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[vpp.git] / docs / whatisvpp / scalar-vs-vector-packet-processing.rst

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-.. _scalar_vector:
-
-==================================
-Scalar vs Vector packet processing
-==================================
-
-FD.io VPP is developed using vector packet processing, as opposed to
-scalar packet processing.
-
-Vector packet processing is a common approach among high performance packet
-processing applications such FD.io VPP and `DPDK <https://en.wikipedia.org/wiki/Data_Plane_Development_Kit>`_.
-The scalar based approach tends to be favoured by network stacks that
-don't necessarily 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.
-
-Press next for more on Packet Processing Graphs.