74 lines
3.0 KiB
ReStructuredText
74 lines
3.0 KiB
ReStructuredText
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.. _what-is-vector-packet-processing:
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=================================
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What is vector packet processing?
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=================================
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FD.io VPP is developed using vector packet processing concepts, as opposed to
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scalar packet processing, these concepts are explained in the following sections.
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Vector packet processing is a common approach among high performance `Userspace
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<https://en.wikipedia.org/wiki/User_space>`_ packet processing applications such
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as developed with FD.io VPP and `DPDK
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<https://en.wikipedia.org/wiki/Data_Plane_Development_Kit>`_. The scalar based
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aproach tends to be favoured by Operating System `Kernel
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<https://en.wikipedia.org/wiki/Kernel_(operating_system)>`_ Network Stacks and
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Userspace stacks that don't have strict performance requirements.
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**Scalar Packet Processing**
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A scalar packet processing network stack typically processes one packet at a
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time: an interrupt handling function takes a single packet from a Network
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Inteface, and processes it through a set of functions: fooA calls fooB calls
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fooC and so on.
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.. code-block:: none
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+---> fooA(packet1) +---> fooB(packet1) +---> fooC(packet1)
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+---> fooA(packet2) +---> fooB(packet2) +---> fooC(packet2)
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...
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+---> fooA(packet3) +---> fooB(packet3) +---> fooC(packet3)
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Scalar packet processing is simple, but inefficent in these ways:
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* When the code path length exceeds the size of the Microprocessor's instruction
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cache (I-cache), `thrashing
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<https://en.wikipedia.org/wiki/Thrashing_(computer_science)>`_ occurs as the
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Microprocessor is continually loading new instructions. In this model, each
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packet incurs an identical set of I-cache misses.
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* The associated deep call stack will also add load-store-unit pressure as
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stack-locals fall out of the Microprocessor's Layer 1 Data Cache (D-cache).
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**Vector Packet Processing**
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In contrast, a vector packet processing network stack processes multiple packets
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at a time, called 'vectors of packets' or simply a 'vector'. An interrupt
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handling function takes the vector of packets from a Network Inteface, and
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processes the vector through a set of functions: fooA calls fooB calls fooC and
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so on.
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.. code-block:: none
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+---> fooA([packet1, +---> fooB([packet1, +---> fooC([packet1, +--->
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packet2, packet2, packet2,
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... ... ...
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packet256]) packet256]) packet256])
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This approach fixes:
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* The I-cache thrashing problem described above, by ammoritizing the cost of
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I-cache loads across multiple packets.
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* The ineffeciences associated with the deep call stack by recieving vectors
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of up to 256 packets at a time from the Network Interface, and processes them
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using a directed graph of node. The graph scheduler invokes one node dispatch
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function at a time, restricting stack depth to a few stack frames.
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The further optimizations that this approaches enables are pipelining and
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prefetching to minimize read latency on table data and parallelize packet loads
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needed to process packets.
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