32. Event Device Library

The DPDK Event device library is an abstraction that provides the application with features to schedule events. This is achieved using the PMD architecture similar to the ethdev or cryptodev APIs, which may already be familiar to the reader.

The eventdev framework introduces the event driven programming model. In a polling model, lcores poll ethdev ports and associated Rx queues directly to look for a packet. By contrast in an event driven model, lcores call the scheduler that selects packets for them based on programmer-specified criteria. The Eventdev library adds support for an event driven programming model, which offers applications automatic multicore scaling, dynamic load balancing, pipelining, packet ingress order maintenance and synchronization services to simplify application packet processing.

By introducing an event driven programming model, DPDK can support both polling and event driven programming models for packet processing, and applications are free to choose whatever model (or combination of the two) best suits their needs.

Step-by-step instructions of the eventdev design is available in the API Walk-through section later in this document.

32.1. Event struct

The eventdev API represents each event with a generic struct, which contains a payload and metadata required for scheduling by an eventdev. The rte_event struct is a 16 byte C structure, defined in libs/librte_eventdev/rte_eventdev.h.

32.1.1. Event Metadata

The rte_event structure contains the following metadata fields, which the application fills in to have the event scheduled as required:

  • flow_id - The targeted flow identifier for the enq/deq operation.
  • event_type - The source of this event, eg RTE_EVENT_TYPE_ETHDEV or CPU.
  • sub_event_type - Distinguishes events inside the application, that have the same event_type (see above)
  • op - This field takes one of the RTE_EVENT_OP_* values, and tells the eventdev about the status of the event - valid values are NEW, FORWARD or RELEASE.
  • sched_type - Represents the type of scheduling that should be performed on this event, valid values are the RTE_SCHED_TYPE_ORDERED, ATOMIC and PARALLEL.
  • queue_id - The identifier for the event queue that the event is sent to.
  • priority - The priority of this event, see RTE_EVENT_DEV_PRIORITY.

32.1.2. Event Payload

The rte_event struct contains a union for payload, allowing flexibility in what the actual event being scheduled is. The payload is a union of the following:

  • uint64_t u64
  • void *event_ptr
  • struct rte_mbuf *mbuf

These three items in a union occupy the same 64 bits at the end of the rte_event structure. The application can utilize the 64 bits directly by accessing the u64 variable, while the event_ptr and mbuf are provided as convenience variables. For example the mbuf pointer in the union can used to schedule a DPDK packet.

32.1.3. Queues

An event queue is a queue containing events that are scheduled by the event device. An event queue contains events of different flows associated with scheduling types, such as atomic, ordered, or parallel.

32.1.3.1. Queue All Types Capable

If RTE_EVENT_DEV_CAP_QUEUE_ALL_TYPES capability bit is set in the event device, then events of any type may be sent to any queue. Otherwise, the queues only support events of the type that it was created with.

32.1.3.2. Queue All Types Incapable

In this case, each stage has a specified scheduling type. The application configures each queue for a specific type of scheduling, and just enqueues all events to the eventdev. An example of a PMD of this type is the eventdev software PMD.

The Eventdev API supports the following scheduling types per queue:

  • Atomic
  • Ordered
  • Parallel

Atomic, Ordered and Parallel are load-balanced scheduling types: the output of the queue can be spread out over multiple CPU cores.

Atomic scheduling on a queue ensures that a single flow is not present on two different CPU cores at the same time. Ordered allows sending all flows to any core, but the scheduler must ensure that on egress the packets are returned to ingress order on downstream queue enqueue. Parallel allows sending all flows to all CPU cores, without any re-ordering guarantees.

32.1.4. Ports

Ports are the points of contact between worker cores and the eventdev. The general use-case will see one CPU core using one port to enqueue and dequeue events from an eventdev. Ports are linked to queues in order to retrieve events from those queues (more details in Linking Queues and Ports below).

32.2. API Walk-through

This section will introduce the reader to the eventdev API, showing how to create and configure an eventdev and use it for a two-stage atomic pipeline with a single core for TX. The diagram below shows the final state of the application after this walk-through:

Fig. 32.1 Sample eventdev usage, with RX, two atomic stages and a single-link to TX.

A high level overview of the setup steps are:

  • rte_event_dev_configure()
  • rte_event_queue_setup()
  • rte_event_port_setup()
  • rte_event_port_link()
  • rte_event_dev_start()

32.2.1. Init and Config

The eventdev library uses vdev options to add devices to the DPDK application. The --vdev EAL option allows adding eventdev instances to your DPDK application, using the name of the eventdev PMD as an argument.

For example, to create an instance of the software eventdev scheduler, the following vdev arguments should be provided to the application EAL command line:

./dpdk_application --vdev="event_sw0"

In the following code, we configure eventdev instance with 3 queues and 6 ports as follows. The 3 queues consist of 2 Atomic and 1 Single-Link, while the 6 ports consist of 4 workers, 1 RX and 1 TX.

const struct rte_event_dev_config config = {
        .nb_event_queues = 3,
        .nb_event_ports = 6,
        .nb_events_limit  = 4096,
        .nb_event_queue_flows = 1024,
        .nb_event_port_dequeue_depth = 128,
        .nb_event_port_enqueue_depth = 128,
};
int err = rte_event_dev_configure(dev_id, &config);

The remainder of this walk-through assumes that dev_id is 0.

32.2.2. Setting up Queues

Once the eventdev itself is configured, the next step is to configure queues. This is done by setting the appropriate values in a queue_conf structure, and calling the setup function. Repeat this step for each queue, starting from 0 and ending at nb_event_queues - 1 from the event_dev config above.

struct rte_event_queue_conf atomic_conf = {
        .schedule_type = RTE_SCHED_TYPE_ATOMIC,
        .priority = RTE_EVENT_DEV_PRIORITY_NORMAL,
        .nb_atomic_flows = 1024,
        .nb_atomic_order_sequences = 1024,
};
int dev_id = 0;
int queue_id = 0;
int err = rte_event_queue_setup(dev_id, queue_id, &atomic_conf);

The remainder of this walk-through assumes that the queues are configured as follows:

  • id 0, atomic queue #1
  • id 1, atomic queue #2
  • id 2, single-link queue

32.2.3. Setting up Ports

Once queues are set up successfully, create the ports as required. Each port should be set up with its corresponding port_conf type, worker for worker cores, rx and tx for the RX and TX cores:

struct rte_event_port_conf rx_conf = {
        .dequeue_depth = 128,
        .enqueue_depth = 128,
        .new_event_threshold = 1024,
};
struct rte_event_port_conf worker_conf = {
        .dequeue_depth = 16,
        .enqueue_depth = 64,
        .new_event_threshold = 4096,
};
struct rte_event_port_conf tx_conf = {
        .dequeue_depth = 128,
        .enqueue_depth = 128,
        .new_event_threshold = 4096,
};
int dev_id = 0;
int port_id = 0;
int err = rte_event_port_setup(dev_id, port_id, &CORE_FUNCTION_conf);

It is now assumed that:

  • port 0: RX core
  • ports 1,2,3,4: Workers
  • port 5: TX core

32.2.4. Linking Queues and Ports

The final step is to “wire up” the ports to the queues. After this, the eventdev is capable of scheduling events, and when cores request work to do, the correct events are provided to that core. Note that the RX core takes input from eg: a NIC so it is not linked to any eventdev queues.

Linking all workers to atomic queues, and the TX core to the single-link queue can be achieved like this:

uint8_t port_id = 0;
uint8_t atomic_qs[] = {0, 1};
uint8_t single_link_q = 2;
uint8_t tx_port_id = 5;
uin8t_t priority = RTE_EVENT_DEV_PRIORITY_NORMAL;

for(int i = 0; i < 4; i++) {
        int worker_port = i + 1;
        int links_made = rte_event_port_link(dev_id, worker_port, atomic_qs, NULL, 2);
}
int links_made = rte_event_port_link(dev_id, tx_port_id, &single_link_q, &priority, 1);

32.2.5. Starting the EventDev

A single function call tells the eventdev instance to start processing events. Note that all queues must be linked to for the instance to start, as if any queue is not linked to, enqueuing to that queue will cause the application to backpressure and eventually stall due to no space in the eventdev.

int err = rte_event_dev_start(dev_id);

32.2.6. Ingress of New Events

Now that the eventdev is set up, and ready to receive events, the RX core must enqueue some events into the system for it to schedule. The events to be scheduled are ordinary DPDK packets, received from an eth_rx_burst() as normal. The following code shows how those packets can be enqueued into the eventdev:

const uint16_t nb_rx = rte_eth_rx_burst(eth_port, 0, mbufs, BATCH_SIZE);

for (i = 0; i < nb_rx; i++) {
        ev[i].flow_id = mbufs[i]->hash.rss;
        ev[i].op = RTE_EVENT_OP_NEW;
        ev[i].sched_type = RTE_SCHED_TYPE_ATOMIC;
        ev[i].queue_id = 0;
        ev[i].event_type = RTE_EVENT_TYPE_ETHDEV;
        ev[i].sub_event_type = 0;
        ev[i].priority = RTE_EVENT_DEV_PRIORITY_NORMAL;
        ev[i].mbuf = mbufs[i];
}

const int nb_tx = rte_event_enqueue_burst(dev_id, port_id, ev, nb_rx);
if (nb_tx != nb_rx) {
        for(i = nb_tx; i < nb_rx; i++)
                rte_pktmbuf_free(mbufs[i]);
}

32.2.7. Forwarding of Events

Now that the RX core has injected events, there is work to be done by the workers. Note that each worker will dequeue as many events as it can in a burst, process each one individually, and then burst the packets back into the eventdev.

The worker can lookup the events source from event.queue_id, which should indicate to the worker what workload needs to be performed on the event. Once done, the worker can update the event.queue_id to a new value, to send the event to the next stage in the pipeline.

int timeout = 0;
struct rte_event events[BATCH_SIZE];
uint16_t nb_rx = rte_event_dequeue_burst(dev_id, worker_port_id, events, BATCH_SIZE, timeout);

for (i = 0; i < nb_rx; i++) {
        /* process mbuf using events[i].queue_id as pipeline stage */
        struct rte_mbuf *mbuf = events[i].mbuf;
        /* Send event to next stage in pipeline */
        events[i].queue_id++;
}

uint16_t nb_tx = rte_event_enqueue_burst(dev_id, port_id, events, nb_rx);

32.2.8. Egress of Events

Finally, when the packet is ready for egress or needs to be dropped, we need to inform the eventdev that the packet is no longer being handled by the application. This can be done by calling dequeue() or dequeue_burst(), which indicates that the previous burst of packets is no longer in use by the application.

An event driven worker thread has following typical workflow on fastpath:

while (1) {
        rte_event_dequeue_burst(...);
        (event processing)
        rte_event_enqueue_burst(...);
}

32.3. Summary

The eventdev library allows an application to easily schedule events as it requires, either using a run-to-completion or pipeline processing model. The queues and ports abstract the logical functionality of an eventdev, providing the application with a generic method to schedule events. With the flexible PMD infrastructure applications benefit of improvements in existing eventdevs and additions of new ones without modification.