From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id B04F043C3B; Thu, 29 Feb 2024 17:04:02 +0100 (CET) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id C219842E5C; Thu, 29 Feb 2024 17:03:45 +0100 (CET) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by mails.dpdk.org (Postfix) with ESMTP id 9655C42D78 for ; Thu, 29 Feb 2024 17:03:39 +0100 (CET) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C1CC61516; Thu, 29 Feb 2024 08:04:17 -0800 (PST) Received: from ampere-altra-2-1.usa.Arm.com (ampere-altra-2-1.usa.arm.com [10.118.91.158]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id DC89C3F6C4; Thu, 29 Feb 2024 08:03:38 -0800 (PST) From: Paul Szczepanek To: dev@dpdk.org Cc: Paul Szczepanek , Honnappa Nagarahalli Subject: [PATCH v6 3/4] docs: add pointer compression to the EAL guide Date: Thu, 29 Feb 2024 16:03:07 +0000 Message-Id: <20240229160308.365277-4-paul.szczepanek@arm.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20240229160308.365277-1-paul.szczepanek@arm.com> References: <20230927150854.3670391-2-paul.szczepanek@arm.com> <20240229160308.365277-1-paul.szczepanek@arm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Documentation added in the EAL guide for the new utility functions for pointer compression showing example code and potential usecases. Signed-off-by: Paul Szczepanek Reviewed-by: Honnappa Nagarahalli --- .../prog_guide/env_abstraction_layer.rst | 142 ++++++++++++++++++ 1 file changed, 142 insertions(+) diff --git a/doc/guides/prog_guide/env_abstraction_layer.rst b/doc/guides/prog_guide/env_abstraction_layer.rst index 6debf54efb..f04d032442 100644 --- a/doc/guides/prog_guide/env_abstraction_layer.rst +++ b/doc/guides/prog_guide/env_abstraction_layer.rst @@ -1192,3 +1192,145 @@ will not be deallocated. Any successful deallocation event will trigger a callback, for which user applications and other DPDK subsystems can register. + +.. _pointer_compression: + +Pointer Compression +------------------- + +Use ``rte_ptr_compress_16()`` and ``rte_ptr_decompress_16()`` to compress and +decompress pointers into 16-bit offsets. Use ``rte_ptr_compress_32()`` and +``rte_ptr_decompress_32()`` to compress and decompress pointers into 32-bit +offsets. + +Compression takes advantage of the fact that pointers are usually located in a +limited memory region (like a mempool). By converting them to offsets from a +base memory address they can be stored in fewer bytes. How many bytes are needed +to store the offset is dictated by the memory region size and alignment of +objects the pointers point to. + +For example, a pointer which is part of a 4GB memory pool can be stored as 32 +bit offset. If the pointer points to memory that is 8 bytes aligned then 3 bits +can be dropped from the offset and a 32GB memory pool can now fit in 32 bits. + +For performance reasons these requirements are not enforced programmatically. +The programmer is responsible for ensuring that the combination of distance +from the base pointer and memory alignment allow for storing of the offset in +the number of bits indicated by the function name (16 or 32). Start of mempool +memory would be a good candidate for the base pointer. Otherwise any pointer +that precedes all pointers, is close enough and has the same alignment as the +pointers being compressed will work. + +.. note:: + + Performance gains depend on the batch size of pointers and CPU capabilities + such as vector extensions. It's important to measure the performance + increase on target hardware. A test called ``ring_perf_autotest`` in + ``dpdk-test`` can provide the measurements. + +Example usage +~~~~~~~~~~~~~ + +In this example we send pointers between two cores through a ring. While this +is a realistic use case the code is simplified for demonstration purposes and +does not have error handling. + +.. code-block:: c + + #include + #include + #include + #include + + #define ITEMS_ARRAY_SIZE (1024) + #define BATCH_SIZE (128) + #define ALIGN_EXPONENT (3) + #define ITEM_ALIGN (1<