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doc: Add info on vhost tx retries.

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Add documentation about vhost tx retries and external
configuration that can help reduce/avoid them.

Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>
This commit is contained in:
Kevin Traynor
2019-06-27 12:12:30 +01:00
committed by Ian Stokes
parent d5d0c94551
commit 6d6513bfc6
1 changed files with 36 additions and 0 deletions
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36
Documentation/topics/dpdk/vhost-user.rst
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@@ -75,6 +75,42 @@ type for all known use cases; the only limitation is that vhost-user client
mode ports require QEMU version 2.7. Ports of type vhost-user are currently
deprecated and will be removed in a future release.
vhost tx retries
~~~~~~~~~~~~~~~~
When sending a batch of packets to a vhost-user or vhost-user-client interface,
it may happen that some but not all of the packets in the batch are able to be
sent to the guest. This is often because there is not enough free descriptors
in the virtqueue for all the packets in the batch to be sent. In this case
there will be a retry, with a default maximum of 8 occurring. If at any time no
packets can be sent, it may mean the guest is not accepting packets, so there
are no (more) retries.
.. note::
Maximum vhost tx batch size is defined by NETDEV_MAX_BURST, and is currently
as 32.
Tx Retries may be reduced or even avoided by some external configuration, such
as increasing the virtqueue size through the ``rx_queue_size`` parameter
introduced in QEMU 2.7.0 / libvirt 2.3.0::
<interface type='vhostuser'>
<mac address='56:48:4f:53:54:01'/>
<source type='unix' path='/tmp/dpdkvhostclient0' mode='server'/>
<model type='virtio'/>
<driver name='vhost' rx_queue_size='1024' tx_queue_size='1024'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x10' function='0x0'/>
</interface>
The guest application will also need need to provide enough descriptors. For
example with ``testpmd`` the command line argument can be used::
--rxd=1024 --txd=1024
The guest should also have sufficient cores dedicated for consuming and
processing packets at the required rate.
.. _dpdk-vhost-user:
vhost-user