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doc/vhost: update zero copy performance tip

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In VM2NIC case zero copy may need some tuning to get best performance.
This patch describes the zero copy starved case and provides a tuning
tip.

Signed-off-by: Junjie Chen <junjie.j.chen@intel.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>
This commit is contained in:
Junjie Chen 2018-04-26 16:13:20 +08:00 committed by Ferruh Yigit
parent 28abade0a7
commit a24e7032f7
1 changed files with 11 additions and 7 deletions
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18
doc/guides/prog_guide/vhost_lib.rst
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@ -65,14 +65,18 @@ The following is an overview of some key Vhost API functions:
* zero copy is really good for VM2VM case. For iperf between two VMs, the
boost could be above 70% (when TSO is enableld).
* for VM2NIC case, the ``nb_tx_desc`` has to be small enough: <= 64 if virtio
indirect feature is not enabled and <= 128 if it is enabled.
* For zero copy in VM2NIC case, guest Tx used vring may be starved if the
PMD driver consume the mbuf but not release them timely.
This is because when dequeue zero copy is enabled, guest Tx used vring will
be updated only when corresponding mbuf is freed. Thus, the nb_tx_desc
has to be small enough so that the PMD driver will run out of available
Tx descriptors and free mbufs timely. Otherwise, guest Tx vring would be
starved.
For example, i40e driver has an optimization to maximum NIC pipeline which
postpones returning transmitted mbuf until only tx_free_threshold free
descs left. The virtio TX used ring will be starved if the formula
(num_i40e_tx_desc - num_virtio_tx_desc > tx_free_threshold) is true, since
i40e will not return back mbuf.
A performance tip for tuning zero copy in VM2NIC case is to adjust the
frequency of mbuf free (i.e. adjust tx_free_threshold of i40e driver) to
balance consumer and producer.
* Guest memory should be backended with huge pages to achieve better
performance. Using 1G page size is the best.