Multi-processor SoCs

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Multi-processor SoCs

• Yijing Chen

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Why multiprocessor systems-on-chips

• Single processors may be sufficient for
  low-performance applications that are typical of
  early microcontrollers, but an increasing number
  of applications require multiprocessors to meet
  their performance goals.
• Multiprocessor systems-on-chips (MPSoC) are one
  of the key applications of VLSL technology today.
• MPSoC are increasingly used to build complex
  integrated system. A MPSoC is more than just a
  rack of processors shrunk down to a single chip.

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Introduce Multiprocessor

• Definition Multiprocessor is Parallel processors
  with a single shared address.
• Microprocessor is now the most cost-effective
  processor.
• Multiprocessors have the highest absolute
  performance-faster than the fastest uniprocessor.

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Several conceptions

• Parallel processing program a single program
  that runs on multiple processors simultaneously.
• Cluster a set of computers connected over a
  local area network (LAN) that function as a
  single large multiprocessor.
• Shared memory a memory for a parallel processor
  with a single address space, implying implicit
  communication with loads and stores.

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Multiprocessosrs communication mode

• Signal address offer the programmer a single
  memory address space that all processors share.
  Processors communicate through shared variables
  in memory, with all processors capable of
  accessing any memory location via loads and
  stores.
• Message passing Communicating between multiple
  processors by explicitly sending and receiving
  information.

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Two types single address access

• Uniform momory access multiprocessors (or
  symmetric multiprocessors) which takes the same
  time to access main memory no matter which
  processor requests it and no matter which word is
  requested.
• Nonuniform memory access multiprocessors some
  memory accesses are faster than others depending
  on which processor asks for which word.
• For nonuniform memory access machines can scale
  to larger sizes and hence are potentially higher
  performance.

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Two basic constructed organizations

• Processors connected by a single bus
• Processors connected by a network

Options in communication style and physical
connection for multiprocessors as the number of
processors varies.
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Multiprocessors connected by a single bus
Typical size is between 2 and 32 processors
Coche coherency consistency in the value of data
between the versions In the caches of several
processors.
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Multiprocessors connected by a network
Note compare with last one, the multiprocessor
connection is no longer Between memory and the
processor.
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Multiprocessors Programming

• Why is it difficult to write multiprocessor
  programs that are fast, especially as the number
  of processors increases.
• Because of the programming difficulty,
  most parallel processing success stories are a
  result of software wizards developing a parallel
  subsystem that presents a sequential interface.
• you must get good performance and
  efficiency from the parallel program on a
  multiprocessor
• Why it is difficult to write parallel processing
  programs is that the programmer must know a good
  deal about the hardware.

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System-on-chips designs constraints

• Not simply high computation rates, but real-time
  performance that meets deadlines.
• Low power or energy consumption.
• Low cost.

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Concept of MPSoC

• Multiprocessor system on chips (MPSoC) are not
  chip multiprocessors.
• Chip multiprocessors are components that take
  advantage of increased transistor densities to
  put more processors on a single chip, but they
  dont try to leverage application needs
• MPSoC are custom architectures that balance the
  constraints of VLSI technology with an
  applications needs.

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Memory system of MPSoC

• Heterogeneous memory systems some blocks of
  memory may be accessible by only one or a few
  processors. Heterogenous memory systems are
  harder to program because the programmer must
  keep in mind what processors can access what
  memory blocks
• Irregular memory structures are often necessary
  in MPSoCs. One reason that designers resort to
  specialized memory is to support real-time
  performance.

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Challenges and Opportunities

• MPSoCs combine the difficulties of building
  complex hardware systems and complex software
  systems.
• Methodology is critical to MPSoC design.
  Methodologies that work offer many advantages.
  They decrease the time it takes to design a
  system they also make it easier to predict how
  long the design will take and how many resources
  it will require. Methodology also codify
  techniques for improving performance and power
  consumption that developers can apply to many
  different designs.
• Methodology will necessarily be a moving target
  for the next decade.

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Challenges and Opportunities

• MPSoC hardware architectures present challenges
  in all aspects of the multiprocessor processing
  elements, memory, and interconnects.
• Configurable processors with customized
  instruction sets are one way to improve the
  characteristics of processing elements hardware/
  software codesign of accelerators is another
  technique.

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Future work

• A critical question in MPSoC architectures is the
  balance between programmability and efficiency.
  It is more difficult to program multiprocessors
  than uniprocessors, and it is more difficult to
  program heterogeneous multiprocessors than
  homogeneous multiprocessors.
• Hardware/ software interface codesign.

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Question?