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-:orphan:
-
-.. _what-is-vector-packet-processing:
-
-=================================
-What is vector packet processing?
-=================================
-
-FD.io VPP is developed using vector packet processing concepts, as opposed to
-scalar packet processing, these concepts are explained in the following sections.
-
-Vector packet processing is a common approach among high performance `Userspace
-<https://en.wikipedia.org/wiki/User_space>`_ packet processing applications such
-as developed with FD.io VPP and `DPDK
-<https://en.wikipedia.org/wiki/Data_Plane_Development_Kit>`_. The scalar based
-approach tends to be favoured by Operating System `Kernel
-<https://en.wikipedia.org/wiki/Kernel_(operating_system)>`_ Network Stacks and
-Userspace stacks that don't have strict performance requirements.
-
-**Scalar Packet Processing**
-
-A scalar packet processing network stack typically processes one packet at a
-time: an interrupt handling function takes a single packet from a Network
-Interface, and processes it through a set of functions: fooA calls fooB calls
-fooC and so on.
-
-.. code-block:: none
-
- +---> fooA(packet1) +---> fooB(packet1) +---> fooC(packet1)
- +---> fooA(packet2) +---> fooB(packet2) +---> fooC(packet2)
- ...
- +---> fooA(packet3) +---> fooB(packet3) +---> fooC(packet3)
-
-
-Scalar packet processing is simple, but inefficient in these ways:
-
-* When the code path length exceeds the size of the Microprocessor's instruction
- cache (I-cache), `thrashing
- <https://en.wikipedia.org/wiki/Thrashing_(computer_science)>`_ occurs as the
- Microprocessor is continually loading new instructions. In this model, each
- packet incurs an identical set of I-cache misses.
-* The associated deep call stack will also add load-store-unit pressure as
- stack-locals fall out of the Microprocessor's Layer 1 Data Cache (D-cache).
-
-**Vector Packet Processing**
-
-In contrast, a vector packet processing network stack processes multiple packets
-at a time, called 'vectors of packets' or simply a 'vector'. An interrupt
-handling function takes the vector of packets from a Network Interface, and
-processes the vector through a set of functions: fooA calls fooB calls fooC and
-so on.
-
-.. code-block:: none
-
- +---> fooA([packet1, +---> fooB([packet1, +---> fooC([packet1, +--->
- packet2, packet2, packet2,
- ... ... ...
- packet256]) packet256]) packet256])
-
-This approach fixes:
-
-* The I-cache thrashing problem described above, by amortizing the cost of
- I-cache loads across multiple packets.
-
-* The inefficiencies associated with the deep call stack by receiving vectors
- of up to 256 packets at a time from the Network Interface, and processes them
- using a directed graph of node. The graph scheduler invokes one node dispatch
- function at a time, restricting stack depth to a few stack frames.
-
-The further optimizations that this approaches enables are pipelining and
-prefetching to minimize read latency on table data and parallelize packet loads
-needed to process packets.
-