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diff --git a/src/spdk/dpdk/doc/guides/contributing/design.rst b/src/spdk/dpdk/doc/guides/contributing/design.rst new file mode 100644 index 000000000..d3dd694b6 --- /dev/null +++ b/src/spdk/dpdk/doc/guides/contributing/design.rst @@ -0,0 +1,177 @@ +.. SPDX-License-Identifier: BSD-3-Clause + Copyright 2018 The DPDK contributors + +Design +====== + +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: + + .. code-block:: c + + #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. + +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. + +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. + +Library Statistics +------------------ + +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. + + +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: + +.. code-block:: console + + # 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. + + +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. + + +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. + +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. |