Increasing the Energy Efficiency of TLS Systems

Using Intermediate Checkpointing

Salman Khan1, Nikolas Ioannou2, Polychronis Xekalakis3 and Marcelo Cintra2

1 University of Manchester2 University of Edinburgh

3 Intel Labs Barcelona - UPC

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Introduction

Power efficiency, complexity and time-to-market reasons lead to CMPs

Problem:– No benefits for sequential applications– Even for mostly parallel applications Amdahl’s

Law limits performance gains with many cores Solution: Thread Level Speculation(TLS)

– But performance through TLS costs in energy

Can we reduce the wastefulness of re-execution due to misspeculation

without losing performance?

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Key Contributions

Propose checkpointing to improve efficiency of speculative execution

Evaluate dependence prediction techniques to guide checkpoint placement

Our approach results in an energy saving of up to 14%, with 7% on average over normal TLS execution, with no significant effect on speedup.

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Outline

Introduction Checkpointing Dependence Predictors Checkpointing Policy Experimental Setup and Results Conclusions

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Thread Level Speculation

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Thread Level Speculation with Checkpointing

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Outline

Introduction Checkpointing Dependence Predictors Checkpointing Policy Experimental Setup and Results Conclusions

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Placing Checkpoints

Stride Dependence Prediction

– Address based– Program Counter Based– Hybrid

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Dependence Prediction

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Hybrid Dependence Predictor

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Outline

Introduction Checkpointing Dependence Predictors Checkpointing Policy Experimental Setup and Results Conclusions

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Placing Checkpoints

Limited number of checkpoints Placing a checkpoint has a cost Checkpointing on every positive

prediction results in too many checkpoints

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Outline

Introduction Checkpointing Dependence Predictors Checkpointing Policy Experimental Setup and Results Conclusions

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Setup Simulator, Compiler and Benchmarks:

– SESC (http://sesc.sourceforge.net/)– POSH (Liu et al. PPoPP ‘06)– Spec 2000 Int.

Architecture:– Four way CMP, 4-Issue cores– 16KB L1 Data (multi-versioned) and

Instruction Caches– 1MB unified L2 Caches– Cycles from Violation to Kill/Restart: 12– Cycles to Spawn: 12

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Measuring Dependence Prediction

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ICS 2009 16

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Wasted Instructions: Unnecessarily squashed instructions.

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Outline

Introduction Checkpointing Dependence Predictors Checkpointing Policy Experimental Setup and Results Conclusions

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Conclusions

Effective checkpointing improves the efficiency of TLS

Placing checkpoints by stride is not sufficient to reduce waste significantly

Checkpointing using dependence predication obtains energy saving of up to 14%, with 7% on average over normal TLS execution, with no significant effect on speedup.HiPC 2011 21

Read the paper for…

Complete results Microarchitectural issues that

arise from checkpointing running tasks

Modified squash/restart mechanism that is needed to avoid performance degradation from checkpointing

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