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#! /bin/bash
export LD_LIBRARY_PATH=`pwd`
gdb --args ./_t-rex-64-debug $@
*/ .highlight .na { color: #336699 } /* Name.Attribute */ .highlight .nb { color: #003388 } /* Name.Builtin */ .highlight .nc { color: #bb0066; font-weight: bold } /* Name.Class */ .highlight .no { color: #003366; font-weight: bold } /* Name.Constant */ .highlight .nd { color: #555555 } /* Name.Decorator */ .highlight .ne { color: #bb0066; font-weight: bold } /* Name.Exception */ .highlight .nf { color: #0066bb; font-weight: bold } /* Name.Function */ .highlight .nl { color: #336699; font-style: italic } /* Name.Label */ .highlight .nn { color: #bb0066; font-weight: bold } /* Name.Namespace */ .highlight .py { color: #336699; font-weight: bold } /* Name.Property */ .highlight .nt { color: #bb0066; font-weight: bold } /* Name.Tag */ .highlight .nv { color: #336699 } /* Name.Variable */ .highlight .ow { color: #008800 } /* Operator.Word */ .highlight .w { color: #bbbbbb } /* Text.Whitespace */ .highlight .mb { color: #0000DD; font-weight: bold } /* Literal.Number.Bin */ .highlight .mf { color: #0000DD; font-weight: bold } /* Literal.Number.Float */ .highlight .mh { color: #0000DD; font-weight: bold } /* Literal.Number.Hex */ .highlight .mi { color: #0000DD; font-weight: bold } /* Literal.Number.Integer */ .highlight .mo { color: #0000DD; font-weight: bold } /* Literal.Number.Oct */ .highlight .sa { color: #dd2200; background-color: #fff0f0 } /* Literal.String.Affix */ .highlight .sb { color: #dd2200; background-color: #fff0f0 } /* Literal.String.Backtick */ .highlight .sc { color: #dd2200; background-color: #fff0f0 } /* Literal.String.Char */ .highlight .dl { color: #dd2200; background-color: #fff0f0 } /* Literal.String.Delimiter */ .highlight .sd { color: #dd2200; background-color: #fff0f0 } /* Literal.String.Doc */ .highlight .s2 { color: #dd2200; background-color: #fff0f0 } /* Literal.String.Double */ .highlight .se { color: #0044dd; background-color: #fff0f0 } /* Literal.String.Escape */ .highlight .sh { color: #dd2200; background-color: #fff0f0 } /* Literal.String.Heredoc */ .highlight .si { color: #3333bb; background-color: #fff0f0 } /* Literal.String.Interpol */ .highlight .sx { color: #22bb22; background-color: #f0fff0 } /* Literal.String.Other */ .highlight .sr { color: #008800; background-color: #fff0ff } /* Literal.String.Regex */ .highlight .s1 { color: #dd2200; background-color: #fff0f0 } /* Literal.String.Single */ .highlight .ss { color: #aa6600; background-color: #fff0f0 } /* Literal.String.Symbol */ .highlight .bp { color: #003388 } /* Name.Builtin.Pseudo */ .highlight .fm { color: #0066bb; font-weight: bold } /* Name.Function.Magic */ .highlight .vc { color: #336699 } /* Name.Variable.Class */ .highlight .vg { color: #dd7700 } /* Name.Variable.Global */ .highlight .vi { color: #3333bb } /* Name.Variable.Instance */ .highlight .vm { color: #336699 } /* Name.Variable.Magic */ .highlight .il { color: #0000DD; font-weight: bold } /* Literal.Number.Integer.Long */
: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.