Speculative execution

Speculative execution is an optimization technique where a computer system performs some task that may not be actually needed. The main idea is to do work before it is known whether that work will be needed at all, so as to prevent a delay that would have to be incurred by doing the work after it is known whether it is needed. If it turns out the work was not needed after all, any changes made by the work are reverted and the results are ignored.

The objective is to provide more concurrency if extra resources are available. This approach is employed in a variety of areas, including branch prediction in pipelined processors, prefetching memory and files, and optimistic concurrency control in database systems.[1][2][3]

Overview

Modern pipelined microprocessors use speculative execution to reduce the cost of conditional branch instructions using schemes that predict the execution path of a program based on the history of branch executions.[2] In order to improve performance and utilization of computer resources, instructions can be scheduled at a time when it has not yet been determined that the instructions will need to be executed, ahead of a branch.[4]

In compiler optimization for multiprocessing systems, speculative execution involves an idle processor executing code in the next processor block, in case there is no dependency on code that could be running on other processors. The benefit of this scheme is reducing response time for individual processors and the overall system. However, there is a net penalty for the average case, since in the case of a bad bet, the pipelines should be flushed.[5] The compiler is limited in issuing speculative execution instruction, since it requires hardware assistance to buffer the effects of speculatively-executed instructions. Without hardware support, the compiler could only issue speculative instructions which have no side effects in case of wrong speculation.[6]

Variants

Eager execution

Main article: Eager evaluation

Eager execution is a form of speculative execution where both sides of the conditional branch are executed; however, the results are committed only if the predicate is true. With unlimited resources, eager execution (also known as oracle execution) would in theory provide the same performance as perfect branch prediction. With limited resources eager execution should be employed carefully since the number of resources needed grows exponentially with each level of branches executed eagerly.[7]

Predictive execution

Predictive execution is a form of speculative execution where some outcome is predicted and execution proceeds along the predicted path until the actual result is known. If the prediction is true, the predicted execution is allowed to commit, however if there is a misprediction, execution has to be unrolled and re-executed. Common forms of this include branch predictors, and memory dependence prediction. A generalized form is sometimes referred to as data prediction.

Lazy execution

Main article: Lazy evaluation

Lazy execution does not speculate. The incorporation of speculative execution into implementations of the Haskell programming language is a current research topic. Eager Haskell is designed around the idea of speculative execution. Recent versions of GHC support a kind of speculative execution with an abortion mechanism to back out in case of a bad choice called optimistic execution.[8]

See also

References

  1. ↑ Lazy and Speculative Execution Butler Lampson Microsoft Research OPODIS, Bordeaux, France 12 December 2006
  2. 1 2 International Business Machines Corporation. Research Division; Prabhakar Raghavan; Hadas Schachnai; Mira Yaniv (1998). Dynamic schemes for speculative execution of code. IBM. Retrieved 18 January 2011.
  3. ↑ Kung, H. T.; John T. Robinson (June 1981). "On optimistic methods for concurrency control". ACM Trans. Database Syst. 6.
  4. ↑ Bernd Krieg-Brückner (1992). ESOP '92: 4th European Symposium on Programming, Rennes, France, February 26-28, 1992 : proceedings. Springer. pp. 56–57. ISBN 978-3-540-55253-6. Retrieved 18 January 2011.
  5. ↑ Phillip A. Laplante (2004). Real-time systems design and analysis. Wiley-IEEE. p. 391. ISBN 978-0-471-22855-4. Retrieved 21 January 2011.
  6. ↑ David J. Lilja; Peter L. Bird (1 January 1994). The interaction of compilation technology and computer architecture. Springer. p. 16. ISBN 978-0-7923-9451-8. Retrieved 21 January 2011.
  7. ↑ Jurij Šilc; Borut Robič; Theo Ungerer (1999). Processor architecture: from dataflow to superscalar and beyond. Springer. pp. 148–150. ISBN 978-3-540-64798-0. Retrieved 21 January 2011.
  8. ↑ Optimistic Evaluation: a fast evaluation strategy for non-strict programs

External links

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