Voltage Emergency Prediction: Using Signatures to Reduce Operating Margins

Vijay Janapa Reddi, Meeta S. Gupta, Glenn Holloway, Gu-Yeon Wei, Michael D. Smith, David Brooks

1Presented by: Divya Ramesh and John Chung

Why Do We Care?

● Feature size reduction => increased sensitivity to voltage fluctuations

● Voltage fluctuations cause timing issues and reduce transistor lifetimes

● Conservative operating margins lead to degraded performance

● Timing margins can be up to 20% of supply voltage

2

Proposed Solutions

● Voltage Emergency Prediction

● Signature-based Voltage Emergency Reduction

● Efficient Predictor Implementation

3

Previous Work and Limitations

4Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

Goals

5Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

A 10,000 FT View

6Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

How Does it Work?

I know nothing...

7Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

How Does it Work?

I know nothing...

Emergency!

8Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

How Does it Work?

Emergency!

Predictor needs to remember this moment...

Recover, Resume execution

9Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

How Does it Work?

Predictor needs to remember this moment...

Recover, Resume execution

Roger that!

10Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

How Does it Work?

I can prevent it!

Eme….!

11Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

What Does the Predictor Need to Know?

Flush in 2 causes an emergency sometimes, but does not other times.

12Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

Flush in 2 causes an emergency sometimes, but does not other times.

Combination of contexts can signal the future emergency!

13Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

What Does the Predictor Need to Know?

Flush in 2 sometimes causes emergency, but sometimes does not

→ Combination of contexts can signal the future emergency!

Voltage emergencies are quite repetitive and stable thus enabling their PREDICTION!

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How Do We Get Predictions?● Predict possible emergencies from event history register

● Capture interleaved sequence of control flow instructions and architectural events that give rise to an emergency

15Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

Contents of Signals for Prediction ● Program Control Flow : Out-of-Order Issue > In-order Fetch and Decode >

In-order Commit

● Microarchitectural events increase the accuracy even more!

16Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

Size of Entries for Prediction

More entries => better accuracy. But, cost of prediction might get expensive!

17Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

Signature size =4

Hardware Overhead Reduction

● Signature encoding: 3 bit encoding + Anchor PC (the program counter for the most recently taken branch)

● Signature compaction: Measure similarity with Manhattan distance → compact signatures with similarity higher than 0.9

○ 67% signature reduction for 403. gcc

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Predictor Accuracy Evaluation

19Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

Comparison of Schemes

20Reddi, Vijay Janapa, et al. "Voltage emergency prediction: Using signatures to reduce operating margins." High Performance Computer Architecture, 2009. HPCA 2009. IEEE 15th International Symposium on. IEEE, 2009.

Discussion (1)

Voltage emergency prediction suffers from cold-start problem, which requires that the chip to first experience emergency before the predictor can learn predictions.

● Is the cold-start problem be trivial, with no need to be cared? ● Can we pre-train the predictor to avoid cold-start problem?

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Discussion (2)

Compare checkpoint recovery schemes with prediction-based throttling schemes.

● What are some of the advantages and disadvantages of each approach?

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Discussion (3)

Does the velocity of voltage drop matter?

● Does voltage drop differ between different types of emergencies?● Can this technique consider the velocity of the voltage drop?

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