[dpdk-dev] [PATCH] doc/vhost: update zero copy perfromance tip
Junjie Chen
junjie.j.chen at intel.com
Thu Apr 26 10:13:20 CEST 2018
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 at intel.com>
---
doc/guides/prog_guide/vhost_lib.rst | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
index ef9a178..92dcdb5 100644
--- a/doc/guides/prog_guide/vhost_lib.rst
+++ b/doc/guides/prog_guide/vhost_lib.rst
@@ -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.
-
- 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 zero copy in VM2NIC case, guest Tx used vring may be starved if the
+ PMD driver consume the mbuf but not release them timely.
+
+ 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.
--
1.8.3.1
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