13.1. 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.

13.1.1. 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.

13.1.1.1. 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, e.g. 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.

13.1.1.2. 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
struct rte_event_vector *vec

These four 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, mbuf, vec are provided as a convenience variables. For example the mbuf pointer in the union can used to schedule a DPDK packet.

13.1.1.3. Event Vector

The rte_event_vector struct contains a vector of elements defined by the event type specified in the rte_event. The event_vector structure contains the following data:

nb_elem - The number of elements held within the vector.

Similar to rte_event the payload of event vector is also a union, allowing flexibility in what the actual vector is.

struct rte_mbuf *mbufs[0] - An array of mbufs.
void *ptrs[0] - An array of pointers.
uint64_t u64s[0] - An array of uint64_t elements.

The size of the event vector is related to the total number of elements it is configured to hold, this is achieved by making rte_event_vector a variable length structure. A helper function is provided to create a mempool that holds event vector, which takes name of the pool, total number of required rte_event_vector, cache size, number of elements in each rte_event_vector and socket id.

rte_event_vector_pool_create("vector_pool", nb_event_vectors, cache_sz,
                             nb_elements_per_vector, socket_id);

The function rte_event_vector_pool_create creates mempool with the best platform mempool ops.

13.1.1.4. 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.

13.1.1.4.1. 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.

13.1.1.4.2. 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.

13.1.1.4.3. Single Link Flag

There is a SINGLE_LINK flag which allows an application to indicate that only one port will be connected to a queue. Queues configured with the single-link flag follow a FIFO like structure, maintaining ordering but it is only capable of being linked to a single port (see below for port and queue linking details).

13.1.1.5. 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).

13.1.2. 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 one core each for RX and TX. RX and TX cores are shown here for illustration, refer to Eventdev Adapter documentation for further details. The diagram below shows the final state of the application after this walk-through:

../../_images/eventdev_usage.svg — Fig. 13.3 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()

13.1.2.1. 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.

13.1.2.2. 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,
};
struct rte_event_queue_conf single_link_conf = {
        .event_queue_cfg = RTE_EVENT_QUEUE_CFG_SINGLE_LINK,
};
int dev_id = 0;
int atomic_q_1 = 0;
int atomic_q_2 = 1;
int single_link_q = 2;
int err = rte_event_queue_setup(dev_id, atomic_q_1, &atomic_conf);
int err = rte_event_queue_setup(dev_id, atomic_q_2, &atomic_conf);
int err = rte_event_queue_setup(dev_id, single_link_q, &single_link_conf);

As shown above, queue IDs are as follows:

id 0, atomic queue #1

id 1, atomic queue #2

id 2, single-link queue

These queues are used for the remainder of this walk-through.

13.1.2.3. Setting up Ports

Once queues are set up successfully, create the ports as required.

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 rx_port_id = 0;
int worker_port_id;
int err = rte_event_port_setup(dev_id, rx_port_id, &rx_conf);

for (worker_port_id = 1; worker_port_id <= 4; worker_port_id++) {
        int err = rte_event_port_setup(dev_id, worker_port_id, &worker_conf);
}

int tx_port_id = 5;
int err = rte_event_port_setup(dev_id, tx_port_id, &tx_conf);

As shown above:

port 0: RX core

ports 1,2,3,4: Workers

port 5: TX core

These ports are used for the remainder of this walk-through.

13.1.2.4. 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 e.g.: 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 rx_port_id = 0;
uint8_t tx_port_id = 5;
uint8_t atomic_qs[] = {0, 1};
uint8_t single_link_q = 2;
uint8_t priority = RTE_EVENT_DEV_PRIORITY_NORMAL;
int worker_port_id;

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

13.1.2.5. Linking Queues to Ports with link profiles

An application can use link profiles if supported by the underlying event device to setup up multiple link profile per port and change them run time depending up on heuristic data. Using Link profiles can reduce the overhead of linking/unlinking and wait for unlinks in progress in fast-path and gives applications the ability to switch between preset profiles on the fly.

An example use case could be as follows.

Config path:

uint8_t lq[4] = {4, 5, 6, 7};
uint8_t hq[4] = {0, 1, 2, 3};

if (rte_event_dev_info.max_profiles_per_port < 2)
    return -ENOTSUP;

rte_event_port_profile_links_set(0, 0, hq, NULL, 4, 0);
rte_event_port_profile_links_set(0, 0, lq, NULL, 4, 1);

Worker path:

uint8_t profile_id_to_switch;

while (1) {
    deq = rte_event_dequeue_burst(0, 0, &ev, 1, 0);
    if (deq == 0) {
        profile_id_to_switch = app_find_profile_id_to_switch();
        rte_event_port_profile_switch(0, 0, profile_id_to_switch);
        continue;
    }

    // Process the event received.
}

13.1.2.6. Event Pre-scheduling

Event pre-scheduling improves scheduling performance by assigning events to event ports in advance when dequeues are issued. The rte_event_dequeue_burst operation initiates the pre-schedule operation, which completes in parallel without affecting the dequeued event flow contexts and dequeue latency. On the next dequeue operation, the pre-scheduled events are dequeued and pre-schedule is initiated again.

An application can use event pre-scheduling if the event device supports it at either device level or at a individual port level. The application must check pre-schedule capability by checking if rte_event_dev_info.event_dev_cap has the bit RTE_EVENT_DEV_CAP_PRESCHEDULE or RTE_EVENT_DEV_CAP_PRESCHEDULE_ADAPTIVE set, if present pre-scheduling can be enabled at device configuration time by setting appropriate pre-schedule type in rte_event_dev_config.preschedule.

The following pre-schedule types are supported:

RTE_EVENT_PRESCHEDULE_NONE - No pre-scheduling.
RTE_EVENT_PRESCHEDULE - Always issue a pre-schedule when dequeue is issued.
RTE_EVENT_PRESCHEDULE_ADAPTIVE - Issue pre-schedule when dequeue is issued and there are no forward progress constraints.

Event devices that support RTE_EVENT_DEV_CAP_PER_PORT_PRESCHEDULE capability allow applications to modify pre-scheduling at a per port level at runtime in fast-path. To modify event pre-scheduling at a given event port, the application can use rte_event_port_preschedule_modify() API. This API can be called even if the event device does not support per port pre-scheduling, it will be treated as a no-op.

rte_event_port_preschedule_modify(dev_id, port_id, RTE_EVENT_PRESCHEDULE);
// Dequeue events from the event port with normal dequeue() function.
rte_event_port_preschedule_modify(dev_id, port_id, RTE_EVENT_PRESCHEDULE_NONE);
// Disable pre-scheduling if thread is about to be scheduled out
// and issue dequeue() to drain pending events.

Application may provide a hint to the eventdev PMD to pre-schedule the next event without releasing the current flow context. Event device that support this feature advertises the capability via the RTE_EVENT_DEV_CAP_PRESCHEDULE_EXPLICIT flag. If pre-scheduling is already enabled at a event device or event port level or if the capability is not supported then the hint is ignored.

rte_event_port_preschedule(dev_id, port_id, RTE_EVENT_PRESCHEDULE);

13.1.2.7. 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);

Note

EventDev needs to be started before starting the event producers such as event_eth_rx_adapter, event_timer_adapter, event_crypto_adapter and event_dma_adapter.

13.1.2.8. 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 = atomic_q_1;
        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, rx_port_id, ev, nb_rx);
if (nb_tx != nb_rx) {
        for(i = nb_tx; i < nb_rx; i++)
                rte_pktmbuf_free(mbufs[i]);
}

13.1.2.9. 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, worker_port_id, events, nb_rx);

13.1.2.10. 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(...);
}

13.1.2.11. Quiescing Event Ports

To migrate the event port to another lcore or while tearing down a worker core using an event port, rte_event_port_quiesce() can be invoked to make sure that all the data associated with the event port are released from the worker core, this might also include any prefetched events.

A flush callback can be passed to the function to handle any outstanding events.

rte_event_port_quiesce(dev_id, port_id, release_cb, NULL);

Note

Invocation of this API does not affect the existing port configuration.

13.1.2.12. Independent Enqueue Capability

This capability applies to eventdev devices that expects all forwarded events to be enqueued in the same order as they are dequeued. For dropped events, their releases should come at the same location as the original event was expected. The eventdev device has this restriction as it uses the order to retrieve information about the original event that was sent to the CPU. This contains information like atomic flow ID to release the flow lock and ordered events sequence number to restore the original order.

This capability only matters to eventdevs supporting burst mode. On ports where the application is going to change enqueue order, RTE_EVENT_PORT_CFG_INDEPENDENT_ENQ support should be enabled.

Example code to inform PMD that the application plans to use independent enqueue order on a port:

if (capability & RTE_EVENT_DEV_CAP_INDEPENDENT_ENQ)
    port_config = port_config | RTE_EVENT_PORT_CFG_INDEPENDENT_ENQ;

13.1.2.13. Stopping the EventDev

A single function call tells the eventdev instance to stop processing events. A flush callback can be registered to free any inflight events using rte_event_dev_stop_flush_callback_register() function.

int err = rte_event_dev_stop(dev_id);

Note

The event producers such as event_eth_rx_adapter, event_timer_adapter, event_crypto_adapter and event_dma_adapter need to be stopped before stopping the event device.

13.1.3. 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.