cs 152: programming language paradigms may 7 class meeting department of computer science san jose...

CS 152: Programming Language Paradigms

May 7 Class Meeting

Department of Computer ScienceSan Jose State University

Spring 2014Instructor: Ron Mak

www.cs.sjsu.edu/~mak

SJSU Dept. of Computer ScienceSpring 2014: May 7

CS 152: Programming Language Paradigms© R. Mak

2

Multithreaded Programming is Hard!

The execution order of the threads is not guaranteed.

Your program must work no matter what the order is. You must explicitly synchronize the threads. A thread must remember to signal other threads

to prevent the program from deadlocking.

It is easy to corrupt shared objects.

A thread must prevent another thread from “sneaking in” before it is finished with its operations.

You must manage critical regions.



3

Multithreaded Programming is Hard! cont’d

Debugging multithreaded programs is difficult.

Different errors (or none at all) may occur on each run.

Errors may be timing related.

However, multithreaded programming is a critical job skill.

Learn multiprogramming or be obsolete!_



4

Class Exercise: Multithreaded Café

A small busy café serves coffee and burgers.

At most three customers at a time can be inside the café .

Only one server is working behind the counter.

Customers are served in the order that they enter._



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Multithreaded Café, cont’d

Coffee customers enter through any one of three coffee doors.

At least one every minute. Once a server starts serving a coffee customer,

it takes up to two minutes.

Burger customers enter through the one burger door.

At least one every five minutes. Once a server starts serving a burger customer,

it takes up to five minutes.



6

Multithreaded Café: The Burger Rule

Once a burger customer enters, the server needs to concentrate on making the burger.

Other customers already in the café can leave after they’ve been served.

No other customers may enter until the burger customer is served and leaves._



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Multithreaded Café: Java Program

Write a multithreaded Java program to simulate the activities of the café.

One second of simulation run time equals one minute of cafe activity._



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Multithreaded Café: Java Program, cont’d

Run the simulation until

5 coffee customers have entered through each coffee entrance and have been served.

5 burger customers have entered through the burger entrance and have been served._



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Multithreaded Café: Java Program, cont’d

Print a line for each event as it occurs. An event is a coffee or burger customer

entering or leaving.

Include in each printed line:

Timestamp of elapsed simulated minutes and seconds.

The current number of customers in the café. The event and its sequence number.

You can print a negative sequence number for a leaving event.



10

Multithreaded Café: Design Questions

How to represent the cafe? How to represent customers?



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Multithreaded Café: Design Questions, cont’d

What threads? Locks? Condition objects? Shared data? Critical regions?



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Multithreaded Café: Design Questions, cont’d

How to implement the burger rule? How to start the simulation?



13

Message Passing

Message passing is another mechanism for thread synchronization and communication using the distributed model of parallel processing.

Threads are synchronized by waiting for messages to arrive._



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Message Passing

In its most basic form, message passing consists of two operations, send and receive.

Example C code:

(Assumes every sender knows its receiver and vice versa.)

void send(Process to, Message msg);void receive(Process from, Message msg);



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Message Passing: Go Language

Google’s Go language has built-in support for parallel programming via message passing.

The go statement starts a goroutine (thread).

Example:

starts function func as a goroutine.

go func()



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A channel type provides a synchronized channel between goroutines.

Example:

Send a message msg over channel ch:

Receive a message from channel ch:

Both block until the other goroutine is ready to communicate.

chan ch

ch <- msg

<- ch



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The select statement waits on a set of channels for communication from any channel._

cs 152: programming language paradigms may 7 class meeting department of computer science san jose...

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