Design Issues of Prefetching Strategies for Heterogeneous Software DSM

Author : Ssu-Hsuan Lu, Chien-Lung Chou, Kuang-Jui Wang, Hsiao-Hsi Wang, and Kuan-Ching Li

Speaker : Chien-Lung Chou

Date : 2006/05/18

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Outline

Introduction Motivation Related Work Proposed Method Performance Evaluation Conclusions and Future Work

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Introduction

In Distributed Shared Memory (DSM) systems, it induces:• large number of page faults.• large number of communication time.

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Introduction (cont.)

Page faults and communication time are major overheads in DSM systems.

We need additional strategies to reduce page faults and communication time are:• Home migration• Write vector• Prefetching

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Introduction (cont.)

Most traditional prefetching strategies can provide good performance in homogeneous cluster platforms.

However, the performance of such strategies may be worse in heterogeneous environment.

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MotivationHomogeneous cluster platform.

Program finish

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Motivation (cont.)Heterogeneous cluster platform

Program finish

Waiting Time

Better Resources

WorseResources

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Motivation (cont.)

We need to concern about heterogeneous environment.• More and more personal computers will be

perform computations collectively.• Large number of advanced techniques will be

develop in this environment.• We will usually meet this environment in

future.

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Related Work

History Prefetching Strategy.• It permits home nodes sending data to remote

nodes in advance.

It has some disadvantages:• Accumulated Waiting Phenomenon.• Waiting Synchronization Phenomenon.• Misprefetch.• Home nodes have too much work.

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Related Work (cont.)

Effective Prefetch Strategy • Filtering Unnecessary Prefetches.• Distributing Prefetch Overhead.• Load Balancing with Barrier Synchronization.

Agent Home of prefetching strategy• It will find a node that will help home nodes to

transfer prefetching data.• Thus, it reduces overhead of home nodes.

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Proposed Method

Host 1 Host 4Host 2 Host 3Time

Idle Time

Prefetching

Enter Barrier

LeaveBarrier

ProgramFinish

Home node

Home node

Idle Time

Home node

Home node

Prefetching Strategy in Heterogeneous Environments

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Proposed Method (cont.)

According to above disadvantages, we propose the method that allows• home nodes are adjusted to suitable place.• high speed processors to execute prefetch in

advance.• low speed processors to leave the barrier early.

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Proposed Method (cont.)

First, we distribute home pages to nodes that have better resources.

These nodes are suitable to be home nodes because they have better performance.

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Proposed Method (cont.)

Second, we observe that hosts that have worse resources finish work later, so we adjust policy of prefetching strategy.

Originally, all hosts leave barrier at the same time.

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Proposed Method (cont.)

In our method, the hosts that have worse resources leave barrier after requesting prefetching data.

The hosts that have better resources leave barrier after sending prefetching pages to hosts that have worse resources.

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Proposed Method (cont.)

Third, we also observe that hosts that have better resources spend large amount of idle time during barrier in heterogeneous environment.

It raises execution time and barrier time.

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Proposed Method (cont.)

We utilize idle time in barrier of hosts that have better resources to perform prefetching to each other.

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Proposed Method (cont.)

Our Proposed Method

Host 1 Host 4Host 2 Host 3Time

Serve Prefetch

Enter Barrier

LeaveBarrier

ProgramFinish

Home node

Home node

Non-home node

Non-home node

Part of Prefetch

Request Prefetch

Idle Time

Idle Time

LeaveBarrier

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Performance Evaluation

Experimental Platform - Hardware

Node ID CPU Memory Network

1 Intel P4 2.4GHz 256MB Fast Ethernet

2 Intel P4 1.7GHz 256MB Fast Ethernet

3 Intel P3 500MHz 640MB Fast Ethernet

4 Intel P2 350MHz 128MB Fast Ethernet

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Performance Evaluation (cont.)

Experimental Platform - Software• Linux Fedora Core 3.• Kernel 2.6.9.• JIAJIA DSM software.

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Host 1 Host 2 Host 3 Host 4

Idle Time in Barrier

JIAJIA 70.71 67.79 27.00 0.36

History Prefetch

72.72 71.05 30.45 0.78

Effective Prefetch

68.31 67.42 26.60 0.63

Agent Home

70.72 68.61 28.11 0.63

Our Method

62.14 62.16 26.77 0.56

Performance Evaluation (cont.)

The Idle Time in Barrier for IS Application

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Host 1 Host 2 Host 3 Host 4

Idle Time in Barrier

JIAJIA 3.75 7.35 8.67 8.21

History Prefetch

3.83 6.25 7.49 7.02

Effective Prefetch

3.92 6.29 7.43 6.97

Agent Home

4.34 6.03 7.03 6.57

Our Method

2.37 2.38 4.91 4.35

Performance Evaluation (cont.)

The Idle Time in Barrier for Merge Application

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Performance Evaluation (cont.)

Performance Benefits

50%

60%

70%

80%

90%

100%

110%

120%

130%

EP IS LU MERGE

Spee

dup

J IAJIA History Prefetch Effective Prefetch Agent Home Our Method

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Conclusions and Future Work

In heterogeneous environment, benefits of original prefetching strategies are limited.

In this paper, we utilized idle time to improve overall performance.

In the best situation, our proposed method could reduce idle time in barrier of about 60%.

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Conclusions and Future Work (cont.)

In the future, we will make effort to find a method to optimize the use of idle time in barrier.

In addition, we will also investigate the parallel program execution with issues about dynamic CPU loads include in our next development stage.

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Thank You!!