13-15 November 2018
America/Vancouver timezone

The Path to DPDK Speeds for AF_XDP

13 Nov 2018, 15:10
Junior-Ballroom-C (Sheraton Vancouver Wall Center)


Sheraton Vancouver Wall Center



Björn Töpel (Intel) Magnus Karlsson (Intel)


AF_XDP is a new socket type for raw frames to be introduced in 4.18
(in linux-next at the time of writing). The current code base offers
throughput numbers north of 20 Mpps per application core for 64-byte
packets on our system, however there are a lot of optimizations that
could be performed in order to increase this even further. The focus
of this paper is the performance optimizations we need to make in
AF_XDP to get it to perform as fast as DPDK.

We present optimization that fall into two broad categories: ones that
are seamless to the application and ones that requires additions to
the uapi. In the first category we examine the following:

  • Loosen the requirement for having an XDP program. If the user does
    not need an XDP program and there is only one AF_XDP socket bound to
    a particular queue, we do not need an XDP program. This should cut
    out quite a number of cycles from the RX path.

  • Wire up busy poll from user space. If the application writer is
    using epoll() and friends, this has the potential benefit of
    removing the coherency communication between the RX (NAPI) core and
    the application core as everything is now done on a single
    core. Should improve performance for a number of use cases. Maybe it
    is worth revisiting the old idea of threaded NAPI in this context

  • Optimize for high instruction cache usage through batching as has
    been explored in for example Cisco's VPP stack and Edward Cree in
    his net-next RFC "Handle multiple received packets at each stage".

In the uapi extensions category we examine the following

  • Support a new mode for NICs with in-order TX completions. In this
    mode, the completion queue would not be used. Instead the
    application would simply look at the pointer in the TX queue to see
    if a packet has been completed. In this mode, we do not need any
    backpreassure between the completion queue and the TX queue and we
    do not need to populate or publish anything in the completion queue
    as it is not used. Should improve the performance of TX for in-order
    NICs significantly.

  • Introduce the "type-writer" model where each chunk can contain
    multiple packets. This is the model that e.g., Chelsio has in its
    NICs. But experiments show that this mode also can provide better
    performance for regular NICs as there are fewer transactions on the
    queues. Requires a new flag to be introduced in the options field of
    the descriptor.

With these optimization, we believe we can reach our goal of close to
40 Mpps of throughput for 64-byte packets in zero-copy mode. Full
analysis with performance numbers will be presented in the final

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