10. OPDL Eventdev Poll Mode Driver

The OPDL (Ordered Packet Distribution Library) eventdev is a specificimplementation of the eventdev API. It is particularly suited to packetprocessing workloads that have high throughput and low latency requirements.All packets follow the same path through the device. The order in whichpackets follow is determined by the order in which queues are set up.Events are left on the ring until they are transmitted. As a result packetsdo not go out of order

10.1. Features

The OPDL eventdev implements a subset of features of the eventdev API;

Queues
  • Atomic
  • Ordered (Parallel is supported as parallel is a subset of Ordered)
  • Single-Link
Ports
  • Load balanced (for Atomic, Ordered, Parallel queues)
  • Single Link (for single-link queues)

10.2. Configuration and Options

The software eventdev is a vdev device, and as such can be created from the application code, or from the EAL command line:

  • Call rte_vdev_init("event_opdl0") from the application
  • Use --vdev="event_opdl0" in the EAL options, which will call rte_vdev_init() internally

Example:

./your_eventdev_application --vdev="event_opdl0"

10.2.1. Single Port Queue

It is possible to create a Single Port Queue RTE_EVENT_QUEUE_CFG_SINGLE_LINK. Packets dequeued from this queue do not need to be re-enqueued (as is the case with an ordered queue). The purpose of this queue is to allow for asynchronous handling of packets in the middle of a pipeline. Ordered queues in the middle of a pipeline cannot delete packets.

10.2.2. Queue Dependencies

As stated the order in which packets travel through queues is static in nature. They go through the queues in the order the queues are setup at initialisation rte_event_queue_setup(). For example if an application sets up 3 queues, Q0, Q1, Q2 and has 3 associated ports P0, P1, P2 and P3 then packets must be

  • Enqueued onto Q0 (typically through P0), then
  • Dequeued from Q0 (typically through P1), then
  • Enqueued onto Q1 (also through P1), then
  • Dequeued from Q2 (typically through P2), then
  • Enqueued onto Q3 (also through P2), then
  • Dequeued from Q3 (typically through P3) and then transmitted on the relevant eth port

10.3. Limitations

The opdl implementation has a number of limitations. These limitations are due to the static nature of the underlying queues. It is because of this that the implementation can achieve such high throughput and low latency

The following list is a comprehensive outline of the what is supported and the limitations / restrictions imposed by the opdl PMD

  • The order in which packets moved between queues is static and fixed (dynamic scheduling is not supported).
  • NEW, RELEASE are not explicitly supported. RX (first enqueue) implicitly adds NEW event types, and TX (last dequeue) implicitly does RELEASE event types.
  • All packets follow the same path through device queues.
  • Flows within queues are NOT supported.
  • Event priority is NOT supported.
  • Once the device is stopped all inflight events are lost. Applications should clear all inflight events before stopping it.
  • Each port can only be associated with one queue.
  • Each queue can have multiple ports associated with it.
  • Each worker core has to dequeue the maximum burst size for that port.
  • For performance, the rte_event flow_id should not be updated once packetis enqueued on RX.

10.3.1. Validation & Statistics

Validation can be turned on through a command line parameter

--vdev="event_opdl0,do_validation=1,self_test=1"

If validation is turned on every packet (as opposed to just the first in each burst), is validated to have come from the right queue. Statistics are also produced in this mode. The statistics are available through the eventdev xstats API. Statistics are per port as follows:

  • claim_pkts_requested
  • claim_pkts_granted
  • claim_non_empty
  • claim_empty
  • total_cycles