dark silicon phenomenon

This project and the research leading to these results has received funding from the European Community's Seventh Framework Programme [FP7/2007-2013] under grant agreement n° 318693

Gulay Yalcin, Anita Sobe, Alexey Voronin, Jons-Tobias Wamhoff, Derin Harmanci, Adrián Cristal, Osman Unsal, Pascal Felber, Christof Fetzer

PDP2014, Turin, Italy

13 February 2014

Combining Error Detection and Transactional Memory for Energy-Efficient Computing below

Safe Operation Margin

Combining Error Detection and TM for Energy-Efficient Computing below Safe Operation Margin

Dark Silicon Phenomenon

Number of transistors can be increased.In order to stay within a chip’s power budget, some must remain “dark”.

One solution: Downscale the voltage.

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How about Reliability?

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When the Vdd is reduced, the error rate increases exponentially [1].

[1] Dan Ernst et al. “Razor: A Low-Power Pipeline Based on Circuit-Level Timing Speculation.” In Proceedings of the 36th annual IEEE/ACM International Symposium on Microarchitecture, pages 7–18, 2003

Our goal is:Investigating the edge cases on voltage reduction while the error recovery still leads to a reduced energy consumption.


Agenda / Overview

MotivationExperiment: Scaling Vdd in a Real System

Basics of ReliabilityError Recovery with TMError Detection Schemes

AnalysisConclusion

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Reducing Vdd in a Real System

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AMD FX-61006-core CPU CPU-heavy executionEvery 10 seconds reduce Vdd by 12.5mVMonitor

Incorrect Result System Crash Machine Check Architecture

The system encounters errors which can not be corrected by MCA even only after 10% reduction in Vdd

Errors are in instruction cache (37%), execution unit (61%) and others (less than 2%).


Basics of Reliability

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Transactional Memory can provide a lightweight Coordinated Local

Checkpoitning [2]

[2] Gulay Yalcin et al. “FaulTM: Fault Tolerance Using Hardware Transactional Memory , DATE 2013


TM provides checkpointing/rollback

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Processor 1

Checkpoint (Log Area)

Checkpoint (Log Area)Checkpoint

(Log Area)Checkpoint (Log Area)Checkpoint

(Log Area)

P2P3

P4Pn

TM write-sets log the tentative memory updates.

Synchronize checkpoints

Data-Versioning provides a synchronization mechanism between

checkpoints.


Error Detection Schemes - Replication

Execute instruction streams multiple timesCompare the results of executionsLess comparison with TM. Dual/Triple Modular Redundancy+ High Error Detection Rate- High Energy Overhead

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Error Detection Schemes-Assertions/Invariants

Assertions: Conditions referring to the current and previous state of the program.Check the stateAdding manually or automatic TM facilitates inserting invariantsEx:

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Error Detection Schemes - Symptoms

Monitor program executions to inspect if there is a symptom of hardware faults.Symptoms:

Mispredictions in high confidence branches,high OS activity,fatal traps (e.g. undefined instruction code)

Reliability at a low cost

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Error Detection Schemes- Encoded Processing

Apply software coding (ECC-like) techniquesThe redundancy is added by applying arithmetic codes to the values.Arithmetic codes: AN, ANBDmem etc.With TM, the validation of a code word can be deferred until a TX commits.Ex:

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Comparing Error Detection Schemes

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Analysis

Gem5 full system simulator 1GHz in-order cores 4 coresX86 ISA64KB L1 data and instruction cachesUnified 2MB L2 cache

SPLASH2 benchmark suite.

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Energy Analysis

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E ≈ C x Vdd 2

Vdd

Error-free Overhead

RecoveryOverhead

Fault Injection

TX size

Error Detection Rate


Energy Reduction

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Reliability of the System

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Conclusion

The energy consumption of CPUs can be reduced if we have efficient hardware support for Transactional Memory and for Error Detection.

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Future Work: Combining DMR and Symptoms

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Thanks!

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dark silicon phenomenon

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