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Online Computing and Predicting Architectural Vulnerability Factor of Microprocessor Structures. Songjun Pan Yu Hu Xiaowei Li {pansongjun, huyu, lxw}@ict.ac.cn Key Laboratory of Computer System and Architecture, I nstitute of C omputing T echnology, Chinese Academy of Sciences

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online computing and predicting architectural vulnerability factor of microprocessor structures
Online Computing and Predicting Architectural Vulnerability Factor of Microprocessor Structures

Songjun Pan Yu Hu Xiaowei Li

{pansongjun, huyu, lxw}@ict.ac.cn

Key Laboratory of Computer System and Architecture,

Institute of Computing Technology, Chinese Academy of Sciences

Nov. 17, 2009

1

PRDC2009

outline
Outline

Background and Motivation

AVF Computing and Predicting

Experimental Results

Conclusions

PRDC2009 Nov. 17

soft errors in microprocessors
Soft Errors in Microprocessors

Soft errors

Caused by neutrons and alpha particles

Technology Scaling

Smaller transistors

Lower threshold voltage

Higher soft error rate

It is important to analyze the vulnerability of different structures to soft errors

PRDC2009 Nov. 17

reliability analysis method
Reliability Analysis Method

Compute Architectural Vulnerability Factor (AVF) for different structures [Mukerjee, MICRO’03]

Reflect the vulnerability of a structure to soft errors

AVF: The probability a soft error in a structure would result in an external visible failure

AVF = 0% branch predictor

AVF ≈ 100% program counter

Higher AVF, higher vulnerable to soft errors

AVF computing is important!

PRDC2009 Nov. 17

offline avf computing methods
Offline AVF Computing Methods

Offline methods [Mukerjee, MICRO’03]

Analyze ACE bit/un-ACE bit

Sim-SODA method [Fu, Workshop of ISCA’06]

Compute AVF for different structures (IQ, ROB, register file)

Accurate AVF result

Guide reliability estimation at early design stage

PRDC2009 Nov. 17

motivation of our work
Motivation of Our Work

Traditional protection schemes (AR-SMT, SRT) for soft errors result in a high performance overhead [Mukerjee, ISCA’02]

AVF varies significantly across different workloads and individual structures [Li, DSN’05]

The AVF information can be used to guide the protection of microprocessors

PRDC2009 Nov. 17

online methods are needed
Online Methods Are Needed

AVF>AVFth

AVF>AVFth

AVF

AVFth

Time

Active protection scheme

  • Dynamically tuning, make a trade-off between reliability and performance
  • Offline methods are not enough
  • Online AVF computing methods are needed
    • Computing AVF during program execution

PRDC2009 Nov. 17

our contributions
Our Contributions

Propose an occupancy-based online AVF computing method

Predict the AVF based on history information to guide reliability design

Demonstrate the efficiency of our method

PRDC2009 Nov. 17

schematic diagram
Schematic Diagram

Online AVF computing and predicting architecture

Load/Store Queue

Activated

Protection

scheme

Issue Queue

FU

Front-End

AVF

Computing&

Predicting

Extra Bit

AVF>AVFth

Reorder Buffer

Extra Bit

Register File

PRDC2009 Nov. 17

occupancy based method
Occupancy-based online AVF computing

The percentage of entries have been taken during a program execution

Occupancy-based Method

OUT

Reorder Buffer

Occupancy

Cycle 1

Cycle 2

Cycle 3

Cycle 4

4/9

5/9

3/9

0/9

IN

PRDC2009 Nov. 17

key observations
Key Observations

Efficient to get the occupancy information during program execution

Assuming the occupancy of a structure as the AVF of that structure

This method takes all the bits as ACE bits, which results in a conservative AVF

We need to further refine the AVF result

PRDC2009 Nov. 17

refine avf computing
Refine AVF Computing
  • Instruction types
    • NOP instructions (NOP): not affect the program output
    • Dynamic dead instructions (DDI)
      • Not affect the program output, BUT
      • Need a long time to differentiate
    • ACE instructions (ACE):affect the program output
  • Refine AVF result
    • Exclude NOP instructions
    • Counter dynamic dead instructions as ACE instructions

PRDC2009 Nov. 17

our online computing method
Our Online Computing Method

Flag Bit

OUT

AVF

Reorder Buffer

NOP

3/9

0

2/9

Cycle 1

Cycle 4

Cycle 2

Cycle 3

NOP

IN

Interval

Computing AVF online at interval granularity

PRDC2009 Nov. 17

avf predicting
AVF Predicting

Interval length: 1000 cycles

latest

interval

next

interval

interval

Time

L3

L2

L1

N1

  • Activate a protection scheme when AVF > AVFth
  • Predicting the next interval’s AVF based on the history AVF information
    • Algorithm 1: last-value based
    • Algorithm 2: average of the latest three interval’s value

PRDC2009 Nov. 17

overall flowchart
Overall Flowchart

Decode

Repeat

NOP?

Yes

No

FLAG=0

FLAG=1

Cycle++

Record Occupancy

End of interval

No

AVF<AVFth

Yes

AVF computing and predicting

AVF>AVFth

Activate Protection scheme

PRDC2009 Nov. 17

experimental setup
Experimental Setup
  • Simulated machine configurations
    • 4 integer ALUs, 2 integer multipliers, 2 float ALUs
    • IQ/ROB/LSQ 20/80/64 entries
    • Hybrid, 4K global + 2-level 1K local + 4K choice branch predictor
    • 64KB 2-way L1 data cache, 2MB direct mapped L2 cache
  • Workload
    • SPEC2000 Integer benchmark suite
    • Simulate 100M instructions starting from each SimPoint.

PRDC2009 Nov. 17

experimental results 1 4
Experimental Results (1/4)

Online computed AVF for IQ、ROB, and LSQ

PRDC2009 Nov. 17

experimental results 2 4
Experimental Results (2/4)
  • Different Configurations - IQ/ROB/LSQ entries
    • Base : 20/80/64
    • 20/10/64 40/80/64 20/80/8

Config2

Config4

Config3

AVF Results for different configurations

PRDC2009 Nov. 17

experimental results 3 4
Experimental Results (3/4)

AVF results with different predicting algorithms

Algorithm 2: higher prediction accuracy

IQ

ROB

PRDC2009 Nov. 17

experimental results 4 4
Experimental Results (4/4)

AVF for IQ, ROB, and LSQ during executing crafty and gap

AVFth

AVFth

crafty

gap

PRDC2009 Nov. 17

conclusions
Conclusions

We propose an occupancy-based method to compute and predict AVF online

Our method can compute AVF efficiently, the difference between our method and an offline method are 0.10, 0.01, and 0.039 respectively

Our method is also independent of the microprocessor configurations.

Our method combines AVF to activate protection scheme, ensuring high reliability while with less performance overhead.

PRDC2009 Nov. 17

slide22
Thanks!

Q&A

PRDC2009 Nov. 17

slide24

T = 3

T = 2

T = 1

T = 4

ACE% = 0/4

ACE% = 1/4

ACE% = 2/4

ACE% = 3/4

Average number of ACE bits in a cycle

Total number of bits in the structure

( 2 + 1 + 0 + 3 ) / 4

4

=

=

Vulnerability of a structure

ACE bit and un-ACE bit

PRDC2009 Nov. 17

interval length
Interval length

Choose an appropriate interval length

Single cycle / length of the entire application

  • Interval length: 1000 cycles

PRDC2009 Nov. 17

refine avf computing1
Refine AVF Computing
  • Instruction types
    • NOP instructions (NOP) / Dynamic dead instructions (DDI) / ACE instructions (ACE)
  • NOP 10.7% DDI 15.8% ACE 73.5%

PRDC2009 Nov. 17

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