COMP346 Winter Tutorial 4 Synchronization Semaphores

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1 COMP346 Winter 2015 Tutorial 4 Synchronization Semaphores 1

2 Topics Synchronization in Details Semaphores Introducing Semaphore.java 2

3 Synchronization What is it? An act of communication between unrelated processes to sync their activities to achieve some goals and solve some common problems of multiprogramming: Mutual exclusion and critical sections Specific execution order must be maintained Improper sync may cause a very big problem in all Operating Systems a deadlock. 3

4 Synchronization Improper synchronization leads to a corruption to shared file / object / data. Deadlock, one process holds a lock for long long time while other process is waiting for that lock. Proper synchronization helps to avoid race conditions. Our goal is to Synchronize between process and Maximize concurrency. Make the CPU busy doing useful tasks most of the time. 4

5 A Typical Example Shared account (between spouses John and Jane) Current account balance: $400 John and Jane happened to be at the ATM in different places, but at almost the same time. Let say John wants to withdraw $200, and Jane wants to withdraw $300. First scenario: both of them see the current balance of $400 and relatively at the same time perform request to withdraw. John goes first 5

6 A Typical Example (2) A DB system takes $400, subtracts $200, and gets interrupted (e.g. network congestion), thus the remaining balance of $200 wasn t recorded yet. Jane s request goes through and the system writes down $100 balance remaining. Then John s request finally goes through, and system updates the balance to $200. The bank is a victim in that case because John and Jane were able to withdraw $500 and have $200 remaining, when initially account s balance was $400! 6

7 A Typical Example (4) class John extends Thread run() balance = ATM.getBalance(); if(balance >= $200) ATM.withdraw($200); class ATM int withdraw(amount) if(amount <= balance) balance = balance amount; return amount; class Jane extends Thread run() balance = ATM.getBalance(); if(balance >= $300) ATM.withdraw($300); A trouble may occur at the points marked with the red arrows. The code MUST NOT be interrupted at those places. 7

8 Solution: Use Semaphores Obvious: make the critical section part atomic. One way of doing it: Semaphores Semaphores are system-wide OS objects (also resources) used to Protect critical section (mutexes for Mutual Exclusion), Coordinate other process activities. Semaphores are NOT shared memory segments! But they both are often used together. 8

9 Semaphores for CS There are two main operations on semaphores Wait() and Signal(). A process wishing to enter the critical section tries to acquire the semaphore (a lock in a human world) by calling Wait(sem),P(). If the lock isn t there (i.e. in use ), the execution of the process calling Wait() is suspended (put asleep). Otherwise, it acquires the semaphore and does the critical section stuff. When a process is over with the critical section, it notifies the rest of the processes waiting on the same semaphore that they can go in by calling Signal(sem),V(). The awakened process goes back to the ready queue to compete again to enter the critical section. 9

10 A Typical Example Solution class John extends Thread run() mutex.wait(); balance = ATM.getBalance(); if(balance >= $200) ATM.withdraw($200); mutex.signal(); class ATM Semaphore mutex = 1; int withdraw(amount) if(amount <= balance) balance = balance amount; return amount; class Jane extends Thread run() mutex.wait(); balance = ATM.getBalance(); if(balance >= $300) ATM.withdraw($300); mutex.signal(); A trouble may occur at the points marked with red. The code MUST NOT be interrupted at those places. 10

11 Introducing the Semaphore Class NOTE: Operations Signal and Wait are guaranteed to be atomic! class Semaphore private int value; public Semaphore(int value) this.value = value; public Semaphore() this(0);... 11

12 Introducing the Semaphore Class (2)... public synchronized void Wait() while(this.value <= 0) try wait(); catch(interruptedexception e) System.out.println ("Semaphore::Wait() + e.getmessage());... this.value--; e.printstacktrace(); 12

13 Introducing the Semaphore Class (3)... public synchronized void Signal() ++this.value; notify(); 13

14 public class Semaphore public Semaphore() value = 0; public Semaphore(int v) value = v; public synchronized void P( ) while( value <= 0) try wait( ); catch(interruptedexception e) value--; public synchronized void V( ) ++value; notify( ); private int value; 14

15 Semaphore Initial Values The mutex is initialized to one to allow only one thread into the critical section at any time 15

16 Barrier Synchronization Take a look at the typical problem: process P1 process P2 <phase I> <phase I> sync <phase II> sync <phase II> all processes must finish their phase I before any of them starts phase II processes must proceed to their phase II in a specific order, for example: 1, 2, 3 This is called barrier synchronization. 16

17 Barrier Example Everyone meet at University at 7:00; once you get there, stay there until all team members shows up, and then we ll figure out what we re doing next Until all team members shows up is a barrier point 17

18 public class Barrier private int elements; public Barrier(int elements) this.elements = elements; //Await means the execution unit has reached to a barrier point public synchronized void await() throws InterruptedException elements; //If I am the last then notify all waiting ones if(elements == 0) notifyall(); //Otherwise, waits until all show up else while(elements > 0) wait(); 18

19 Barrier using Semaphore await mutex.wait() count = count + 1 if count == N barrier.signal() mutex.signal barrier.wait() barrier.signal() 19

20 JDK CyclicBarrier java.util.concurrent.cyclicbarrier 20

21 class BarrierExample static class MyThread1 implements Runnable public MyThread1(Barrier barrier) this.barrier = barrier; public void run() try Thread.sleep(1000); System.out.println("MyThread1 waiting on barrier"); barrier.await(); System.out.println("Barrier has been reached"); catch (InterruptedException ie) System.out.println(ie); private Barrier barrier; 21

22 static class MyThread2 implements Runnable Barrier barrier; public MyThread2(Barrier barrier) this.barrier = barrier; public void run() try Thread.sleep(3000); System.out.println("MyThread2 waiting for barrier\n"); barrier.await(); System.out.println( Barrier reached \n"); catch (InterruptedException ie) System.out.println(ie); 22

23 public static void main(string[] args)throws InterruptedException /* * MyThread1 MyThread2 * * BR.await();... *... BR.await(); */ Barrier BR = new Barrier(); Thread t1 = new Thread(new BarrierExample.MyThread1(BR)); Thread t2 = new Thread(new BarrierExample.MyThread2(BR)); t1.start(); t2.start(); t1.join(); t2.join(); 23

24 References 6/ oncurrency/sync.html 24

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