Discerning the dominant out-of-order performance advantage: Is it speculation or dynamism?

Daniel S. McFarlin, Charles Tucker, Craig Zilles

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

In this paper, we set out to study the performance advantages of an Out-of-Order (OOO) processor relative to in-order processors with similar execution resources. In particular, we try to tease apart the performance contributions from two sources: the improved schedules enabled by OOO hardware speculation support and its ability to generate different schedules on different occurrences of the same instructions based on operand and functional unit availability. We find that the ability to express good static schedules achieves the bulk of the speedup resulting from OOO. Specifically, of the 53% speedup achieved by OOO relative to a similarly provisioned inorder machine, we find that 88% of that speedup can be achieved by using a single "best" static schedule as suggested by observing an OOO schedule of the code. We discuss the ISA mechanisms that would be required to express these static schedules. Furthermore, we find that the benefits of dynamism largely come from two kinds of events that influence the application's critical path: load instructions that miss in the cache only part of the time and branch mispredictions. We find that much of the benefit of OOO dynamism can be achieved by the potentially simpler task of addressing these two behaviors directly. Categories and Subject Descriptors D.3.4 [Software]: Programming Languages-Processors:Compilers, Optimization, C.0 [Computer Systems Organization]: General-Hardware/software interfaces General Terms Performance.

Original languageEnglish (US)
Title of host publicationASPLOS 2013 - 18th International Conference on Architectural Support for Programming Languages and Operating Systems
Pages241-251
Number of pages11
DOIs
StatePublished - Apr 5 2013
Event18th International Conference on Architectural Support for Programming Languages and Operating Systems, ASPLOS 2013 - Houston, TX, United States
Duration: Mar 16 2013Mar 20 2013

Publication series

NameInternational Conference on Architectural Support for Programming Languages and Operating Systems - ASPLOS

Other

Other18th International Conference on Architectural Support for Programming Languages and Operating Systems, ASPLOS 2013
CountryUnited States
CityHouston, TX
Period3/16/133/20/13

Keywords

  • Dynamic scheduling
  • Optimization
  • Speculation

ASJC Scopus subject areas

  • Software
  • Information Systems
  • Hardware and Architecture

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