doc: update af_xdp documentation on MTU limitations
PATCH 1/1 doc: update af_xdp documentation on MTU limitations
Explain how kernel driver RX buffer sizes affect the maximum MTU size in practice.
Signed-off-by Ciara Loftus <[email protected]>
--- doc/guides/nics/af_xdp.rst | 26 +++++++++++++++++++++++--- 1 file changed, 23 insertions(+), 3 deletions(-)
doc/guides/nics/af_xdp.rst +23 -3
@@ -18,9 +18,6 @@ packets through the socket which would bypass the kernel network stack. Current implementation only supports single queue, multi-queues feature will be added later. -Note that MTU of AF_XDP PMD is limited due to XDP lacks support for -fragmentation. - AF_XDP PMD enables need_wakeup flag by default if it is supported. This need_wakeup feature is used to support executing application and driver on the same core efficiently. This feature not only has a large positive performance
@@ -57,3 +54,26 @@ The following example will set up an af_xdp interface in DPDK: .. code-block:: console --vdev net_af_xdp,iface=ens786f1 + +Limitations +----------- + +- **MTU** + + The MTU of the AF_XDP PMD is limited due to the XDP requirement of one packet + per page. In the PMD we report the maximum MTU for zero copy to be equal + to the page size less the frame overhead introduced by AF_XDP (XDP HR = 256) + and DPDK (frame headroom = 320). With a 4K page size this works out at 3520. + However in practice this value may be even smaller, due to differences between + the supported RX buffer sizes of the underlying kernel netdev driver. + + For example, the largest RX buffer size supported by the underlying kernel driver + which is less than the page size (4096B) may be 3072B. In this case, the maximum + MTU value will be at most 3072, but likely even smaller than this, once relevant + headers are accounted for eg. Ethernet and VLAN. + + To determine the actual maximum MTU value of the interface you are using with the + AF_XDP PMD, consult the documentation for the kernel driver. + + Note: The AF_XDP PMD will fail to initialise if an MTU which violates the driver's + conditions as above is set prior to launching the application.
Signature · 1 line
2.25.0
Nice doc about MTU of AF_XDP, thanks for the update.
Quoted 52 lines · Ciara Loftus
Explain how kernel driver RX buffer sizes affect the maximum MTU size in practice.
Signed-off-by Ciara Loftus <[email protected]>---
doc/guides/nics/af_xdp.rst| 26 +++++++++++++++++++++++---1 file changed, 23 insertions(+), 3 deletions(-)doc/guides/nics/af_xdp.rst +0
-0@@ -18,9 +18,6 @@ packets through the socket which would bypass the kernel network stack.Current implementation only supports single queue, multi-queues feature will be added later.
doc/guides/nics/af_xdp.rst +0
-3-Note that MTU of AF_XDP PMD is limited due to XDP lacks support for -fragmentation. -
AF_XDPPMD enablesneed_wakeupflag by default if it is supported. Thisneed_wakeupfeature is used to support executing application and driver on the same core efficiently. This feature not only has a large positive performancedoc/guides/nics/af_xdp.rst +0
-0@@ -57,3 +54,26 @@ The following example will set up an af_xdp interface in DPDK:.. code-block:: console
--vdev net_af_xdp,iface=ens786f1doc/guides/nics/af_xdp.rst +23
-0+ +Limitations +----------- + +- **MTU** + + The MTU of the AF_XDP PMD is limited due to the XDP requirement of one packet + per page. In the PMD we report the maximum MTU for zero copy to be equal + to the page size less the frame overhead introduced by AF_XDP (XDP HR = 256) + and DPDK (frame headroom = 320). With a 4K page size this works out at 3520. + However in practice this value may be even smaller, due to differences between + the supported RX buffer sizes of the underlying kernel netdev driver. + + For example, the largest RX buffer size supported by the underlying kernel driver + which is less than the page size (4096B) may be 3072B. In this case, the maximum + MTU value will be at most 3072, but likely even smaller than this, once relevant + headers are accounted for eg. Ethernet and VLAN. + + To determine the actual maximum MTU value of the interface you are using with the + AF_XDP PMD, consult the documentation for the kernel driver. + + Note: The AF_XDP PMD will fail to initialise if an MTU which violates the driver's + conditions as above is set prior to launching the application.Signature · 1 line
2.25.0
Reviewed-by Xiaolong Ye <[email protected]>
Ferruh Yigit
AppliedQuoted 49 lines · Ye Xiaolong
Nice doc about MTU of
AF_XDP, thanks for the update.Quoted 46 lines · Ciara Loftus
Explain how kernel driver RX buffer sizes affect the maximum MTU size in practice.
Signed-off-by Ciara Loftus <[email protected]>---
doc/guides/nics/af_xdp.rst| 26 +++++++++++++++++++++++---1 file changed, 23 insertions(+), 3 deletions(-)doc/guides/nics/af_xdp.rst +0
-0@@ -18,9 +18,6 @@ packets through the socket which would bypass the kernel network stack.Current implementation only supports single queue, multi-queues feature will be added later.
doc/guides/nics/af_xdp.rst +0
-3-Note that MTU of AF_XDP PMD is limited due to XDP lacks support for -fragmentation. -
AF_XDPPMD enablesneed_wakeupflag by default if it is supported. Thisneed_wakeupfeature is used to support executing application and driver on the same core efficiently. This feature not only has a large positive performancedoc/guides/nics/af_xdp.rst +0
-0@@ -57,3 +54,26 @@ The following example will set up an af_xdp interface in DPDK:.. code-block:: console
--vdev net_af_xdp,iface=ens786f1doc/guides/nics/af_xdp.rst +20
-0+ +Limitations +----------- + +- **MTU** + + The MTU of the AF_XDP PMD is limited due to the XDP requirement of one packet + per page. In the PMD we report the maximum MTU for zero copy to be equal + to the page size less the frame overhead introduced by AF_XDP (XDP HR = 256) + and DPDK (frame headroom = 320). With a 4K page size this works out at 3520. + However in practice this value may be even smaller, due to differences between + the supported RX buffer sizes of the underlying kernel netdev driver. + + For example, the largest RX buffer size supported by the underlying kernel driver + which is less than the page size (4096B) may be 3072B. In this case, the maximum + MTU value will be at most 3072, but likely even smaller than this, once relevant + headers are accounted for eg. Ethernet and VLAN. + + To determine the actual maximum MTU value of the interface you are using with the + AF_XDP PMD, consult the documentation for the kernel driver.
Quoted 9 lines
Quoted 6 lines
doc/guides/nics/af_xdp.rst +3
-0+ + Note: The AF_XDP PMD will fail to initialise if an MTU which violates the driver's + conditions as above is set prior to launching the application.Signature · 1 line
2.25.0Reviewed-by Xiaolong Ye <[email protected]>
Applied to dpdk-next-net/master, thanks.