Multiprocessing & The .Net Parallel Extensions

Guy Ben HaimSenior Application

Engineer

Intelguy.ben.haim@intel.com

www.intel.com/software/

Asaf ShellySenior Consultant

Pacific SoftwareAsaf.Shelly@gmail.com

www.AsyncOp.com

Session Objectives and Agenda• Multicore• Parallel Software• .Net Parallel Extensions• Q&A• Summary

What is Multicore

Pentium

Pentium

Pentium

Pentium

Pentium Processor

Dual Core

Quad Core

Moore’s Law – GHz to MulticorePerformance

2006

Intel MC Assistance•Threading•Multi-tasking•Training •Tools

Performance ThroughPerformance ThroughMulti-CoreMulti-Core

Performance ThroughPerformance Throughfrequencyfrequency

-+

Intel Processor Advancement

Multiple execution cores ramping across Intel platforms

Why Multi Core?

Power

Performance

2 GHz

100%

CPU that is 20% Faster

Power

Performance

2.4 GHz 2 GHz

174%

100%

113%100%

CPU that is 20% Slower

Power

Performance

1.6 GHz

100%

2 GHz

50%

87%

2.4 GHz

174%

100%

113%

Multi Core: Energy Efficient Performance

Power

Performance

1.6 GHz

100%

2 GHz

100%

174%

2.4 GHz

174%

100%

113%

174%

What does it mean Multi What does it mean Multi Cores?Cores?

PerformancePerformance

Start Thinking Parallel

• Software Today• Instructions – Assembly – Making it workInstructions – Assembly – Making it work

• Thinking like a CPUThinking like a CPU

• Functions – C, Pascal, Basic – Faster CodeFunctions – C, Pascal, Basic – Faster Code• Procedural ThinkingProcedural Thinking

• Objects – C++, Java, C#, Delphi, VB – Manage Objects – C++, Java, C#, Delphi, VB – Manage CodeCode• OOD, OOP – Thinking in objectsOOD, OOP – Thinking in objects

• Tasks - ? – Optimize RuntimeTasks - ? – Optimize Runtime• Thinking Parallel

Situation Today

• Experts, Freelance Specialists, Skilled Groups

• API is not intuitive• Hard to understand execution flow• Problematic Design Patterns• Little awareness of tools• Hidden Problems• Hard to test and debug

Understanding Parallel Computing• Resources• Ownership• Global data / Shared data• Collisions and Race Conditions

• Task Design• Conjunction Points

Task Oriented Design

Modify

Write

Open

Modify

Scan ScanScanScan ScanScan

Simple For

for ( int y = 0; y < bmp.Height; y++ ){ for ( int x = 0; x < bmp.Width; x++ ) { Pixels[ x, y ] = bmp.GetPixel( x, y );

}}

Parallel For

Parallel.For( 0, bmp.Height, y =>{ for ( int x = 0; x < bmp.Width; x++ ) { Pixels[ x, y ] = bmp.GetPixel( x, y );

}});

.Net Parallel Extensions - Performance

Parallel Class

• Parallel.For• Parallel.Do• Parallel.ForEach• Inplace code / Function• Object Type

Parallel Do

Parallel Quick Sort:

void QuicksortParallel( , , ){ int pivot = Partition(arr, left, right); Parallel.Do( () => QuicksortParallel(arr, left, pivot - 1), () => QuicksortParallel(arr, pivot + 1,

right));}

PLINQ

.Net Parallel Extensions – PLINQ

Task Parallel Library

• Parallel For, Do, ForEach• PLINQ• Tasks over Threads• Tasks over Cores• TaskManager• Conjunction Points

.Net Parallel Extensions – Tasks Parallel Library

.Net Parallel Extensions – RayTracer

Tips

• Shared are Globals• Parallel Loops are not loops• Define data as Loop internal• Race Conditions are still here• Don’t use Locks!!• Don’t use MUTEXs

Threading Tools

Intel® Thread Checker•Used to create correct multi-threaded code

Intel® Thread Profiler•Used to analyze performance

Intel Software Solutions Group :

http://www.intel.com/software

Data Race example• Serial program

• What is value of A_SUM:

A_Sum = 4 R

S1: x = 1.0; y = 2.0 ; A1 = 0;S2: A1 = x * y;S3: A_SUM = 2 * A1;

x

y A1

Data Race example (Cont.)• Initiate x = 1.0; y = 2.0 ; A1 = 0;

Thread1A1 = x * y

Thread2A_SUM = 2 * A1

• What is value of x if:• Thread1 runs before Thread2?• Thread2 runs before Thread1?

Execution order is not guaranteed

x

y

A_Sum = 4

A_Sum = 0

A1

Intel® Thread Checker Diagnostics

Source Code Viewer

Performance Profile

0

1

2

3

1 2 3 4 Threads

Sp

eed

up

Possible causes for this scalability profile:1. Insufficient parallel work2. Load imbalance3. Synchronization overhead4. Memory bandwidth limitations

Finding Serial and Parallel Time

Load Imbalance

Multi Threading should be Multi Threading should be managed managed Programming should consider load imbalanceProgramming should consider load imbalance

Load Imbalance• Unequal work loads lead to idle threads and

wasted time

Busy

Idle

Time

Thread 0

Thread 1

Thread 2

Thread 3

Start threads

Join thread

s

Synchronization

Programming should consider Synchronizations issuesProgramming should consider Synchronizations issues

Synchronization

• By definition, synchronization serializes execution• Lock contention means more idle time for threads

Busy Idle In Critical

Thread 0

Thread 1

Thread 2

Thread 3

Time

Real example : Before fix

Serial Parallel

Switching Overhead

Real example: After fix

Serial Parallel

2 X Speed Up

Summary

Parallelize or Perish !

Do we really want Parallel Code?Do users even care?

2005 2007 2008 2010

Change In Mindset

Everything is stopped. Waiting for the Everything is stopped. Waiting for the photographerphotographer

Everyone is working independentlyEveryone is working independently

Change In Mindset

Developers are writing functionsDevelopers are writing functions

Developers are managing tasksDevelopers are managing tasks

Change In Mindset

Doing things the way we always haveDoing things the way we always have

Things are going to be differentThings are going to be different

Keep yourself in the loop

• Public event by Pacific Software

• Register to the User Group

• Asynchronous Operations Web Site has all the online resources that you need... and morehttp://www.AsyncOp.com

• Register to my five day course titled Multiprocessing Traps and Pitfalls

• Use our poster to let people know that you know

Resources

• Download the Microsoft .Net Parallel Extensionshttp://www.microsoft.com/downloads/details.aspx?FamilyID=e848dc1d-5be3-4941-8705-024bc7f180ba&displaylang=en

• Asynchronous Operations Web Site http://www.AsyncOp.com

• Intel’s Multicorehttp://www.intel.com/multi-core

• Pacificsoft Training and Consultinghttp://www.Pacificsoft.com

• Microsoft Forum for Parallel Computinghttp://forums.microsoft.com/MSDN/default.aspx?ForumGroupID=551&SiteID=1

Make a difference

• Let us know what you think

• Feedback for the .Net Parallel Extensions Dev team

• Video blog about parallel computing

• Fill the feedback form …

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