1. Synchronization, part 3 Monitors, classical sync. problems
Operating Systems, 132
Practical Session 7
Synchronization, part 3 Monitors, classical sync. problems

2. Java synchronized methods
Monitor
Monitor – a synchronization primitive.
A monitor is a collection of procedures, variables and data structures, grouped together.
Mutual Exclusion – only one process can be active within a monitor at any given time.
Programming language construct! The compiler of the language will know that monitors procedures are different than other procedures, and will treat them differently. That means that the compiler is in charge of the mutual exclusion implementation.
Java synchronized methods

3. A quick recap Condition variables
A way for processes to block when they can’t continue.
Despite its name, it is used to indicate an event and not as a regular valued variable. A CV is not a counter!
Two operations: wait, signal. Wait: causes the process to block, and allows entry of other threads to the monitor. Signal: More than just one alternative:
Hoare type monitors: The signaler yields the monitor to the released thread. Signal will be the last operation within the monitor, which wakes up waiting processes (waiting on the same variable). This is not true for Java.
Mesa type monitors: The signaling process is allowed to continue.
Wakes up a single thread that is waiting on this object's monitor. If any threads are waiting on this object, one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. A thread waits on an object's monitor by calling one of the wait methods. The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened thread enjoys no reliable privilege or disadvantage in being the next thread to lock this object.

4. The Sleeping Barber
Write a solution to the sleeping barber problem using monitors and condition variables.
Reminder, the sleeping barber:
The barber cuts peoples hair in his shop, which has 2 doors – entrance and exit. When people are in his shop, he gives them a hair cut, one at a time. When none are in his shop, he sleeps on his chair.
When a customer arrives and finds the barber sleeping, he awakens him and sits in the barber’s chair to receive his haircut. After the cut is done, the barber sees the customer out through the exit door.
If the barber is busy when a customer arrives, the customer waits in one of the chairs in the shop. If all are occupied, he goes away.
After serving a customer the barber looks to see if any are waiting and if so proceeds to serve one of them. Otherwise, he sleeps again in his chair.

5. The Sleeping Barber barbershop: monitor
waiting : integer := 0; % customers waiting for haircut
customers : condition; % used by barber, wait for a customer
barber : condition; % used by customer, wait for barber
procedure seek-customer( ) % called by the barber
begin
if waiting==0 then
WAIT (customers); % sleeps if no customers
cut-hair();
waiting = waiting-1; % one less customer waiting
SIGNAL (barber); % free a waiting customer
end seek-customer;

6. The Sleeping Barber procedure get-haircut( ) % called by a customer
begin
% is there a free chair to sit and wait?
% if no free chairs just go away
if waiting < chairs then {
waiting = waiting+1; % one more customer waiting
SIGNAL (customers) % if the barber is asleep
WAIT (barber); % wait for turn with the barber }
end get-haircut;
end barbershop; % End of monitor

7. Java and monitors – Exercise
Write a code snippet in Java which will enforce a FIFO waking order (i.e., create a class in Java that will allow a programmer fair synchronization)

8. Java and monitors – Solution
class WaitObject { boolean released = false; // this flag avoids race!!! synchronized void doWait() { while (! released) { try { wait(); } catch (InterruptedException ie) {} // ignore it } } synchronized void doNotify(){ if (! released){ released = true; notify(); } } }

9. Java and monitors – Solution
class CriticalSection { // critical section that preserves FIFO private Vector _waiting; // wait list private boolean _busy; // someone in critical section public CriticalSection() { // constructor _waiting = new Vector(); // create wait list _busy = false; // no one is in the CS now. } public void enter() { WaitObject my_lock = null; synchronized(this){ if (! _busy) { _busy = true; return; }else { my_lock = new WaitObject(); // create my unique lock _waiting.add(my_lock); // add to waiting list } } my_lock.doWait(); // wait on lock }

10. Java and monitors – Solution
public synchronized void leave() { if (_waiting.size() > 0) { // someone is waiting WaitObject o = (WaitObject)_waiting.elementAt(0); _waiting.removeElementAt(0); o.doNotify(); } else { _busy = false; }

11. Java and monitors – Discussion
Does this code guarantee a FIFO waking order which is equivalent to the order in which threads reached the critical section entrance?
public void enter() { WaitObject my_lock = null; synchronized(this){ … } my_lock.doWait(); } It does, however, guarantee a waking order on those threads already sleeping.
NO!
What happens when multiple threads attempt to enter at the same time?

12. Hoare monitors, Moed A 2010
Monitor ProducerConsumer 1. condition full, empty 2. integer count initially 0 3. procedure insert(item: integer) 4. begin 5. if count=N then wait(full) 6. insert_item(item) 7. count=count+1 8. if count > 0 then signal(empty) 9. end 10. procedure remove: integer 11. begin 12. if count=0 then wait(empty) 13. remove=remove_item() 14. count=count if count < N then signal(full) 16. end end Monitor

13. Hoare monitors, Moed A 2010
Recall that a monitor is a Hoare typed monitor if the following conditions hold:
Whenever a ‘signal’ is received by a thread t it is guaranteed that thread t will be the next thread to enter the monitor
‘signal’ is the last action taken within the monitor and a thread executing it will immediately release the monitor afterward
Consider the above implementation for the producer consumer problem with N items and prove or disprove the following claim: The above implementation is correct even if used with non Hoare typed monitor

14. Hoare monitors, Moed A 2010
The following scenario contradicts the above claim: Assume an N sized buffer, already full. Now, consider a producer p1 attempting to add a new item. Reaching line 5 p1 must wait for some consumer to remove an item before it may continue. Now, we introduce a consumer c1 which successfully executes the remove function – including line 15 (signal). However, unlike a regular Hoare typed monitor, instead of having p1 take control of the monitor a new producer p2 is added. The monitor is taken by p2 which executes the entire insert() function. Finally, p1 is granted CPU time and continues its run (line 6 to the end of the function). Result: an extra item is added to the already full buffer

15. The one-way tunnel problem
Allows any number of processes in the same direction
If there is traffic in the opposite direction – have to wait
A special case of readers/writers

16. The one way tunnel (exam 2004)
The one way tunnel solution: int count[2]; Semaphore mutex=1, busy=1; Semaphore waiting[2]={1,1};
void arrive(int direction){
down(&waiting[direction]);
down(&mutex);
count[direction]+=1;
if (count[direction]==1){
up(&mutex);
down(&busy);
} else up(&mutex);
up(&waiting[direction]); }
void leave(int direction){
down(&mutex);
count[direction]-=1;
if (count[direction]==0){
up(&busy); }
up(&mutex);

17. The one way tunnel (exam 2004)
Add changes to the one way tunnel solution so that vehicles from direction “0” are always preferred. Vehicles from direction “1” will only enter the tunnel if no vehicles at “0” are waiting.
Add changes to the one way tunnel solution so that there will be no starvation. If vehicles are present on both “0” and “1” they will take alternate turns in entering the tunnel. When there are vehicles coming from only one direction, they can pass through with no limitations.
Notes:
you may only use integers and binary semaphores.
You may change the solution for a single direction at a time.

18. The one way tunnel (exam 2004)
1. Code for direction 0: Semaphore new_mutex=1; void arrive() { down(&waiting[0]); down(&mutex); count[0]++; if(count[0] == 1) { up(&mutex); down(&new_mutex); down(&busy); up(&new_mutex); { else up(&mutex); up(waiting[0]); }
Code for direction 1:
void arrive(int direction) { down(waiting[1]); down(&new_mutex); down(&mutex); count[1]++; if(count[1] == 1) { up(&mutex); up(waiting[1]); down(busy); { else { up(&mutex); up(waiting[1]); { up(&new_mutex); }
The leave function remains the same
Note: direction 0 attempts to do a down to new_mutex only when it attempts to grab the tunnel. In contrast, any access to the tunnel by direction 1 processes is regulated with downs and ups on new_mutex.

19. The one way tunnel (exam 2004)
For both directions: void arrive(int direction) { down(waiting[direction]); down(new_mutex); down(mutex); count[direction]++; if(count[direction] == 1) { up(mutex); up(waiting[direction]); down(busy); { else { up(mutex); up(waiting[direction]); { up(new_mutex); }
void leave(int direction){
down(&mutex);
count[direction]-=1;
if (count[direction]==0){
up(&busy); }
up(&mutex);
Assuming semaphore fairness (new_mutex)

20. One way, convoy (midterm 2008)
In the following question you must implement a solution to the convoy problem using only semaphores (and regular variables). In this problem, each thread represent a vehicle. The vehicles must go through a one way tunnel, but unlike the tunnel problem, here vehicles may only cross the tunnel in groups of exactly 5 (all in the same direction). A group of another 5 vehicles (from the same or opposite direction) may cross the tunnel again, only after the previous group of 5 vehicles comes out of it. The general code structure is as follows:
Variable Declaration
PrepareToCross(int direction)
CROSS
DoneWithCrossing(int direction)

21. One way, convoy (midterm 2008)
Implement PrepareToCross() and DoneWithCrossing(). For your implementation you may only use semaphores (counting or binary) and regular variables. No busy-waiting is allowed.
We say a thread is passing the tunnel if it completed its call to PrepareToCross and haven’t called DoneWithCrossing or if it is still in PrepareToCross but is no longer waiting on a semaphore, and when it will receive CPU time it may complete the procedure without waiting.
Your implementation must satisfy the following conditions:
Mutual Exclusion – threads from different direction may never be in the tunnel at the same time.
Threads may only cross in groups of 5. When the first is leaving PrepareToCross, there are exactly 4 others which are passing the tunnel as well.
Progress – Whenever there are 5 (or more) threads from the same direction waiting to cross the tunnel, then eventually, they will.

22. One way, convoy (midterm 2008)
We will use the following: Counting Semaphore waitingToCross[]={5,5} Counting Semaphore barrier[]={0,0} Binary Semaphore busy=1 Binary Semaphore mutex=1 int waitingCount[]={0,0} int passed=0

23. One way, convoy (midterm 2008)
PrepareToCross(int i){ down(&waitingToCross[i]); down(&mutex); waitingCount[i]++; If (waitingCount[i]<5){ up(&mutex); down(&barrier[i]); } else { waitingCount[i]=0; down(&busy); for (int k=0; k<4; k++) up(&barrier[i]); } up(&waitingToCross[i]);
DoneWithCrossing(int i){ down(&mutex); passed++; if (passed==5){ passed=0; up(&busy); } up(&mutex);

24. A quick recap Message passing
Used on distributed systems (when there is no shared memory).
Uses send(), and receive() system calls.
Introduces a new set of problems, such as acknowledgments, sequencing, addressing, authentication, etc’…

25. Reader/Writer problem with MP
Write a solution to the reader/writer problem using Message Passing.
Assume the following:
Three groups of processes: readers, writer, manager.
Multiple readers may access the DB simultaneously.
A writer needs exclusive access to the DB.
Readers have preference.

26. Reader/Writer problem with MP
Reader: while (true){ SEND (manager, start_read); RECEIVE (manager, msg); % wait for confirmation read_db(); SEND (manager, end_read); use_data(); } Writer: while (true){ generate_data(); SEND (manager, start_write); RECEIVE (manager, msg); % wait for confirmation write_to_db(); SEND (manager, end_write);

27. Reader/Writer problem with MP
Manager: int readers_count=0; % number of readers accessing DB boolean writing=false; % writing flag Message msg; Queue readQ, writeQ; % Queues for waiting readers and writers ProcessID src; % pid while (true){ src = RECEIVE(msg); switch msg.type{ case (start_read): if (not writing){ send(src, ok); readers_count++; } else readQ.add(src);

28. Reader/Writer problem with MP
case (end_read): readers_count--; if (readers_count==0 && not writeQ.empty){ src=writeQ.remove; SEND (src, ok); writing = true; } case (start_write): if (readers_count==0 && not writing){ SEND (src, ok); writing = true; } else writeQ.add(src);

29. Reader/Writer problem with MP
case (end_write): writing = false; if (readQ.empty && not writeQ.empty){ src = writeQ.remove; SEND(src, ok); writing = true; } else { while (not readQ.empty){ src = readQ.remove; send(src, ok); readers_count++; } } } % switch } % while