Performance Evaluation of Cache Replacement Policies for the SPEC CPU2000 Benchmark Suite

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Performance Evaluation of Cache Replacement
Policies for the SPEC CPU2000 Benchmark Suite

• Hussein Al-Zoubi

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Overview

• Introduction
• Common cache replacement policies
• Experimental methodology
• Evaluating cache replacement policies questions
  and answers
• Conclusion

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Introduction

• Increasing speed gap between processor and memory
  
• Modern processors include multiple levels of
  caches, cache associativity increases
• Replacement policy Which block to discard when
  the cache is full

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Introduction...cont.

• Optimal Replacement (OPT) algorithm replace
  cache memory block whose next reference farthest
  away in the future, infeasible
• State-of-the-art processors employ various
  policies

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Introduction...cont.

• Random
• LRU (Least Recently Used)
• Round-robin (FIFO First-In-First-Out)
• PLRU (Pseudo Least Recently Used) reduce the
  hardware cost by approximating the LRU mechanism

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Introduction...cont.

• Our goal explore and evaluate common cache
  replacement policies
• how existing policies relate to OPT
• effect on instruction and data caches
• how good are pseudo techniques in approximating
  true LRU

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Common cache replacement policies LRU
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Common cache replacement policiescont.

• Random policy simpler, but at the expense
  performance. Linear Feedback Shift Register
  (LFSR)
• Round Robin (or FIFO) replacement replacing
  oldest block in cache memory. Circular counter

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Common cache replacement policiescont.
PLRUt
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Common cache replacement policiescont.
PLRUm
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Experimental methodology

• sim-cache and sim-cheetah simulators
• Alpha version of the SimpleScalar
• original simulators modified to support
  additional pseudo-LRU replacement policies
• sim-cache simulator modified to print interval
  statistics per specified number of instructions

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Evaluating cache replacement policies questions
and answers

• Q How much associativity is enough for
  state-of-the-art benchmarks?
• A For data cache, performance gain for
  transition from a direct mapped to a two-way set
  associative cache
• For instruction cache, OPT replacement policy
  benefits from increased associativity. realistic
  policies dont exploit more than 8 ways, or in
  some cases even more than 2 ways

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Evaluating cache replacement policies questions
and answerscont.

• Q How much space is there for improvement for
  each specific benchmark and cache configuration?

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Evaluating cache replacement policies questions
and answerscont.

• Q Do replacement policies behave differently for
  different types of memory references, such as
  instruction and data?
• A In general, LRU policy has better performance
  than FIFO and Random with some exceptions

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Evaluating cache replacement policies questions
and answerscont.

• Q Can dynamic change of replacement policy
  reduce the total number of cache misses?
• A If one policy better than the other, it stays
  consistently better

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Evaluating cache replacement policies questions
and answerscont.

• Can we use most recently used information for
  cache way prediction?

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Evaluating cache replacement policies questions
and answerscont.

• Q How good are pseudo LRU techniques at
  approximating true LRU?
• A PLRUm and PLRUt very efficient in
  approximating LRU policy and close to LRU during
  whole program execution

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Conclusion

• Eliminating cache misses extremely important for
  improving overall processor performance
• Cache replacement policies gain more significance
  in set associative caches
• Gap between LRU and OPT replacement policies, up
  to 50, new research to close the gap is
  necessary