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DPDK, Data Plane Development Kit

# DPDK Overview

Data Plane Development Kit (DPDK) is a set of libraries that allows network interface controller (NIC) drivers to use user space memory buffers to send and receive data over a network. These libraries underpin one of the network transport options in UHD.

In UHD, the DPDK-based transport will fork off I/O threads that implement the network services, and these I/O threads will service the NICs on cores provided in your configuration. The cores will be completely consumed by the I/O thread. Together with DPDK's polling-mode drivers, this virtually eliminates context switching in UHD's transport layer, which enables us to stream higher sample rates.

# DPDK Setup

DPDK is currently only available on Linux platforms, requires an input-output memory management unit (IOMMU), and must be run on a multicore processor. The following subsections will talk through the steps required to setup DPDK on your computer.

## DPDK Installation Instructions

As a new and developing technology, the DPDK APIs are unstable. UHD requires version 18.11(deprecated), 19.11, 20.11 or 21.11. Adjacent non-LTS releases of DPDK may work, but are not recommended.

On Ubuntu 20.04, Fedora 33-36, or Debian Bullseye/Buster (via backports), DPDK is available in your distribution's repositories. For example, on Debian systems, it can be obtained with the following command:

    sudo apt install dpdk dpdk-dev

Otherwise, you'll need to follow the build guide at https://doc.dpdk.org/guides-21.11/linux_gsg/build_dpdk.html . The software releases can be found at https://core.dpdk.org/download/. Version >=20.11 builds with appropriate options when using the default build procedure.

Note that if you are using a Mellanox NIC, the MLNX-OFED drivers available from https://www.mellanox.com/products/infiniband-drivers/linux/mlnx_ofed may be needed to support dpdk.

Note that if you are building and installing older versions of DPDK from source, you will need to change its configuration to build shared libraries in order for UHD to link successfully. After running make config or make defconfig, open the build/.config file in an editor and find the following line:

    CONFIG_RTE_BUILD_SHARED_LIB=n

Change the n to a y to enable the building of shared libraries, then type make to start the build.

If you are using a Mellanox ConnectX 4 or later NIC, you also need to update the following line:

    CONFIG_RTE_LIBRTE_MLX5_PMD=n

## System Configuration

The official documentation regarding system configuration can be found at https://doc.dpdk.org/guides-21.11/linux_gsg/sys_reqs.html.

First, you'll need to enable the IOMMU and set up some hugepages. DPDK will completely take over all available hugepages, so don't allocate all your memory to them- the rest of UHD and the application need memory too.

For example, on a system with 16 GB of RAM, a generous appropriation of 512x 2 MiB pages was more than sufficient, and you likely won't need that much.

For best results, hugepages should be enabled at boot. For example, using an Intel IOMMU with Ubuntu 19.04 IOMMU drivers, the following line was needed in our Grub config.

    iommu=pt intel_iommu=on hugepages=2048

The setup of the IOMMU and hugepages is system-specific, so consult the kernel documentation for more info. After you reboot, you should see /sys/kernel/iommu_groups populated.

Next, many of the NIC drivers are implemented atop vfio-pci, so you'll need to load that driver with the following command:

    modprobe vfio-pci

Mellanox ConnectX 4 and later NICs do not use the vfio-pci driver, so this step is not necessary for them. They will use the standard driver in conjunction with libibverbs.

For NICs that require vfio-pci (like Intel's X520), you'll want to use the dpdk-devbind.py script to the vfio-pci driver. This script is shipped with DPDK and installed to $prefix/share/dpdk/usertools. If the NIC uses the vfio-pci driver, and the package was installed apt-get, then a typical invocation might be

    /usr/share/dpdk/usertools/dpdk-devbind.py --bind=vfio-pci ens6f0

If successful, the script might provide an updated status like this:

    /usr/share/dpdk/usertools/dpdk-devbind.py -s

    Network devices using DPDK-compatible driver
    ============================================
    0000:02:00.0 '82599ES 10-Gigabit SFI/SFP+ Network Connection 10fb' drv=vfio-pci unused=ixgbe
    [...]

See https://doc.dpdk.org/guides-21.11/linux_gsg/linux_drivers.html#binding-and-unbinding-network-ports-to-from-the-kernel-modules for more details.

With the hugepages, IOMMU, and drivers set up, the system is ready for DPDK to use.

## NIC Configuration

Configuration of the NIC can be controlled via device arguments via the usual methods, but the UHD configuration file is the recommended location.

In order to run, you'll need to set the permissions for your user to take over the vfio-pci devices, the hugepages, and the scheduler's settings for the threads (at a minimum). You may consider running you applications as root, at least while becoming familiar with DPDK. If you use a per-user config file, make sure it's in the correct location.

The config file will have 2 different components. First are the global DPDK options:

    ;When present in device args, use_dpdk indicates you want DPDK to take over the UDP transports
    ;The value here represents a config, so you could have another section labeled use_dpdk=myconf
    ;instead and swap between them
    [use_dpdk=1]
    ;dpdk_mtu is the NIC's MTU setting
    ;This is separate from MPM's maximum packet size
    dpdk_mtu=9000
    ;dpdk_driver is the -d flag for the DPDK EAL. If DPDK doesn't pick up the driver for your NIC
    ;automatically, you may need this argument to point it to the folder where it can find the drivers
    ;Note that DPDK will attempt to load _everything_ in that folder as a driver, so you may want to
    ;create a separate folder with symlinks to the librte_pmd_* and librte_mempool_* libraries.
    dpdk_driver=/usr/local/lib/x86_64-linux-gnu/dpdk/pmds-21.0/
    ;dpdk_corelist is the -l flag for the DPDK EAL. See more at the link
    ; https://doc.dpdk.org/guides-21.11/linux_gsg/build_sample_apps.html#running-a-sample-application
    ;Note if you use multiple SFP ports in a streaming application simultaneously,
    ;you can specify multiple cores in the core list (e.g. 0, 1, 2) and then assign
    ;them each to the separate SFP port/NIC.
    dpdk_corelist=0,1
    ;dpdk_num_mbufs is the total number of packet buffers allocated
    ;to each direction's packet buffer pool
    ;This will be multiplied by the number of NICs, but NICs on the same
    ;CPU socket share a pool. When using Mellanox NICs, this value must be greater
    ;than the dpdk_num_desc value in the next section.
    dpdk_num_mbufs=4096
    ;dpdk_mbuf_cache_size is the number of buffers to cache for a CPU
    ;The cache reduces the interaction with the global pool
    dpdk_mbuf_cache_size=64

The other sections fall under per-NIC arguments. The key for NICs is the MAC address, and it must be in a particular format. Hex digits must all be lower case, and octets must be separated by colons. Here is an example:

    [dpdk_mac=3c:fd:fe:a2:a9:09]
    ;dpdk_lcore selects the lcore that this NIC's driver will run on
    ;Multiple NICs may occupy one lcore, but the I/O thread will completely
    ;consume that lcore's CPU. When using dual SFP configuration, using a
    ;different dpdk_lcore value for each SFP connection is recommended and
    ;will result in better streaming performance. Also, 0 is reserved for
    ;the master thread (i.e.the initial UHD thread that calls init() for DPDK).
    ;Attempting to use it as an I/O thread will only result in hanging.
    ;Note also that by default, the lcore ID will be the same as the CPU ID.
    dpdk_lcore = 1
    ;dpdk_ipv4 specifies the IPv4 address, and both the address and
    ;subnet mask are required (and in this format!). DPDK uses the
    ;netmask to create a basic routing table. Routing to other networks
    ;(i.e. via gateways) is not permitted.
    dpdk_ipv4 = 192.168.10.1/24
    ;dpdk_num_desc is the number of descriptors in each DMA ring.
    ;Must be a power of 2.
    dpdk_num_desc=4096

    [dpdk_mac=3c:fd:fe:a2:a9:0a]
    ;Using a separate dpdk_lcore value for each SFP connection/MAC entry
    ;can possibly result in improved streaming performance. E.g. dpdk_lcore = 2.
    dpdk_lcore = 1
    dpdk_ipv4 = 192.168.20.1/24

# Using DPDK in UHD

Once DPDK is installed and configured on your system, it can be used with UHD. The following steps will describe how to stream using DPDK. DPDK is currently only available on the following devices:

- E320 (but not E31x) device
- N3xx devices
- X3xx devices
- X4xx devices

## Enabling DPDK with UHD Device Args

Add the following to your device args in order to indicate that a DPDK-based UDP transport shall be used instead of the kernel's UDP stack.

    --args="use_dpdk=1"

Device discovery via DPDK is not currently implemented, so the device args mgmt_addr, addr, and second_addr (if applicable) must all be specified at runtime. There is no mechanism for MPM's TCP/IP control traffic to flow over a link that is occupied by DPDK, so mgmt_addr must point to a link that is not used for CHDR, such as N310's RJ45 port.

## DPDK Link Detection

When DPDK is enabled and the driver is initializing, the status of all DPDK-enabled links is checked to verify that all links are active before the driver proceeds. The links are checked every 250 ms until all links have reported that they are up or until the link timeout expires, which by default is 1000 ms. If any of the links have not reported as being up by the time the timeout expires, a runtime error is thrown.

Users may choose to override the link timeout in cases where particular systems and/or network cards take longer to establish stable DPDK links. The timeout can be overridden by passing dpdk_link_timeout=N in the device arguments, where N is the desired timeout time in milliseconds, or by adding a dpdk_link_timeout entry to the UHD configuration file.

## Troubleshooting

With Linux kernels 5.10 and beyond, we have observed periodic underruns on systems that otherwise have no issues. These Linux kernel versions are the default for Ubuntu 20.04.3 LTS and later. The underrun issue is due to the RT_RUNTIME_SHARE feature being disabled by default in newer versions of the Linux kernel. The following procedure can be used to enable RT_RUNTIME_SHARE. Note, this process was tested on Linux kernel version 5.13. The procedure may be slightly different on other kernel versions. To determine the Linux kernel version of your system, in a terminal issue the command uname -r.

    sudo -s

    cd /sys/kernel/debug/sched/

    cat features
        GENTLE_FAIR_SLEEPERS START_DEBIT NO_NEXT_BUDDY LAST_BUDDY CACHE_HOT_BUDDY WAKEUP_PREEMPTION NO_HRTICK NO_HRTICK_DL NO_DOUBLE_TICK NONTASK_CAPACITY TTWU_QUEUE SIS_PROP NO_WARN_DOUBLE_CLOCK RT_PUSH_IPI **NO_RT_RUNTIME_SHARE** NO_LB_MIN ATTACH_AGE_LOAD WA_IDLE WA_WEIGHT WA_BIAS UTIL_EST UTIL_EST_FASTUP NO_LATENCY_WARN ALT_PERIOD BASE_SLICE

    echo RT_RUNTIME_SHARE > features

    cat features
        GENTLE_FAIR_SLEEPERS START_DEBIT NO_NEXT_BUDDY LAST_BUDDY CACHE_HOT_BUDDY WAKEUP_PREEMPTION NO_HRTICK NO_HRTICK_DL NO_DOUBLE_TICK NONTASK_CAPACITY TTWU_QUEUE SIS_PROP NO_WARN_DOUBLE_CLOCK RT_PUSH_IPI **RT_RUNTIME_SHARE** NO_LB_MIN ATTACH_AGE_LOAD WA_IDLE WA_WEIGHT WA_BIAS UTIL_EST UTIL_EST_FASTUP NO_LATENCY_WARN ALT_PERIOD BASE_SLICE