Csc407 software architecture summer 2006 design patterns iii
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CSC407: Software Architecture Summer 2006 Design Patterns III. Greg Wilson BA 4234 [email protected] Concurrency Patterns. Programming is hard Concurrent programming is a zillion times harder People aren’t good at thinking about many things at once

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Csc407 software architecture summer 2006 design patterns iii

CSC407: Software ArchitectureSummer 2006Design Patterns III

Greg Wilson

BA 4234

[email protected]


Concurrency patterns
Concurrency Patterns

  • Programming is hard

  • Concurrent programming is a zillion times harder

    • People aren’t good at thinking about many things at once

    • Race conditions make bugs intermittent

  • But concurrency is what’s going to keep us on Moore’s Curve

    • Every processor and system you care about already does many things at once


Concurrency patterns1
…Concurrency Patterns

  • Makes the use of design patterns even more important

    • Every pattern is the distilled experience of dozens of your predecessors

    • Where “experience” is what we call our mistakes when we’re in a job interview

  • Nothing wrong with inventing entirely new solutions from scratch, as long as you have time to debug them


Active object
Active Object

  • An active object has its own thread of execution

  • Instead of “calling methods”, other objects “send messages”

    • The latter terminology is sometimes used for regular objects as well

  • Must make sure that some messages are only accepted under the right conditions

    • E.g., only print jobs one at a time

  • The basis of many other patterns



Reactor
Reactor

  • Problem: want to wait for any of several different events of different kinds

    • E.g., incoming socket connection or timeout or signal

  • Solution: spawn one child for each different kind of event

    • Most OS’s can multiplex events of a single kind

  • Children all communicate with parent in the same way

    • An example of the Adapter pattern


Leader follower
Leader/Follower

  • Problem: the more inter-thread communication you have, the less efficient your system is

    • Having one thread wait for connections, then pass requests to other threads that talk to the database, is probably OK

    • Doing this in a real-time system may not be

  • Solution: threads take turns being the leader or followers

    • A two-state state machine




Thread pool and task farm
Thread Pool and Task Farm

  • The preceding diagram refers to a Thread Pool

    • Another design pattern

    • Name explains itself…

  • Sometimes used as a synonym for Task Farm

    • We will use the latter to mean a system with:

    • A source that generates jobs

    • A pool of workers that execute them

    • A sink that collects results


Task farm
…Task Farm

  • The preceding diagram refers to a Thread Pool

    • Another design pattern

    • Name explains itself…

  • Sometimes used as a synonym for Task Farm

    • We will use the latter to mean a system with:

    • A source that generates jobs

    • A pool of workers that execute them

    • A sink that collects results




Task farm3
…Task Farm

  • One of the few models for massively parallel computing that actually works

    • Seen any good movies lately?

  • “Commodity computing” is a recurring hot topic

    • Major obstacles are security and privacy


SIMD

  • Single Instruction, Multiple Data

    • I.e., apply a single instruction to many values at once

  • At the hardware level, can be implemented by:

    • Pipelining (your GPU)

    • Lots of Little Processors (LOLP)

  • Also makes sense as a programming model

    • MATLAB, anyone?



…SIMD

  • The other massively parallel programming model that people can actually understand

    • I.e., debug

  • But what do you do about conditionals?

# Get absolute value of A

Array[zillion, zillion] A

where A < 0:

A = -A


…SIMD

  • What about function calls?

where A < 0:

A = f(A)

  • If mask is inherited, author of f() has no idea which writes will take effect

  • If mask is not inherited, caller of f() has no idea what values will be mutated


Yes this is really hard
Yes, This Is Really Hard

  • Problem: want to initialize data safely in a multithreaded program

class Foo {

private Helper helper = null;

public Helper getHelper() {

if (helper == null) {

helper = new Helper();

}

return helper;

}

}


Really hard
…Really Hard

class Foo {

private Helper helper = null;

public Helper getHelper() {

if (helper == null) {

synchronized(this) {

if (helper == null) {

helper = new Helper();

}

}

}

return helper;

}

}


Really hard1
…Really Hard

  • But the compiler may assign a value to helper before it has finished constructing the object

    • Second thread to come along sees a non-null reference, and assumes the object is good to use

    • Go ahead—try to debug this…

  • Java 1.4 solution: synchronize getHelper()

    • And take the 100X performance hit

  • Java 1.5 solution: use the volatile keyword


Summary
Summary

  • Knowing lots of design patterns won’t make you a great programmer

  • But it can stop you from being a bad one

  • See c2.com/cgi/wiki?PortlandPatternRepository (the original wiki)

  • Buy a book!

    • Gang of Four started it all

    • Metsker’s and Grand’s are my favorites


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