2. Design

2.1. Environment or Architecture-specific Sources

In DPDK and DPDK applications, some code is specific to an architecture (i686, x86_64) or to an executive environment (freebsd or linux) and so on. As far as is possible, all such instances of architecture or env-specific code should be provided via standard APIs in the EAL.

By convention, a file is common if it is not located in a directory indicating that it is specific. For instance, a file located in a subdir of “x86_64” directory is specific to this architecture. A file located in a subdir of “linux” is specific to this execution environment.

Note

Code in DPDK libraries and applications should be generic. The correct location for architecture or executive environment specific code is in the EAL.

When absolutely necessary, there are several ways to handle specific code:

  • Use a #ifdef with the CONFIG option in the C code. This can be done when the differences are small and they can be embedded in the same C file:

    #ifdef RTE_ARCH_I686
toto();
#else
titi();
#endif

  • Use the CONFIG option in the Makefile. This is done when the differences are more significant. In this case, the code is split into two separate files that are architecture or environment specific. This should only apply inside the EAL library.

Note

As in the linux kernel, the CONFIG_ prefix is not used in C code. This is only needed in Makefiles or shell scripts.

2.1.1. Per Architecture Sources

The following config options can be used:

  • CONFIG_RTE_ARCH is a string that contains the name of the architecture.
  • CONFIG_RTE_ARCH_I686, CONFIG_RTE_ARCH_X86_64, CONFIG_RTE_ARCH_X86_64_32 or CONFIG_RTE_ARCH_PPC_64 are defined only if we are building for those architectures.

2.1.2. Per Execution Environment Sources

The following config options can be used:

  • CONFIG_RTE_EXEC_ENV is a string that contains the name of the executive environment.
  • CONFIG_RTE_EXEC_ENV_FREEBSD or CONFIG_RTE_EXEC_ENV_LINUX are defined only if we are building for this execution environment.

2.2. Mbuf features

The rte_mbuf structure must be kept small (128 bytes).

In order to add new features without wasting buffer space for unused features, some fields and flags can be registered dynamically in a shared area. The “dynamic” mbuf area is the default choice for the new features.

The “dynamic” area is eating the remaining space in mbuf, and some existing “static” fields may need to become “dynamic”.

Adding a new static field or flag must be an exception matching many criteria like (non exhaustive): wide usage, performance, size.

2.3. Library Statistics

2.3.1. Description

This document describes the guidelines for DPDK library-level statistics counter support. This includes guidelines for turning library statistics on and off and requirements for preventing ABI changes when implementing statistics.

2.3.2. Mechanism to allow the application to turn library statistics on and off

Each library that maintains statistics counters should provide a single build time flag that decides whether the statistics counter collection is enabled or not. This flag should be exposed as a variable within the DPDK configuration file. When this flag is set, all the counters supported by current library are collected for all the instances of every object type provided by the library. When this flag is cleared, none of the counters supported by the current library are collected for any instance of any object type provided by the library:

# DPDK file config/common_linux, config/common_freebsd, etc.
CONFIG_RTE_<LIBRARY_NAME>_STATS_COLLECT=y/n

The default value for this DPDK configuration file variable (either “yes” or “no”) is decided by each library.

2.3.3. Prevention of ABI changes due to library statistics support

The layout of data structures and prototype of functions that are part of the library API should not be affected by whether the collection of statistics counters is turned on or off for the current library. In practical terms, this means that space should always be allocated in the API data structures for statistics counters and the statistics related API functions are always built into the code, regardless of whether the statistics counter collection is turned on or off for the current library.

When the collection of statistics counters for the current library is turned off, the counters retrieved through the statistics related API functions should have a default value of zero.

2.3.4. Motivation to allow the application to turn library statistics on and off

It is highly recommended that each library provides statistics counters to allow an application to monitor the library-level run-time events. Typical counters are: number of packets received/dropped/transmitted, number of buffers allocated/freed, number of occurrences for specific events, etc.

However, the resources consumed for library-level statistics counter collection have to be spent out of the application budget and the counters collected by some libraries might not be relevant to the current application. In order to avoid any unwanted waste of resources and/or performance impacts, the application should decide at build time whether the collection of library-level statistics counters should be turned on or off for each library individually.

Library-level statistics counters can be relevant or not for specific applications:

  • For Application A, counters maintained by Library X are always relevant and the application needs to use them to implement certain features, such as traffic accounting, logging, application-level statistics, etc. In this case, the application requires that collection of statistics counters for Library X is always turned on.
  • For Application B, counters maintained by Library X are only useful during the application debug stage and are not relevant once debug phase is over. In this case, the application may decide to turn on the collection of Library X statistics counters during the debug phase and at a later stage turn them off.
  • For Application C, counters maintained by Library X are not relevant at all. It might be that the application maintains its own set of statistics counters that monitor a different set of run-time events (e.g. number of connection requests, number of active users, etc). It might also be that the application uses multiple libraries (Library X, Library Y, etc) and it is interested in the statistics counters of Library Y, but not in those of Library X. In this case, the application may decide to turn the collection of statistics counters off for Library X and on for Library Y.

The statistics collection consumes a certain amount of CPU resources (cycles, cache bandwidth, memory bandwidth, etc) that depends on:

  • Number of libraries used by the current application that have statistics counters collection turned on.
  • Number of statistics counters maintained by each library per object type instance (e.g. per port, table, pipeline, thread, etc).
  • Number of instances created for each object type supported by each library.
  • Complexity of the statistics logic collection for each counter: when only some occurrences of a specific event are valid, additional logic is typically needed to decide whether the current occurrence of the event should be counted or not. For example, in the event of packet reception, when only TCP packets with destination port within a certain range should be recorded, conditional branches are usually required. When processing a burst of packets that have been validated for header integrity, counting the number of bits set in a bitmask might be needed.

2.4. PF and VF Considerations

The primary goal of DPDK is to provide a userspace dataplane. Managing VFs from a PF driver is a control plane feature and developers should generally rely on the Linux Kernel for that.

Developers should work with the Linux Kernel community to get the required functionality upstream. PF functionality should only be added to DPDK for testing and prototyping purposes while the kernel work is ongoing. It should also be marked with an “EXPERIMENTAL” tag. If the functionality isn’t upstreamable then a case can be made to maintain the PF functionality in DPDK without the EXPERIMENTAL tag.