Improving Quasi-Dynamic Schedules through Region Slip

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Improving Quasi-Dynamic Schedules through Region
Slip

• Ilie Savga PELAB, IDA Linkoping University

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Dynamic vs. Static Scheduling

• Dynamic schedule instructions continuously,
  based on dynamically variant conditions
  efficient utilization of execution resources-
  extra hardware complexity
• Static rely on the compiler, the scheduling
  function being incorporated into code
  generation requires relatively little hardware
  support- susceptible to degradation in
  performance
• A possible trade-off quasy-dynamic approach

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Quasy-dynamic Scheduling

• Aims to achieve the performance of a
  fully-dynamic approach, while preserving the
  relative simplicity of supporting hardware
• Adapts to phases in run-time behaviour by
  occasionally (not continuously) regenerating and
  rescheduling of regions
• Basically is a non-dynamic mechanism hence, have
  the same impediments as the static scheduling in
  achieving efficient schedules

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Fundamental Impediments to Effective Static
Scheduling

• Region boundariesIntiutively, they form barriers
  across which instructions cannot be moved from
  one region to another
• The decrease of instruction-level parallelism
  towards the end of regionsIntiutively, the
  non-dynamic scheduler knows less potentially
  parallel instructions comparing to the
  full-dynamic one
• Together contribute up to 63 of the total number
  of empty slots in the issue buffer

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Region Slip - a concept for improving
non-dynamic scheduling

• Possible solution allow for overlapping of
  regions, thus
• softening the region boundariesready
  instructions can fill a previous regions empty
  issue slots
• increasing the instruction level
  parallelismthere are more available instructions
  in the combined region
• As the main consequence, there are fewer empty
  issue slots in the buffer
• Region slip is a concept of allowing one static
  region schedule to overlap with another one

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Region Slip - implementation

• In implementation region slip is a
  microarchitectural enhancement, with the most
  complex additional hardware in inserting
  instructions into the issue queue.
• In the presented case, the issue buffer is FIFO
  and is as wide as the number of functional units
• The restriction that all new instructions are
  written only to the tail pointer of the buffer is
  removed by region slip.
• An instruction earliest possible issue slot is
  determined by the later of- its intra-region
  predecessor (maintain program order)- its
  parents (maintain corectness across regions)

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Issue Buffer without Region Slip
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Issue Buffer with Region Slip
LOM
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Evaluation results

• The concept of region slip was applied to a
  low-level dynamic optimization framework rePLay.
• No significant constraints on resources were
  assumed to exist to emphasize the relative
  benefits quiasi-dynamic scheduling over a static
  one
• The relative performance of quasi-dynamic
  scheduling vs. unscheduled and fully-dynamic
  scheduled code without and with region slip was
  assessed using benchmarks
• Region slip was found to be capable of filling
  24 of the empty issue slots previously not
  filled by quasi-dynamic scheduled machine, and
  increasing performance by 26

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Conclusions

• Quasi-dynamic scheduling allows for the
  performance closed to a fully-dynamic approach,
  while maintaining hardware remains significantly
  simplier.
• Its two main problems are region boundaries and
  the decrease of the available ILP towards the end
  of the regions
• By allowing regions to overlap, region slip
  alleviates these problems and drastically
  improves the performance in the most
  unconstrained scenario