DPDK 18.11.0 开发者指南：加速数据包处理

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"DPDK最新开发文档，包括Programmer's Guide Release 18.11.0，提供了DPDK的详细介绍、环境抽象层、服务核心、环形缓冲区和内存池库等内容，旨在帮助开发者理解和优化高性能网络应用程序的性能。" DPDK（Data Plane Development Kit）是一个开放源代码的开发套件，由6WIND和Intel等公司共同开发，专注于在Linux环境下提升数据包处理速度和吞吐量，以提高数据平面应用的效率。它包含了一系列的库和驱动，适用于构建高性能网络服务，如数据中心、网络功能虚拟化(NFV)和云计算环境。 DPDK的核心组件包括环境抽象层(EAL)、核心组件、以太网Poll Mode Driver架构、包转发算法支持以及librte_net等。其中，环境抽象层(EAL)负责处理硬件访问、内存管理和多线程，提供了一个在用户空间运行的执行环境。以太网Poll Mode Driver架构是DPDK的关键部分，通过轮询模式处理网络数据包，避免了中断处理的开销，显著提高了处理速率。环境抽象层还包含了对内存段和内存区域（memzone）的管理，支持多个pthread线程，以及malloc内存分配器的优化。这些特性使DPDK能高效利用系统资源，实现高性能的网络数据处理。服务核心是DPDK中的一个重要概念，它们用于执行特定的服务任务，如初始化、在核心上启用服务以及收集统计信息。环形缓冲区(Ring Library)是DPDK中的一种无锁数据结构，用于在不同进程或线程间高效地传递数据，具有多种应用场景，并且在FreeBSD和Linux上都有实现。内存池库(Mempool Library)则提供了内存对象池管理，支持内存分配和释放的优化，确保内存对齐，同时具有本地缓存功能，用于提高内存对象的获取速度。此外，它还提供了统计信息和处理程序，便于监控和调试。 Mbuf库是DPDK处理网络包的核心组件，设计了用于存储和操作网络数据包的专用缓冲区。它优化了内存布局，提高了数据包处理的效率。 DPDK的这些文档和库详细解释了如何使用DPDK来开发和优化网络应用，提供了丰富的示例和用例，帮助开发者深入理解DPDK的工作原理，从而在实际项目中实现高性能的数据包处理。

Programmer’s Guide, Release 18.11.0

3.1.1 Initialization and Core Launching

Part of the initialization is done by the start function of glibc. A check is also performed at

initialization time to ensure that the micro architecture type chosen in the conﬁg ﬁle is supported

by the CPU. Then, the main() function is called. The core initialization and launch is done

in rte_eal_init() (see the API documentation). It consist of calls to the pthread library (more

speciﬁcally, pthread_self(), pthread_create(), and pthread_setafﬁnity_np()).

Note: Initialization of objects, such as memory zones, rings, memory pools, lpm tables and

hash tables, should be done as part of the overall application initialization on the master lcore.

The creation and initialization functions for these objects are not multi-thread safe. However,

once initialized, the objects themselves can safely be used in multiple threads simultaneously.

3.1.2 Shutdown and Cleanup

During the initialization of EAL resources such as hugepage backed memory can be allocated

by core components. The memory allocated during rte_eal_init() can be released by

calling the rte_eal_cleanup() function. Refer to the API documentation for details.

3.1.3 Multi-process Support

The Linuxapp EAL allows a multi-process as well as a multi-threaded (pthread) deployment

model. See chapter Multi-process Support for more details.

3.1.4 Memory Mapping Discovery and Memory Reservation

The allocation of large contiguous physical memory is done using the hugetlbfs kernel ﬁlesys-

tem. The EAL provides an API to reserve named memory zones in this contiguous memory.

The physical address of the reserved memory for that memory zone is also returned to the

user by the memory zone reservation API.

There are two modes in which DPDK memory subsystem can operate: dynamic mode, and

legacy mode. Both modes are explained below.

Note: Memory reservations done using the APIs provided by rte_malloc are also backed by

pages from the hugetlbfs ﬁlesystem.

• Dynamic memory mode

Currently, this mode is only supported on Linux.

In this mode, usage of hugepages by DPDK application will grow and shrink based on applica-

tion’s requests. Any memory allocation through rte_malloc(), rte_memzone_reserve()

or other methods, can potentially result in more hugepages being reserved from the system.

Similarly, any memory deallocation can potentially result in hugepages being released back to

the system.

Memory allocated in this mode is not guaranteed to be IOVA-contiguous. If large chunks of

IOVA-contiguous are required (with “large” deﬁned as “more than one page”), it is recom-

mended to either use VFIO driver for all physical devices (so that IOVA and VA addresses can

be the same, thereby bypassing physical addresses entirely), or use legacy memory mode.

3.1. EAL in a Linux-userland Execution Environment 8

Programmer’s Guide, Release 18.11.0

For chunks of memory which must be IOVA-contiguous, it is recommended to use

rte_memzone_reserve() function with RTE_MEMZONE_IOVA_CONTIG ﬂag speciﬁed. This

way, memory allocator will ensure that, whatever memory mode is in use, either reserved

memory will satisfy the requirements, or the allocation will fail.

There is no need to preallocate any memory at startup using -m or --socket-mem command-

line parameters, however it is still possible to do so, in which case preallocate memory will be

“pinned” (i.e. will never be released by the application back to the system). It will be possible

to allocate more hugepages, and deallocate those, but any preallocated pages will not be

freed. If neither -m nor --socket-mem were speciﬁed, no memory will be preallocated, and

all memory will be allocated at runtime, as needed.

Another available option to use in dynamic memory mode is --single-file-segments

command-line option. This option will put pages in single ﬁles (per memseg list), as opposed

to creating a ﬁle per page. This is normally not needed, but can be useful for use cases like

userspace vhost, where there is limited number of page ﬁle descriptors that can be passed to

VirtIO.

If the application (or DPDK-internal code, such as device drivers) wishes to receive notiﬁca-

tions about newly allocated memory, it is possible to register for memory event callbacks via

rte_mem_event_callback_register() function. This will call a callback function any

time DPDK’s memory map has changed.

If the application (or DPDK-internal code, such as device drivers) wishes to

be notiﬁed about memory allocations above speciﬁed threshold (and have a

chance to deny them), allocation validator callbacks are also available via

rte_mem_alloc_validator_callback_register() function.

A default validator callback is provided by EAL, which can be enabled with a --socket-limit

command-line option, for a simple way to limit maximum amount of memory that can be used

by DPDK application.

Note: In multiprocess scenario, all related processes (i.e. primary process, and secondary

processes running with the same preﬁx) must be in the same memory modes. That is, if

primary process is run in dynamic memory mode, all of its secondary processes must be run

in the same mode. The same is applicable to --single-file-segments command-line

option - both primary and secondary processes must shared this mode.

• Legacy memory mode

This mode is enabled by specifying --legacy-mem command-line switch to the EAL. This

switch will have no effect on FreeBSD as FreeBSD only supports legacy mode anyway.

This mode mimics historical behavior of EAL. That is, EAL will reserve all memory at startup,

sort all memory into large IOVA-contiguous chunks, and will not allow acquiring or releasing

hugepages from the system at runtime.

If neither -m nor --socket-mem were speciﬁed, the entire available hugepage memory will

be preallocated.

• 32-bit support

Additional restrictions are present when running in 32-bit mode. In dynamic memory mode, by

default maximum of 2 gigabytes of VA space will be preallocated, and all of it will be on master

lcore NUMA node unless --socket-mem ﬂag is used.

3.1. EAL in a Linux-userland Execution Environment 10

Programmer’s Guide, Release 18.11.0

In legacy mode, VA space will only be preallocated for segments that were requested (plus

padding, to keep IOVA-contiguousness).

• Maximum amount of memory

All possible virtual memory space that can ever be used for hugepage mapping in a DPDK

process is preallocated at startup, thereby placing an upper limit on how much memory a

DPDK application can have. DPDK memory is stored in segment lists, each segment is strictly

one physical page. It is possible to change the amount of virtual memory being preallocated at

startup by editing the following conﬁg variables:

• CONFIG_RTE_MAX_MEMSEG_LISTS controls how many segment lists can DPDK have

• CONFIG_RTE_MAX_MEM_MB_PER_LIST controls how much megabytes of memory each

segment list can address

• CONFIG_RTE_MAX_MEMSEG_PER_LIST controls how many segments each segment

can have

• CONFIG_RTE_MAX_MEMSEG_PER_TYPE controls how many segments each memory

type can have (where “type” is deﬁned as “page size + NUMA node” combination)

• CONFIG_RTE_MAX_MEM_MB_PER_TYPE controls how much megabytes of memory each

memory type can address

• CONFIG_RTE_MAX_MEM_MB places a global maximum on the amount of memory DPDK

can reserve

Normally, these options do not need to be changed.

Note: Preallocated virtual memory is not to be confused with preallocated hugepage memory!

All DPDK processes preallocate virtual memory at startup. Hugepages can later be mapped

into that preallocated VA space (if dynamic memory mode is enabled), and can optionally be

mapped into it at startup.

3.1.5 Support for Externally Allocated Memory

It is possible to use externally allocated memory in DPDK, using a set of malloc heap API’s.

Support for externally allocated memory is implemented through overloading the socket ID -

externally allocated heaps will have socket ID’s that would be considered invalid under normal

circumstances. Requesting an allocation to take place from a speciﬁed externally allocated

memory is a matter of supplying the correct socket ID to DPDK allocator, either directly (e.g.

through a call to rte_malloc) or indirectly (through data structure-speciﬁc allocation API’s

such as rte_ring_create).

Since there is no way DPDK can verify whether memory are is available or valid, this re-

sponsibility falls on the shoulders of the user. All multiprocess synchronization is also user’s

responsibility, as well as ensuring that all calls to add/attach/detach/remove memory are done

in the correct order. It is not required to attach to a memory area in all processes - only attach

to memory areas as needed.

The expected workﬂow is as follows:

• Get a pointer to memory area

• Create a named heap

3.1. EAL in a Linux-userland Execution Environment 11

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