CostEffective Register File Soft Error reduction

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Cost-Effective Register File Soft Error reduction

• Pablo Montesinos, Wei Liu and Josep Torellas,
• University of Illinois at Urbana-Champaign

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Overview

• Study of register file vulnerability to
  SDC(Silent Data Corruption)
• Shield cost effective protection to register
  files
• Highighting policies and techniques used in
  shield
• Experiment - Results

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Register File AVF

• RF-AVF is the probability that a fault that
  occurs will lead to error.
• Register lifetime is divided into PreWrite,
  Useful, and PostLastRead parts.
• Based on AVF calculation we can divide lifetime
  of bit into ACE (Architecturally Correct
  Execution) and un-ACE cycles.

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Register File AVF

• During PreWrite Period un-ACE
• If used atleast once after write the reg switches
  to ACE state.
• After last read on reg, switches back to un-ACE
  during PostLastRead

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Highlighting Insights (1)

• The combined -USEFUL time of all registers is
  small

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Highlighting Insights (1)

• The average number of useful (live) registers is
  less than 20 (SPECint) and 17(SPECfp).
• It is thus possible to redue the vulnerability of
  the register file by only protecting a subset of
  carefully chosen registers at a time.

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Highlighting Insights (2)

• Only a few long-lived registers contribute to
  overall Total useful time
• On average less than 10 of register versions are
  long-lived.

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Highlighting Insights (2)

• On average 40 of useful time comes from the few
  long-lived versions.
• In SPECfp, 5 of long-lived versions account for
  46 of the useful time.

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Motivation

• Register files have a very high access rate.
• High temperature thus leading to lesser Qcrit for
  the devices.
• An error in an RF can propagate with hght failure
  probability
• If we isolate a few register versions, predicting
  their life-time, and protect these register
  versions alone, high reliability can be achieved
  with limited overhead.

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Shield - Architecture
Life-Time Prediction
Register Error Check
Shielding Decision
Error Recovery
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Reg-Version Lifetime Prediction
P12 gt Used(1) , Renamed(1) P7 gt
Used(0) , Renamed(1)
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Shielding Decision

• These prediction bits are stored as status in the
  ECC table.
• The decision to shield an incoming register
  version written is by
• Availability of free ECC-Table entry
• Same register present in the ECC table will be
  replaced with new entry.
• Existing reg-version with lesser lifetime than
  incoming reg-version will be replaced.
• Replacement policy

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Register Error Check Recovery

• On a read request the register data is sent to
  the original datapath and shield.
• If the Reg matches with a tag entry, then the
  reg-data is checked for errors at the
  ECC-Checker.
• If Error is detected
• Processor stalls the instruction I reading reg P
• Reg-data is corrected and written into RF
• Oldest read instruction reading reg P in ROB and
  all succeeding instructions is flushed.
• Processor resumes from flushed instruction.

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Experiments- Results

• AVF computation for RF with shield

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Experiments-Results

• AVF of intREG reduced by different replacement
  policies
• LRU 31
• Effective 63
• OptEffective 84 ( pinning of global pointers
  to particular ECC entries Effective
  )
• AVF for fpREG can be reduced maximum by 100,
  because fewer fp-registers are in useful state.

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Power and Area Impact

• Shield only uses 3ECC generators and 3 ECC
  checkers.
• Shield has 45 power overhead over a plain
  register file. (Full ECC has 2X)
• Shield introduces an overall 10 area overhead.

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Conclusion

• A cost-effective architectural technique has been
  proposed to reduce the vulnerability of RF by 84
• The area and power overhead indicated is a
  marginal tradeoff for reliability achieved.

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