6. Software Eventdev Poll Mode Driver

The software eventdev is an implementation of the eventdev API, that provides a wide range of the eventdev features. The eventdev relies on a CPU core to perform event scheduling. This PMD can use the service core library to run the scheduling function, allowing an application to utilize the power of service cores to multiplex other work on the same core if required.

6.1. Features

The software eventdev implements many features in the eventdev API;

Queues

  • Atomic

  • Ordered

  • Parallel

  • Single-Link

Ports

  • Load balanced (for Atomic, Ordered, Parallel queues)

  • Single Link (for single-link queues)

Event Priorities

  • Each event has a priority, which can be used to provide basic QoS

6.2. 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_sw0") from the application

  • Use --vdev="event_sw0" in the EAL options, which will call rte_vdev_init() internally

Example:

./your_eventdev_application --vdev="event_sw0"

6.2.1. Scheduling Quanta

The scheduling quanta sets the number of events that the device attempts to schedule in a single schedule call performed by the service core. Note that is a hint only, and that fewer or more events may be scheduled in a given iteration.

The scheduling quanta can be set using a string argument to the vdev create call:

--vdev="event_sw0,sched_quanta=64"

6.2.2. Credit Quanta

The credit quanta is the number of credits that a port will fetch at a time from the instance’s credit pool. Higher numbers will cause less overhead in the atomic credit fetch code, however it also reduces the overall number of credits in the system faster. A balanced number (e.g. 32) ensures that only small numbers of credits are pre-allocated at a time, while also mitigating performance impact of the atomics.

Experimentation with higher values may provide minor performance improvements, at the cost of the whole system having less credits. On the other hand, reducing the quanta may cause measurable performance impact but provide the system with a higher number of credits at all times.

A value of 32 seems a good balance however your specific application may benefit from a higher or reduced quanta size, experimentation is required to verify possible gains.

--vdev="event_sw0,credit_quanta=64"

6.2.3. Scheduler tuning arguments

The scheduler minimum number of events that are processed can be increased to reduce per event overhead and increase internal burst sizes, which can improve throughput.

  • min_burst specifies the minimum number of inflight events that can be moved to the next stage in the scheduler. Default value is 1.

  • refill_once is a switch that when set instructs the scheduler to deque the events waiting in the ingress rings only once per call. The default behavior is to dequeue as needed.

  • deq_burst is the burst size used to dequeue from the port rings. Default value is 32, and it should be increased to 64 or 128 when setting refill_once=1.

--vdev="event_sw0,min_burst=8,deq_burst=64,refill_once=1"

6.3. Limitations

The software eventdev implementation has a few limitations. The reason for these limitations is usually that the performance impact of supporting the feature would be significant.

6.3.1. “All Types” Queues

The software eventdev does not support creating queues that handle all types of traffic. An eventdev with this capability allows enqueuing Atomic, Ordered and Parallel traffic to the same queue, but scheduling each of them appropriately.

The reason to not allow Atomic, Ordered and Parallel event types in the same queue is that it causes excessive branching in the code to enqueue packets to the queue, causing a significant performance impact.

The RTE_EVENT_DEV_CAP_QUEUE_ALL_TYPES flag is not set in the event_dev_cap field of the rte_event_dev_info struct for the software eventdev.

6.3.2. Distributed Scheduler

The software eventdev is a centralized scheduler, requiring a service core to perform the required event distribution. This is not really a limitation but rather a design decision.

The RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED flag is not set in the event_dev_cap field of the rte_event_dev_info struct for the software eventdev.

6.3.3. Dequeue Timeout

The eventdev API supports a timeout when dequeuing packets using the rte_event_dequeue_burst function. This allows a core to wait for an event to arrive, or until timeout number of ticks have passed. Timeout ticks is not supported by the software eventdev for performance reasons.