Pipelining, Parallelism, and Simplified Circuits

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Pipelining, Parallelism, and Simplified Circuits

• Discrete Math
• April 13, 2006
• Harding University
• Jonathan White

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Outline

• What Pipelining is
• Benefits
• Downsides
• How modern processors use Pipelining
• Parallelism
• Threads
• Circuits
• Pros/Cons

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Pipelining

• Definition
• Pipelining is an implementation technique where
  multiple instructions are overlapped in execution
  on a processor.
• Each stage completes part of an instruction in
  parallel.
• The stages are connected one to the next to form
  a pipe - instructions enter at one end, progress
  through the stages, and exit at the other end.

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Pipelining Laundry Example

• 4 loads of laundry that need to washed, dried,
  and folded.
• 30 minutes to wash, 40 min. to dry, and 20 min.
  to fold.
• We have 1 washer, 1 dryer, and 1 folding station.
• Whats the most efficient way to get the 4 loads
  of laundry done?

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Non Pipelined Laundry

• Wash, dry, fold.
• Then wash, dry, fold.
• Then wash, dry fold.
• Takes a total of 6 hours nothing is done in
  parallel

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Pipelined Laundry

• A better idea would be start the next load
  washing while the first is drying.
• Then, while the first load was being folded, the
  second load would dry and a new load could be put
  in the washer.
• Using this method, the laundry would be done at
  930.

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Processors

• Computers, like laundry, typically perform the
  exact same steps for every instruction
• Fetch an instruction from memory
• Decode the instruction
• Execute the instruction
• Read memory to get input
• Write the result back to memory

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Example of a Basic Non-Pipelined Instruction
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Example of a Pipelined Architecture
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Pipelining Aspects

• The length of the longest step dictates the
  length of the pipeline stages.
• So, the slowest resource affects the entire
  process.
• Whats the slowest process in a processors 5
  steps?
• Pipelining improves performance by increasing
  instruction throughput, as opposed to decreasing
  the execution time of any individual instruction.

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Pipeline Video
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Pipelining Benefits

• For the right instruction language, pipelining
  increases performance linearly with the number of
  pipeline stages.
• Languages are designed to be pipelined now.
• RISC vs CISC architectures
• Pipelining is easy to do with only a few
  additions
• Pipelining makes efficient use of resources.
• Circuits consume similar amounts of power whether
  performing calculations or just waiting.

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Pipelining Downsides

• Pipelining requires additional hardware
• Every instruction must be able to be performed in
  each of the stages
• ie, some instruction require the ALU in more than
  one step.
• Registers to hold data between cycles
• More ALUs are required.
• For example, 1 ALU is needed just to increase the
  program counter.
• Branch prediction and collision avoidance units
  are required.
• Often times, you will have to clear the pipeline
  when youve written code that causes a hazard.
• X Y 4
• Z X 1

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Branch Prediction

• How many times will this loop execute?
• for(int x 0 x
• do something.
• It would be nice for the processor to be able to
  predict that this code will be executed more than
  once
• Some modern processors just assume
• branch will never be taken.
• Also, compilers will often do out of order
• execution of commands to avoid stalling the
• pipe.

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More benefits of Pipelining

• The parallelism is invisible to the programmer.

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Modern processors

• Pentium 4s have a 30 stage pipeline.
• If the pipeline gets too large, there is too much
  overhead (flushing 300 stages is easier than 30).
• However, new processors like the CELL processor
  in the Playstation 3 are moving to multicore
  architectures.
• The pipeline is much smaller between 5 and 10.
• Multicore processors work best for applications
  that run a lot of threaded applications that are
  easily seperable.

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Other Levels of Parallelism

• Threads
• Way for an application to split itself into 2
  separate tasks.
• MS Word
• Logic circuits
• These are naturally parallel

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Pros of Parallelism

• The average throughput is greatly increased.
• Very little time is wasted.
• A lot of things are naturally parallel.

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Cons of Parallelism

• Requires more overhead.
• More power, more components
• For threaded computer programs, either the kernel
  or your program must do some work to switch
  between individual threads.
• At some point, more parallelism actually makes
  things slower.
• You spend too much time switching between tasks
  instead of doing actual work.