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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
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.
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