This patch adds documentation on the usage of the max SIMD bitwidth EAL setting, and how to use it to enable AVX-512 at runtime.
Cc: Anatoly Burakov <anatoly.bura...@intel.com> Cc: John McNamara <john.mcnam...@intel.com> Cc: Marko Kovacevic <marko.kovace...@intel.com> Signed-off-by: Ciara Power <ciara.po...@intel.com> Acked-by: Kevin Laatz <kevin.la...@intel.com> --- v6: Updated enum value. v4: Updated docs to reflect renamed enum. v3: - Added enum value for disabling use of max SIMD to doc. - Added entry to HowTo index. --- doc/guides/howto/avx512.rst | 36 +++++++++++++++++++ doc/guides/howto/index.rst | 1 + doc/guides/linux_gsg/eal_args.include.rst | 16 +++++++++ .../prog_guide/env_abstraction_layer.rst | 32 +++++++++++++++++ 4 files changed, 85 insertions(+) create mode 100644 doc/guides/howto/avx512.rst diff --git a/doc/guides/howto/avx512.rst b/doc/guides/howto/avx512.rst new file mode 100644 index 0000000000..6eb3755775 --- /dev/null +++ b/doc/guides/howto/avx512.rst @@ -0,0 +1,36 @@ +.. SPDX-License-Identifier: BSD-3-Clause + Copyright(c) 2020 Intel Corporation. + + +Using AVX-512 with DPDK +======================= + +AVX-512 is not used by default in DPDK, but it can be selected at runtime by apps through the use of EAL API, +and by the user with a commandline argument. DPDK has a setting for max SIMD bitwidth, +which can be modified and will then limit the vector path taken by the code. + + +Using the API in apps +--------------------- + +Apps can request DPDK uses AVX-512 at runtime, if it provides improved application performance. +This can be done by modifying the EAL setting for max SIMD bitwidth to 512, as by default it is 256, +which does not allow for AVX-512. + +.. code-block:: c + + rte_set_max_simd_bitwidth(RTE_SIMD_512); + +This API should only be called once at initialization, before EAL init. +For more information on the possible enum values to use as a parameter, go to :ref:`max_simd_bitwidth`: + + +Using the command-line argument +--------------------------------------------- + +The user can select to use AVX-512 at runtime, using the following argument to set the max bitwidth:: + + ./app/dpdk-testpmd --force-max-simd-bitwidth=512 + +This will override any further changes to the max SIMD bitwidth in DPDK, +which is useful for testing purposes. diff --git a/doc/guides/howto/index.rst b/doc/guides/howto/index.rst index 5a97ea508c..c2a2c60ddb 100644 --- a/doc/guides/howto/index.rst +++ b/doc/guides/howto/index.rst @@ -20,3 +20,4 @@ HowTo Guides telemetry debug_troubleshoot openwrt + avx512 diff --git a/doc/guides/linux_gsg/eal_args.include.rst b/doc/guides/linux_gsg/eal_args.include.rst index 0fe4457968..a0bfbd1a98 100644 --- a/doc/guides/linux_gsg/eal_args.include.rst +++ b/doc/guides/linux_gsg/eal_args.include.rst @@ -210,3 +210,19 @@ Other options * ``--no-telemetry``: Disable telemetry. + +* ``--force-max-simd-bitwidth=<val>``: + + Specify the maximum SIMD bitwidth size to handle. This limits which vector paths, + if any, are taken, as any paths taken must use a bitwidth below the max bitwidth limit. + For example, to allow all SIMD bitwidths up to and including AVX-512:: + + --force-max-simd-bitwidth=512 + + The following example shows limiting the bitwidth to 64-bits to disable all vector code:: + + --force-max-simd-bitwidth=64 + + To disable use of max SIMD bitwidth limit:: + + --force-max-simd-bitwidth=0 diff --git a/doc/guides/prog_guide/env_abstraction_layer.rst b/doc/guides/prog_guide/env_abstraction_layer.rst index 936c885081..90e43d2c1f 100644 --- a/doc/guides/prog_guide/env_abstraction_layer.rst +++ b/doc/guides/prog_guide/env_abstraction_layer.rst @@ -486,6 +486,38 @@ the desired addressing mode when virtual devices that are not directly attached To facilitate forcing the IOVA mode to a specific value the EAL command line option ``--iova-mode`` can be used to select either physical addressing('pa') or virtual addressing('va'). +.. _max_simd_bitwidth: + + +Max SIMD bitwidth +~~~~~~~~~~~~~~~~~ + +The EAL provides a single setting to limit the max SIMD bitwidth used by DPDK, +which is used in determining the vector path, if any, chosen by a component. +The value can be set at runtime by an application using the 'rte_set_max_simd_bitwidth(uint16_t bitwidth)' function, +which should only be called once at initialization, before EAL init. +The value can be overridden by the user using the EAL command-line option '--force-max-simd-bitwidth'. + +When choosing a vector path, along with checking the CPU feature support, +the value of the max SIMD bitwidth must also be checked, and can be retrieved using the 'rte_get_max_simd_bitwidth()' function. +The value should be compared against the enum values for accepted max SIMD bitwidths: + +.. code-block:: c + + enum rte_max_simd { + RTE_SIMD_DISABLED = 64, + RTE_SIMD_128 = 128, + RTE_SIMD_256 = 256, + RTE_SIMD_512 = 512, + RTE_SIMD_MAX = INT16_MAX + 1, + }; + + if (rte_get_max_simd_bitwidth() >= RTE_SIMD_512) + /* Take AVX-512 vector path */ + else if (rte_get_max_simd_bitwidth() >= RTE_SIMD_256) + /* Take AVX2 vector path */ + + Memory Segments and Memory Zones (memzone) ------------------------------------------ -- 2.22.0