads::
+
+    $ ovs-vsctl set Interface <iface> \
+        other_config:pmd-rxq-affinity=<rxq-affinity-list>
+
+where:
+
+- ``<rxq-affinity-list>`` is a CSV list of ``<queue-id>:<core-id>`` values
+
+For example::
+
+    $ ovs-vsctl set interface dpdk-p0 options:n_rxq=4 \
+        other_config:pmd-rxq-affinity="0:3,1:7,3:8"
+
+This will ensure there are *4* Rx queues and that these queues are configured
+like so:
+
+- Queue #0 pinned to core 3
+- Queue #1 pinned to core 7
+- Queue #2 not pinned
+- Queue #3 pinned to core 8
+
+PMD threads on cores where Rx queues are *pinned* will become *isolated*. This
+means that this thread will only poll the *pinned* Rx queues.
+
+.. warning::
+
+   If there are no *non-isolated* PMD threads, *non-pinned* RX queues will not
+   be polled. Also, if the provided ``<core-id>`` is not available (e.g. the
+   ``<core-id>`` is not in ``pmd-cpu-mask``), the RX queue will not be polled
+   by any PMD thread.
+
+If ``pmd-rxq-affinity`` is not set for Rx queues, they will be assigned to PMDs
+(cores) automatically. Where known, the processing cycles that have been stored
+for each Rx queue will be used to assign Rx queue to PMDs based on a round
+robin of the sorted Rx queues. For example, take the following example, where
+there are five Rx queues and three cores - 3, 7, and 8 - available and the
+measured usage of core cycles per Rx queue over the last interval is seen to
+be:
+
+- Queue #0: 30%
+- Queue #1: 80%
+- Queue #3: 60%
+- Queue #4: 70%
+- Queue #5: 10%
+
+The Rx queues will be assigned to the cores in the following order::
+
+    Core 3: Q1 (80%) |
+    Core 7: Q4 (70%) | Q5 (10%)
+    Core 8: Q3 (60%) | Q0 (30%)
+
+To see the current measured usage history of PMD core cycles for each Rx
+queue::
+
+    $ ovs-appctl dpif-netdev/pmd-rxq-show
+
+.. note::
+
+   A history of one minute is recorded and shown for each Rx queue to allow for
+   traffic pattern spikes. Any changes in the Rx queue's PMD core cycles usage,
+   due to traffic pattern or reconfig changes, will take one minute to be fully
+   reflected in the stats.
+
+Rx queue to PMD assignment takes place whenever there are configuration changes
+or can be triggered by using::
+
+    $ ovs-appctl dpif-netdev/pmd-rxq-rebalance
+
+.. versionchanged:: 2.8.0
+
+   Automatic assignment of Rx queues to PMDs and the two related commands,
+   ``pmd-rxq-show`` and ``pmd-rxq-rebalance``, were added in OVS 2.8.0. Prior
+   to this, behavior was round-robin and processing cycles were not taken into
+   consideration. Tracking for stats was not available.
diff --git a/Documentation/topics/dpdk/vhost-user.rst b/Documentation/topics/dpdk/vhost-user.rst
index ca8a3289f..6f794f296 100644
--- a/Documentation/topics/dpdk/vhost-user.rst
+++ b/Documentation/topics/dpdk/vhost-user.rst
@@ -130,11 +130,10 @@ an additional set of parameters::
     -netdev type=vhost-user,id=mynet2,chardev=char2,vhostforce
     -device virtio-net-pci,mac=00:00:00:00:00:02,netdev=mynet2
 
-In addition,       QEMU must allocate the VM's memory on hugetlbfs. vhost-user
-ports access a virtio-net device's virtual rings and packet buffers mapping the
-VM's physical memory on hugetlbfs. To enable vhost-user ports to map the VM's
-memory into their process address space, pass the following parameters to
-QEMU::
+In addition, QEMU must allocate the VM's memory on hugetlbfs. vhost-user ports
+access a virtio-net device's virtual rings and packet buffers mapping the VM's
+physical memory on hugetlbfs. To enable vhost-user ports to map the VM's memory
+into their process address space, pass the following parameters to QEMU::
 
     -object memory-backend-file,id=mem,size=4096M,mem-path=/dev/hugepages,share=on
     -numa node,memdev=mem -mem-prealloc
@@ -154,18 +153,18 @@ where:
   The number of vectors, which is ``$q`` * 2 + 2
 
 The vhost-user interface will be automatically reconfigured with required
-number of rx and tx queues after connection of virtio device.  Manual
+number of Rx and Tx queues after connection of virtio device.  Manual
 configuration of ``n_rxq`` is not supported because OVS will work properly only
 if ``n_rxq`` will match number of queues configured in QEMU.
 
-A least 2 PMDs should be configured for the vswitch when using multiqueue.
+A least two PMDs should be configured for the vswitch when using multiqueue.
 Using a single PMD will cause traffic to be enqueued to the same vhost queue
 rather than being distributed among different vhost queues for a vhost-user
 interface.
 
 If traffic destined for a VM configured with multiqueue arrives to the vswitch
-via a physical DPDK port, then the number of rxqs should also be set to at
-least 2 for that physical DPDK port. This is required to increase the
+via a physical DPDK port, then the number of Rx queues should also be set to at
+least two for that physical DPDK port. This is required to increase the
 probability that a different PMD will handle the multiqueue transmission to the
 guest using a different vhost queue.
 
-- 
2.14.3



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