8. Enabling Additional Functionality

8.1. High Precision Event Timer (HPET) Functionality

8.1.1. BIOS Support

The High Precision Timer (HPET) must be enabled in the platform BIOS if the HPET is to be used. Otherwise, the Time Stamp Counter (TSC) is used by default. The BIOS is typically accessed by pressing F2 while the platform is starting up. The user can then navigate to the HPET option. On the Crystal Forest platform BIOS, the path is: Advanced -> PCH-IO Configuration -> High Precision Timer -> (Change from Disabled to Enabled if necessary).

On a system that has already booted, the following command can be issued to check if HPET is enabled:

grep hpet /proc/timer_list

If no entries are returned, HPET must be enabled in the BIOS (as per the instructions above) and the system rebooted.

8.1.2. Linux Kernel Support

The DPDK makes use of the platform HPET timer by mapping the timer counter into the process address space, and as such, requires that the HPET_MMAP kernel configuration option be enabled.

Warning

On Fedora, and other common distributions such as Ubuntu, the HPET_MMAP kernel option is not enabled by default. To recompile the Linux kernel with this option enabled, please consult the distributions documentation for the relevant instructions.

8.1.3. Enabling HPET in the DPDK

By default, HPET support is disabled in the DPDK build configuration files. To use HPET, use the following meson build option which will enable the HPET settings at compile time:

meson configure -Duse_hpet=true

For an application to use the rte_get_hpet_cycles() and rte_get_hpet_hz() API calls, and optionally to make the HPET the default time source for the rte_timer library, the new rte_eal_hpet_init() API call should be called at application initialization. This API call will ensure that the HPET is accessible, returning an error to the application if it is not, for example, if HPET_MMAP is not enabled in the kernel. The application can then determine what action to take, if any, if the HPET is not available at run-time.

Note

For applications that require timing APIs, but not the HPET timer specifically, it is recommended that the rte_get_timer_cycles() and rte_get_timer_hz() API calls be used instead of the HPET-specific APIs. These generic APIs can work with either TSC or HPET time sources, depending on what is requested by an application call to rte_eal_hpet_init(), if any, and on what is available on the system at runtime.

8.2. Running DPDK Applications Without Root Privileges

In order to run DPDK as non-root, the following Linux filesystem objects’ permissions should be adjusted to ensure that the Linux account being used to run the DPDK application has access to them:

  • All directories which serve as hugepage mount points, for example, /dev/hugepages
  • If the HPET is to be used, /dev/hpet

When running as non-root user, there may be some additional resource limits that are imposed by the system. Specifically, the following resource limits may need to be adjusted in order to ensure normal DPDK operation:

  • RLIMIT_LOCKS (number of file locks that can be held by a process)
  • RLIMIT_NOFILE (number of open file descriptors that can be held open by a process)
  • RLIMIT_MEMLOCK (amount of pinned pages the process is allowed to have)

The above limits can usually be adjusted by editing /etc/security/limits.conf file, and rebooting.

Additionally, depending on which kernel driver is in use, the relevant resources also should be accessible by the user running the DPDK application.

For vfio-pci kernel driver, the following Linux file system objects’ permissions should be adjusted:

  • The VFIO device file, /dev/vfio/vfio
  • The directories under /dev/vfio that correspond to IOMMU group numbers of devices intended to be used by DPDK, for example, /dev/vfio/50

Note

The instructions below will allow running DPDK with igb_uio or uio_pci_generic drivers as non-root with older Linux kernel versions. However, since version 4.0, the kernel does not allow unprivileged processes to read the physical address information from the pagemaps file, making it impossible for those processes to be used by non-privileged users. In such cases, using the VFIO driver is recommended.

For igb_uio or uio_pci_generic kernel drivers, the following Linux file system objects’ permissions should be adjusted:

  • The userspace-io device files in /dev, for example, /dev/uio0, /dev/uio1, and so on

  • The userspace-io sysfs config and resource files, for example for uio0:

    /sys/class/uio/uio0/device/config
    /sys/class/uio/uio0/device/resource*
    

8.3. Power Management and Power Saving Functionality

Enhanced Intel SpeedStep® Technology must be enabled in the platform BIOS if the power management feature of DPDK is to be used. Otherwise, the sys file folder /sys/devices/system/cpu/cpu0/cpufreq will not exist, and the CPU frequency- based power management cannot be used. Consult the relevant BIOS documentation to determine how these settings can be accessed.

For example, on some Intel reference platform BIOS variants, the path to Enhanced Intel SpeedStep® Technology is:

Advanced
  -> Processor Configuration
  -> Enhanced Intel SpeedStep\ |reg| Tech

In addition, C3 and C6 should be enabled as well for power management. The path of C3 and C6 on the same platform BIOS is:

Advanced
  -> Processor Configuration
  -> Processor C3 Advanced
  -> Processor Configuration
  -> Processor C6

8.4. Using Linux Core Isolation to Reduce Context Switches

While the threads used by a DPDK application are pinned to logical cores on the system, it is possible for the Linux scheduler to run other tasks on those cores also. To help prevent additional workloads from running on those cores, it is possible to use the isolcpus Linux kernel parameter to isolate them from the general Linux scheduler.

For example, if DPDK applications are to run on logical cores 2, 4 and 6, the following should be added to the kernel parameter list:

isolcpus=2,4,6

8.5. Loading the DPDK KNI Kernel Module

To run the DPDK Kernel NIC Interface (KNI) sample application, an extra kernel module (the kni module) must be loaded into the running kernel. The module is found in the kernel/linux sub-directory of the DPDK build directory. It should be loaded using the insmod command:

insmod <build_dir>/kernel/linux/kni/rte_kni.ko

Note

See the “Kernel NIC Interface Sample Application” chapter in the DPDK Sample Applications User Guide for more details.

8.6. Using Linux IOMMU Pass-Through to Run DPDK with Intel® VT-d

To enable Intel® VT-d in a Linux kernel, a number of kernel configuration options must be set. These include:

  • IOMMU_SUPPORT
  • IOMMU_API
  • INTEL_IOMMU

In addition, to run the DPDK with Intel® VT-d, the iommu=pt kernel parameter must be used when using igb_uio driver. This results in pass-through of the DMAR (DMA Remapping) lookup in the host. Also, if INTEL_IOMMU_DEFAULT_ON is not set in the kernel, the intel_iommu=on kernel parameter must be used too. This ensures that the Intel IOMMU is being initialized as expected.

Please note that while using iommu=pt is compulsory for igb_uio driver, the vfio-pci driver can actually work with both iommu=pt and iommu=on.