Embed Size (px)
Citation preview
COMP346/5461 - Operating Systems
Revision 1.3
Feb 3rd, 2010
10/27/2012 Serguei A. Mokhov, [email protected] 1
Topics Synchronization in Details
Semaphores
Introducing Semaphore.java
10/27/2012 Serguei A. Mokhov, [email protected] 2
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.
10/27/2012 Serguei A. Mokhov, [email protected] 3
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.
10/27/2012 Serguei A. Mokhov, [email protected] 4
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…
10/27/2012 Serguei A. Mokhov, [email protected] 5
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!
10/27/2012 Serguei A. Mokhov, [email protected] 6
A Typical Example (4)
10/27/2012 Serguei A. Mokhov, [email protected] 7
class John extends Thread {
run() {
balance = ATM.getBalance();
if(balance >= $200)
ATM.withdraw($200);
}
}
class Jane extends Thread {
run() {
balance = ATM.getBalance();
if(balance >= $300)
ATM.withdraw($300);
}
}
class ATM{ …
int withdraw(amount){
if(amount <= balance) {
balance = balance – amount;
return amount;
}
}
A trouble may occur at
the points marked with
the red arrows. The code
MUST NOT be interrupted
at those places.
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.
10/27/2012 Serguei A. Mokhov, [email protected] 8
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), sem.Wait() (a.k.a. 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 CS stuff.
When a process is over with the CS, it notifies the rest of the processes waiting on the same semaphore that they can go in by calling Signal(sem), Sem.signal() (a.k.a. V()).
The awakened process goes back to the ready queue to compete again to enter the CS.
10/27/2012 Serguei A. Mokhov, [email protected] 9
A Typical Example Solution
10/27/2012 Serguei A. Mokhov, [email protected] 10
class John extends Thread {
run() {
mutex.Wait();
balance = ATM.getBalance();
if(balance >= $200)
ATM.withdraw($200);
mutex.Signal();
}
}
class Jane extends Thread {
run() {
mutex.Wait();
balance = ATM.getBalance();
if(balance >= $300)
ATM.withdraw($300);
mutex.Signal();
}
}
class ATM{ …
Semaphore mutex = 1;
int withdraw(amount) {
if(amount <= balance) {
balance = balance – amount;
return amount;
}
}
A trouble may occur at
the points marked with
red. The code MUST NOT be
interrupted at those places.
Semaphores for Barrier Sync Take a look at the typical problem:
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.
10/27/2012 Serguei A. Mokhov, [email protected] 11
semaphore s1 = 0, s2 = 0;
process P1 process P2
<phase I> <phase I>
V (s1) V (s2)
P (s2) P (s1)
<phase II> <phase II>
Barrier Sync for Three Processes A possible copy-cat attempt:
Why it doesn’t work?
10/27/2012 Serguei A. Mokhov, [email protected] 12
semaphore s1 = 0, s2 = 0, s3 = 0;
process P1 process P2 process P3
<phase I> <phase I> <phase I>
V (s1) V (s2) V (s3)
P (s3) P (s1) P (s2)
<phase II> <phase II> <phase II>
Barrier Sync for Three Processes A possible copy-cat attempt:
Why it doesn’t work?
Scenario: 1-6 , process 2 even hasn’t started!
None of the requirements are met
10/27/2012 Serguei A. Mokhov, [email protected] 13
semaphore s1 = 0, s2 = 0, s3 = 0;
process P1 process P2 process P3
3 <phase I> <phase I> 1 <phase I>
4 V (s1) V (s2) 2 V (s3)
5 P (s3) P (s1) P (s2)
6 <phase II> <phase II> <phase II>
Barrier Sync for Three Processes (2) Another attempt:
What’s wrong now?
10/27/2012 Serguei A. Mokhov, [email protected] 14
semaphore s1 = 0, s2 = 0, s3 = 0;
process P1 process P2 process P3
<phase I> <phase I> <phase I>
P (s1) V (s1) V (s1)
P (s1) P (s2) P (s3)
V (s2) V (s3)
<phase II> <phase II> <phase II>
Barrier Sync for Three Processes (2) Another attempt:
What’s wrong now?
Scenario: 1-10, so far so good, but after…
The second requirement isn’t met
10/27/2012 Serguei A. Mokhov, [email protected] 15
semaphore s1 = 0, s2 = 0, s3 = 0;
process P1 process P2 process P3
7 <phase I> 4 <phase I> 1 <phase I>
8 P (s1) 5 V (s1) 2 V (s1)
9 P (s1) 6 P (s2) 3 P (s3)
10 V (s2) V (s3)
<phase II> <phase II> <phase II>
Barrier Sync for Three Processes (3)
Last attempt:
A bit “optimized”:
10/27/2012 Serguei A. Mokhov, [email protected] 16
semaphore s1 = 0, s2 = 0, s3 = 0;
process P1 process P2 process P3
<phase I> <phase I> <phase I>
P (s1) V (s1) V (s1)
P (s1) P (s2) P (s3)
<phase II> <phase II> <phase II>
V (s2) V (s3)
semaphore s1 = -1, s2 = 0, s3 = 0;
process P1 process P2 process P3
<phase I> <phase I> <phase I>
V (s1) V (s1)
P (s1) P (s2) P (s3)
<phase II> <phase II> <phase II>
V (s2) V (s3)
Barrier Sync: Need for the General Solution
Problem with the proposed solution: # of semaphores == # of processes.
Semaphores as any other resource are limited and take space => overhead
Imagine you need to sync 10 processes in the same manner? 100? 1000? Complete mess and a high possibility of a deadlock!
10/27/2012 Serguei A. Mokhov, [email protected] 17
Barrier Sync: Need for the General Solution (2)
Attempt for the first requirement:
The second requirement is left as an exercise to the curious student :-)
10/27/2012 Serguei A. Mokhov, [email protected] 18
semaphore s1 = -n + 2, s2 = 0;
process P1 process P2 ... process Pn
<phase I> <phase I> ... <phase I>
P (s1) V (s1) ... V (s1)
V (s2) P (s2) ... P (s2)
V (s2) ... V (s2)
<phase II> <phase II> ... <phase II>
Introducing the Semaphore Class
NOTE: Operations Signal and Wait are
guaranteed to be atomic!
10/27/2012 Serguei A. Mokhov, [email protected] 19
class Semaphore { private int value; public Semaphore(int value) { this.value = value; } public Semaphore() { this(0); } ...
Introducing the Semaphore Class (2)
10/27/2012 Serguei A. Mokhov, [email protected] 20
... public synchronized void Wait() { while(this.value <= 0) { try { wait(); } catch(InterruptedException e) { System.out.println ( "Semaphore::Wait() …” + e.getMessage() ); e.printStackTrace(); } } this.value--; } ...
Introducing the Semaphore Class (3)
10/27/2012 Serguei A. Mokhov, [email protected] 21
... public synchronized void Signal() { ++this.value; notify(); } public synchronized void P() { this.Wait(); } public synchronized void V() { this.Signal(); } }
References http://users.encs.concordia.ca/~mokhov/comp346
/
10/27/2012 Serguei A. Mokhov, [email protected] 22
