Improving Energy Efficiency by Making DRAM Less Randomly Accessed

1
Improving Energy Efficiency by Making DRAM Less
Randomly Accessed

• Hai Huang, Kang G. Shin, Charles Lefurgy, Tom
  Keller
• University of Michigan
• IBM Austin Research Lab

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Overview

• Continual increase in the power budget allocated
  to main memory (i.e., DRAM)
• E.g., in a mid-range IBM eServer system, 40 of
  the total system energy is consumed by its main
  memory subsystem
• By passively monitoring memory traffic and
  managing the power, existing power management
  techniques are not fully exploiting deeper
  power-saving states
• gt Actively shape memory traffic to enable
  existing techniques to save more energy

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Passive Monitoring Memory Traffic

• Why is passively monitoring memory traffic
  inefficient?
• Memory accesses are random good for
  performance, bad for energy consumption!
• Idle time between consecutive memory accesses is
  often too short for use of the deeper
  power-saving state
• Randomness is mostly due to OSs arbitrary
  virtual-to-physical mapping

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Example Active vs. Passive
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How to Shape Memory Traffic

• Essentially, we need to artificially create
  disparity in access frequency among different
  memory ranks
• Hot Ranks and Cold Ranks
• Disparity in access frequency can be created by
  finding and migrating frequently-accessed pages
  to a subset of memory ranks
• Hot ranks contain frequently-accessed pages
• Cold ranks contain infrequently-accessed and
  unmapped pages
• Page migration can be done by system software

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Implementation
Rank 0
Hot ranks
MC
page counter
Rank 1
Rank 2
Cold ranks
Rank 3
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Issues with Page Migration

• There is a cost associated with each page
  migration

Memory access frequency Is often highly
skewed!!! 6 pages causes 75 accesses 14 pages
causes 90 accesses Not all pages need to be
migrated
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Evaluation

• Simulators
• Mambo IBM A full-machine simulator,
  cycle-accurate, supports PowerPC architecture
• Memsim IBM Detailed trace-driven main memory
  simulator, written in CSIM
• Workloads
• Low memory-intensive workload SPECjbb bzip
  crafty
• High memory-intensive workload SPECjbb art
  mcf
• SPECjbb simulating 8 warehouses
• SPEC2000 benchmarks using Reference input set

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Low Memory-Intensive Workload
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High Memory-Intensive Workload
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Summary of Results

• Energy
• Actively shaping memory traffic saves 35 more
  energy than passively monitoring
• Performance
• Low memory-intensive workload small impact on
  performance
• High memory-intensive workload significantly
  degrades performance due to more contention on
  hot ranks
• Cost
• Use hardware counters, or
• Software page faults

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Conclusion

• Actively shaping memory traffic allows existing
  power management techniques to more effectively
  save power
• Highly-skewed page accesses are observed
• Alternative main memory design
• Use high-performance/highly-parallel ranks as hot
  ranks
• Use low-performance/low-power ranks as cold ranks
• Allows frequently-accessed pages to be accessed
  faster
• Allows memory ranks that hold infrequently-accesse
  d and unmapped pages to consume less energy