Writing concurrent programs using basic constructs such as locks, monitors, Semaphores, Mutex, and conditional variables

1. Writing Concurrent Programs
Using Basic Constructs
CS4532 Concurrent Programming
Dilum Bandara
Dilum.Bandara@uom.lk
Some sides adapted from Dr. Srinath Perera, WSO2 and
“The Art of Multiprocessor Programming” by Maurice Herlihy & Nir Shavit Slightly

2. Outline
 Locks
 Monitors
 Semaphores
 Mutex
 Conditional Variables
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3. Why Concurrent Programming is
Hard?
 Threads/processes often need to share data
 Data consistency
 Enforced through critical sections/atomic operations
 E.g., preventing race conditions
 Cooperation (synchronization)
 Control the way programs makes progress
 E.g., producer consumer problem
 This is complicated by lack of timing
guarantees
 Cache misses, page faults, context switches
 Threads are scheduled by OS or thread library
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Source: www.grype.ca

4. How Things Can Go Wrong?
int balance = 200;
Thread1{
run(){
if(balance >= 100){
balance = balance - 100;
System.out.println("Withdraw 100, balance "+ balance);
}else{
System.out.println("Insufficient Balance "+ balance);
}
}
}
Thread2{
run(){
if(balance >= 200){
balance = balance - 200;
System.out.println("Withdraw 200, balance "+ balance);
}else{
System.out.println("Insufficient Balance "+ balance);
}
}
} 4

5. Mutual Exclusion & Synchronization
 Mutual Exclusion
 Ensure at most 1 thread can be in critical section at
any given time
 Achieved via Atomic Operations
 Use Locks, Monitors, & Mutexs
 Synchronization
 Making sure different threads are in sync with each
other
 E.g, producer consumer problem, reads & writers problem
 Use Semaphores & some patterns
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6. Withdraw Problem with Locks –
Alice & Bob have a shared account
int ac1 = 200;
int ac2 = 100;
Alice:
if(ac1 >= 100){
ac1 = ac1 - 100;
}
Print (balance)
Bob:
if(ac1 >= 100){
ac1 = ac1 - 100;
}
Print (balance)
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7. Alice & Bob have 2 shared accounts,
how to extend above to 2 accounts?
int ac1 = 200;
int ac2 = 100;
Alice:
if(ac1 >= 100){
ac1 = ac1 - 100;
}
if(ac2 >= 100){
ac2 = ac2 - 100;
}
Print (balance)
Bob:
if(ac1 >= 100){
ac1 = ac1 - 100;
}
if(ac2 >= 100){
ac2 = ac2 - 100;
}
Print (balance) 7

8. Withdraw Problem with Monitors
Account{
int balance;
void synchronized withdraw(int amount){
if(balance >= amount){
balance = balance - amount;
}
}
}
 “Synchronized” keyword of Java does the same thing
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9. Condition Variables
 Allows checking for a condition
 Operations are called wait() & signal()
 Idea is similar to sleep() & wakeup()
 wait() & notify() in Java
 Usually wake up only 1 blocked thread
 Also have a broadcast(), which wakes up everyone
 Threads can wait by calling wait()
 1 of them is released when signal() is called
 Once signaled, the event is lost
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10. Exercise – Condition Variables
 Write a program so that program will always
print A before B, using condition variable(s)
 i.e., wait() & notify()
Thread 1 Thread 2
Print A Print B
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11. Exercise – Condition Variables (Cont.)
CV cv;
Thread 1 Thread 2
Print A cv.wait()
cv.notify() Print B
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Will this work?
What if notify() before wait()?

12. Exercise – Condition Variables (Cont.)
CV cv;
Lock l;
Boolean done = False;
Thread 1 Thread 2
Print A l.lock()
l.lock() if(!done)
done = True cv.wait()
l.unlock() l.unlock()
cv.notify() Print B
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Is the lock essential?
Will this work?

13. Exercise – Condition Variables (Cont.)
CV cv;
Lock l;
Boolean done = False;
Thread 1 Thread 2
Print A Synchronized {
Synchronized { if(!done)
done = True cv.wait()
} }
cv.notify() Print B
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Is the lock essential?
Will this work?

14. Implement a Semaphore with Locks &
Condition Variables
up(){
synchronized(){
val++;
if (val <= 0){
cv.signal();
}
}
}
down(){
synchronized(){
val--;
if(val < 0){
cv.wait();
}
}
}
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CV cv;
int val = 0;
Will this work?

15. Parallel Program Design
 Is signaling required?
 Decide what semaphores you need, if any
 Decide any state you need
 What states need mutual exclusion?
 Problem state vs. signaling state
 Write program
 Test by considering different scenarios
 Sequential case & concurrent case
 Any race conditions & deadlocks?
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