7. Poll Mode Driver

The DPDK includes 1 Gigabit, 10 Gigabit and 40 Gigabit and para virtualized virtio Poll Mode Drivers.

A Poll Mode Driver (PMD) consists of APIs, provided through the BSD driver running in user space, to configure the devices and their respective queues. In addition, a PMD accesses the RX and TX descriptors directly without any interrupts (with the exception of Link Status Change interrupts) to quickly receive, process and deliver packets in the user’s application. This section describes the requirements of the PMDs, their global design principles and proposes a high-level architecture and a generic external API for the Ethernet PMDs.

7.1. Requirements and Assumptions

The DPDK environment for packet processing applications allows for two models, run-to-completion and pipe-line:

  • In the run-to-completion model, a specific port’s RX descriptor ring is polled for packets through an API. Packets are then processed on the same core and placed on a port’s TX descriptor ring through an API for transmission.
  • In the pipe-line model, one core polls one or more port’s RX descriptor ring through an API. Packets are received and passed to another core via a ring. The other core continues to process the packet which then may be placed on a port’s TX descriptor ring through an API for transmission.

In a synchronous run-to-completion model, each logical core assigned to the DPDK executes a packet processing loop that includes the following steps:

  • Retrieve input packets through the PMD receive API
  • Process each received packet one at a time, up to its forwarding
  • Send pending output packets through the PMD transmit API

Conversely, in an asynchronous pipe-line model, some logical cores may be dedicated to the retrieval of received packets and other logical cores to the processing of previously received packets. Received packets are exchanged between logical cores through rings. The loop for packet retrieval includes the following steps:

  • Retrieve input packets through the PMD receive API
  • Provide received packets to processing lcores through packet queues

The loop for packet processing includes the following steps:

  • Retrieve the received packet from the packet queue
  • Process the received packet, up to its retransmission if forwarded

To avoid any unnecessary interrupt processing overhead, the execution environment must not use any asynchronous notification mechanisms. Whenever needed and appropriate, asynchronous communication should be introduced as much as possible through the use of rings.

Avoiding lock contention is a key issue in a multi-core environment. To address this issue, PMDs are designed to work with per-core private resources as much as possible. For example, a PMD maintains a separate transmit queue per-core, per-port. In the same way, every receive queue of a port is assigned to and polled by a single logical core (lcore).

To comply with Non-Uniform Memory Access (NUMA), memory management is designed to assign to each logical core a private buffer pool in local memory to minimize remote memory access. The configuration of packet buffer pools should take into account the underlying physical memory architecture in terms of DIMMS, channels and ranks. The application must ensure that appropriate parameters are given at memory pool creation time. See Mempool Library.

7.2. Design Principles

The API and architecture of the Ethernet* PMDs are designed with the following guidelines in mind.

PMDs must help global policy-oriented decisions to be enforced at the upper application level. Conversely, NIC PMD functions should not impede the benefits expected by upper-level global policies, or worse prevent such policies from being applied.

For instance, both the receive and transmit functions of a PMD have a maximum number of packets/descriptors to poll. This allows a run-to-completion processing stack to statically fix or to dynamically adapt its overall behavior through different global loop policies, such as:

  • Receive, process immediately and transmit packets one at a time in a piecemeal fashion.
  • Receive as many packets as possible, then process all received packets, transmitting them immediately.
  • Receive a given maximum number of packets, process the received packets, accumulate them and finally send all accumulated packets to transmit.

To achieve optimal performance, overall software design choices and pure software optimization techniques must be considered and balanced against available low-level hardware-based optimization features (CPU cache properties, bus speed, NIC PCI bandwidth, and so on). The case of packet transmission is an example of this software/hardware tradeoff issue when optimizing burst-oriented network packet processing engines. In the initial case, the PMD could export only an rte_eth_tx_one function to transmit one packet at a time on a given queue. On top of that, one can easily build an rte_eth_tx_burst function that loops invoking the rte_eth_tx_one function to transmit several packets at a time. However, an rte_eth_tx_burst function is effectively implemented by the PMD to minimize the driver-level transmit cost per packet through the following optimizations:

  • Share among multiple packets the un-amortized cost of invoking the rte_eth_tx_one function.
  • Enable the rte_eth_tx_burst function to take advantage of burst-oriented hardware features (prefetch data in cache, use of NIC head/tail registers) to minimize the number of CPU cycles per packet, for example by avoiding unnecessary read memory accesses to ring transmit descriptors, or by systematically using arrays of pointers that exactly fit cache line boundaries and sizes.
  • Apply burst-oriented software optimization techniques to remove operations that would otherwise be unavoidable, such as ring index wrap back management.

Burst-oriented functions are also introduced via the API for services that are intensively used by the PMD. This applies in particular to buffer allocators used to populate NIC rings, which provide functions to allocate/free several buffers at a time. For example, an mbuf_multiple_alloc function returning an array of pointers to rte_mbuf buffers which speeds up the receive poll function of the PMD when replenishing multiple descriptors of the receive ring.

7.3. Logical Cores, Memory and NIC Queues Relationships

The DPDK supports NUMA allowing for better performance when a processor’s logical cores and interfaces utilize its local memory. Therefore, mbuf allocation associated with local PCIe* interfaces should be allocated from memory pools created in the local memory. The buffers should, if possible, remain on the local processor to obtain the best performance results and RX and TX buffer descriptors should be populated with mbufs allocated from a mempool allocated from local memory.

The run-to-completion model also performs better if packet or data manipulation is in local memory instead of a remote processors memory. This is also true for the pipe-line model provided all logical cores used are located on the same processor.

Multiple logical cores should never share receive or transmit queues for interfaces since this would require global locks and hinder performance.

7.4. Device Identification and Configuration

7.4.1. Device Identification

Each NIC port is uniquely designated by its (bus/bridge, device, function) PCI identifiers assigned by the PCI probing/enumeration function executed at DPDK initialization. Based on their PCI identifier, NIC ports are assigned two other identifiers:

  • A port index used to designate the NIC port in all functions exported by the PMD API.
  • A port name used to designate the port in console messages, for administration or debugging purposes. For ease of use, the port name includes the port index.

7.4.2. Device Configuration

The configuration of each NIC port includes the following operations:

  • Allocate PCI resources
  • Reset the hardware (issue a Global Reset) to a well-known default state
  • Set up the PHY and the link
  • Initialize statistics counters

The PMD API must also export functions to start/stop the all-multicast feature of a port and functions to set/unset the port in promiscuous mode.

Some hardware offload features must be individually configured at port initialization through specific configuration parameters. This is the case for the Receive Side Scaling (RSS) and Data Center Bridging (DCB) features for example.

7.4.3. On-the-Fly Configuration

All device features that can be started or stopped “on the fly” (that is, without stopping the device) do not require the PMD API to export dedicated functions for this purpose.

All that is required is the mapping address of the device PCI registers to implement the configuration of these features in specific functions outside of the drivers.

For this purpose, the PMD API exports a function that provides all the information associated with a device that can be used to set up a given device feature outside of the driver. This includes the PCI vendor identifier, the PCI device identifier, the mapping address of the PCI device registers, and the name of the driver.

The main advantage of this approach is that it gives complete freedom on the choice of the API used to configure, to start, and to stop such features.

As an example, refer to the configuration of the IEEE1588 feature for the Intel® 82576 Gigabit Ethernet Controller and the Intel® 82599 10 Gigabit Ethernet Controller controllers in the testpmd application.

Other features such as the L3/L4 5-Tuple packet filtering feature of a port can be configured in the same way. Ethernet* flow control (pause frame) can be configured on the individual port. Refer to the testpmd source code for details. Also, L4 (UDP/TCP/ SCTP) checksum offload by the NIC can be enabled for an individual packet as long as the packet mbuf is set up correctly. See Hardware Offload for details.

7.4.4. Configuration of Transmit Queues

Each transmit queue is independently configured with the following information:

  • The number of descriptors of the transmit ring
  • The socket identifier used to identify the appropriate DMA memory zone from which to allocate the transmit ring in NUMA architectures
  • The values of the Prefetch, Host and Write-Back threshold registers of the transmit queue
  • The minimum transmit packets to free threshold (tx_free_thresh). When the number of descriptors used to transmit packets exceeds this threshold, the network adaptor should be checked to see if it has written back descriptors. A value of 0 can be passed during the TX queue configuration to indicate the default value should be used. The default value for tx_free_thresh is 32. This ensures that the PMD does not search for completed descriptors until at least 32 have been processed by the NIC for this queue.
  • The minimum RS bit threshold. The minimum number of transmit descriptors to use before setting the Report Status (RS) bit in the transmit descriptor. Note that this parameter may only be valid for Intel 10 GbE network adapters. The RS bit is set on the last descriptor used to transmit a packet if the number of descriptors used since the last RS bit setting, up to the first descriptor used to transmit the packet, exceeds the transmit RS bit threshold (tx_rs_thresh). In short, this parameter controls which transmit descriptors are written back to host memory by the network adapter. A value of 0 can be passed during the TX queue configuration to indicate that the default value should be used. The default value for tx_rs_thresh is 32. This ensures that at least 32 descriptors are used before the network adapter writes back the most recently used descriptor. This saves upstream PCIe* bandwidth resulting from TX descriptor write-backs. It is important to note that the TX Write-back threshold (TX wthresh) should be set to 0 when tx_rs_thresh is greater than 1. Refer to the Intel® 82599 10 Gigabit Ethernet Controller Datasheet for more details.

The following constraints must be satisfied for tx_free_thresh and tx_rs_thresh:

  • tx_rs_thresh must be greater than 0.
  • tx_rs_thresh must be less than the size of the ring minus 2.
  • tx_rs_thresh must be less than or equal to tx_free_thresh.
  • tx_free_thresh must be greater than 0.
  • tx_free_thresh must be less than the size of the ring minus 3.
  • For optimal performance, TX wthresh should be set to 0 when tx_rs_thresh is greater than 1.

One descriptor in the TX ring is used as a sentinel to avoid a hardware race condition, hence the maximum threshold constraints.

Note

When configuring for DCB operation, at port initialization, both the number of transmit queues and the number of receive queues must be set to 128.

7.4.5. Free Tx mbuf on Demand

Many of the drivers do not release the mbuf back to the mempool, or local cache, immediately after the packet has been transmitted. Instead, they leave the mbuf in their Tx ring and either perform a bulk release when the tx_rs_thresh has been crossed or free the mbuf when a slot in the Tx ring is needed.

An application can request the driver to release used mbufs with the rte_eth_tx_done_cleanup() API. This API requests the driver to release mbufs that are no longer in use, independent of whether or not the tx_rs_thresh has been crossed. There are two scenarios when an application may want the mbuf released immediately:

  • When a given packet needs to be sent to multiple destination interfaces (either for Layer 2 flooding or Layer 3 multi-cast). One option is to make a copy of the packet or a copy of the header portion that needs to be manipulated. A second option is to transmit the packet and then poll the rte_eth_tx_done_cleanup() API until the reference count on the packet is decremented. Then the same packet can be transmitted to the next destination interface. The application is still responsible for managing any packet manipulations needed between the different destination interfaces, but a packet copy can be avoided. This API is independent of whether the packet was transmitted or dropped, only that the mbuf is no longer in use by the interface.
  • Some applications are designed to make multiple runs, like a packet generator. For performance reasons and consistency between runs, the application may want to reset back to an initial state between each run, where all mbufs are returned to the mempool. In this case, it can call the rte_eth_tx_done_cleanup() API for each destination interface it has been using to request it to release of all its used mbufs.

To determine if a driver supports this API, check for the Free Tx mbuf on demand feature in the Network Interface Controller Drivers document.

7.4.6. Hardware Offload

Depending on driver capabilities advertised by rte_eth_dev_info_get(), the PMD may support hardware offloading feature like checksumming, TCP segmentation or VLAN insertion.

The support of these offload features implies the addition of dedicated status bit(s) and value field(s) into the rte_mbuf data structure, along with their appropriate handling by the receive/transmit functions exported by each PMD. The list of flags and their precise meaning is described in the mbuf API documentation and in the in Mbuf Library, section “Meta Information”.

7.5. Poll Mode Driver API

7.5.1. Generalities

By default, all functions exported by a PMD are lock-free functions that are assumed not to be invoked in parallel on different logical cores to work on the same target object. For instance, a PMD receive function cannot be invoked in parallel on two logical cores to poll the same RX queue of the same port. Of course, this function can be invoked in parallel by different logical cores on different RX queues. It is the responsibility of the upper-level application to enforce this rule.

If needed, parallel accesses by multiple logical cores to shared queues can be explicitly protected by dedicated inline lock-aware functions built on top of their corresponding lock-free functions of the PMD API.

7.5.2. Generic Packet Representation

A packet is represented by an rte_mbuf structure, which is a generic metadata structure containing all necessary housekeeping information. This includes fields and status bits corresponding to offload hardware features, such as checksum computation of IP headers or VLAN tags.

The rte_mbuf data structure includes specific fields to represent, in a generic way, the offload features provided by network controllers. For an input packet, most fields of the rte_mbuf structure are filled in by the PMD receive function with the information contained in the receive descriptor. Conversely, for output packets, most fields of rte_mbuf structures are used by the PMD transmit function to initialize transmit descriptors.

The mbuf structure is fully described in the Mbuf Library chapter.

7.5.3. Ethernet Device API

The Ethernet device API exported by the Ethernet PMDs is described in the DPDK API Reference.

7.5.4. Extended Statistics API

The extended statistics API allows a PMD to expose all statistics that are available to it, including statistics that are unique to the device. Each statistic has three properties name, id and value:

  • name: A human readable string formatted by the scheme detailed below.
  • id: An integer that represents only that statistic.
  • value: A unsigned 64-bit integer that is the value of the statistic.

Note that extended statistic identifiers are driver-specific, and hence might not be the same for different ports. The API consists of various rte_eth_xstats_*() functions, and allows an application to be flexible in how it retrieves statistics.

7.5.4.1. Scheme for Human Readable Names

A naming scheme exists for the strings exposed to clients of the API. This is to allow scraping of the API for statistics of interest. The naming scheme uses strings split by a single underscore _. The scheme is as follows:

  • direction
  • detail 1
  • detail 2
  • detail n
  • unit

Examples of common statistics xstats strings, formatted to comply to the above scheme:

  • rx_bytes
  • rx_crc_errors
  • tx_multicast_packets

The scheme, although quite simple, allows flexibility in presenting and reading information from the statistic strings. The following example illustrates the naming scheme:rx_packets. In this example, the string is split into two components. The first component rx indicates that the statistic is associated with the receive side of the NIC. The second component packets indicates that the unit of measure is packets.

A more complicated example: tx_size_128_to_255_packets. In this example, tx indicates transmission, size is the first detail, 128 etc. are more details, and packets indicates that this is a packet counter.

Some additions in the metadata scheme are as follows:

  • If the first part does not match rx or tx, the statistic does not have an affinity with either receive of transmit.
  • If the first letter of the second part is q and this q is followed by a number, this statistic is part of a specific queue.

An example where queue numbers are used is as follows: tx_q7_bytes which indicates this statistic applies to queue number 7, and represents the number of transmitted bytes on that queue.

7.5.4.2. API Design

The xstats API uses the name, id, and value to allow performant lookup of specific statistics. Performant lookup means two things;

  • No string comparisons with the name of the statistic in fast-path
  • Allow requesting of only the statistics of interest

The API ensures these requirements are met by mapping the name of the statistic to a unique id, which is used as a key for lookup in the fast-path. The API allows applications to request an array of id values, so that the PMD only performs the required calculations. Expected usage is that the application scans the name of each statistic, and caches the id if it has an interest in that statistic. On the fast-path, the integer can be used to retrieve the actual value of the statistic that the id represents.

7.5.4.3. API Functions

The API is built out of a small number of functions, which can be used to retrieve the number of statistics and the names, IDs and values of those statistics.

  • rte_eth_xstats_get_names(): returns the names of the statistics. When given a NULL parameter the function returns the number of statistics that are available.
  • rte_eth_xstats_get_id_by_name(): Searches for the statistic ID that matches xstat_name. If found, the id integer is set.
  • rte_eth_xstats_get(): Fills in an array of uint64_t values with matching the provided ids array. If the ids array is NULL, it returns all statistics that are available.

7.5.4.4. Application Usage

Imagine an application that wants to view the dropped packet count. If no packets are dropped, the application does not read any other metrics for performance reasons. If packets are dropped, the application has a particular set of statistics that it requests. This “set” of statistics allows the app to decide what next steps to perform. The following code-snippets show how the xstats API can be used to achieve this goal.

First step is to get all statistics names and list them:

struct rte_eth_xstat_name *xstats_names;
uint64_t *values;
int len, i;

/* Get number of stats */
len = rte_eth_xstats_get_names(port_id, NULL, NULL, 0);
if (len < 0) {
    printf("Cannot get xstats count\n");
    goto err;
}

xstats_names = malloc(sizeof(struct rte_eth_xstat_name) * len);
if (xstats_names == NULL) {
    printf("Cannot allocate memory for xstat names\n");
    goto err;
}

/* Retrieve xstats names, passing NULL for IDs to return all statistics */
if (len != rte_eth_xstats_get_names(port_id, xstats_names, NULL, len)) {
    printf("Cannot get xstat names\n");
    goto err;
}

values = malloc(sizeof(values) * len);
if (values == NULL) {
    printf("Cannot allocate memory for xstats\n");
    goto err;
}

/* Getting xstats values */
if (len != rte_eth_xstats_get(port_id, NULL, values, len)) {
    printf("Cannot get xstat values\n");
    goto err;
}

/* Print all xstats names and values */
for (i = 0; i < len; i++) {
    printf("%s: %"PRIu64"\n", xstats_names[i].name, values[i]);
}

The application has access to the names of all of the statistics that the PMD exposes. The application can decide which statistics are of interest, cache the ids of those statistics by looking up the name as follows:

uint64_t id;
uint64_t value;
const char *xstat_name = "rx_errors";

if(!rte_eth_xstats_get_id_by_name(port_id, xstat_name, &id)) {
    rte_eth_xstats_get(port_id, &id, &value, 1);
    printf("%s: %"PRIu64"\n", xstat_name, value);
}
else {
    printf("Cannot find xstats with a given name\n");
    goto err;
}

The API provides flexibility to the application so that it can look up multiple statistics using an array containing multiple id numbers. This reduces the function call overhead of retrieving statistics, and makes lookup of multiple statistics simpler for the application.

#define APP_NUM_STATS 4
/* application cached these ids previously; see above */
uint64_t ids_array[APP_NUM_STATS] = {3,4,7,21};
uint64_t value_array[APP_NUM_STATS];

/* Getting multiple xstats values from array of IDs */
rte_eth_xstats_get(port_id, ids_array, value_array, APP_NUM_STATS);

uint32_t i;
for(i = 0; i < APP_NUM_STATS; i++) {
    printf("%d: %"PRIu64"\n", ids_array[i], value_array[i]);
}

This array lookup API for xstats allows the application create multiple “groups” of statistics, and look up the values of those IDs using a single API call. As an end result, the application is able to achieve its goal of monitoring a single statistic (“rx_errors” in this case), and if that shows packets being dropped, it can easily retrieve a “set” of statistics using the IDs array parameter to rte_eth_xstats_get function.