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diff --git a/docs/whatisvpp/scalar-vs-vector-packet-processing.rst b/docs/whatisvpp/scalar-vs-vector-packet-processing.rst deleted file mode 100644 index ffa54a3f306..00000000000 --- a/docs/whatisvpp/scalar-vs-vector-packet-processing.rst +++ /dev/null @@ -1,69 +0,0 @@ -.. _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. |