: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
aproach 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
Inteface, 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 inefficent 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 Inteface, 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 ammoritizing the cost of
  I-cache loads across multiple packets.

* The ineffeciences associated with the deep call stack by recieving 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.