1. Lecture 25 Syed Mansoor Sarwar
Operating Systems
Lecture 25
Syed Mansoor Sarwar

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Agenda for Today
Review of previous lecture
Dining philosophers problem
High-level synchronization constructs
Critical region
Monitor
Recap of lecture
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Review of Lecture 24
Counting semaphores
Classical synchronization problems
Bounded buffer problem
Readers and writers problem
Dining philosophers problem
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4. Dining Philosophers Problem
Five philosophers who spend their lives just thinking and eating.
Only five chopsticks are available to the philosophers
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5. Dining Philosophers Problem
Each philosopher thinks. When he becomes hungry, he sits down and picks up the two chopsticks that are closest to him and eats.
After a philosopher finishes eating, he puts down the chopsticks and starts to think.
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6. Dining-Philosophers Problem
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7. Dining-Philosophers Problem
Shared data : semaphore chopstick[5];
// Initialized to 1
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8. Dining-Philosophers Problem
Philosopher i
do {
wait(chopstick[i])
wait(chopstick[(i+1)% 5])
eat
signal(chopstick[i]);
signal(chopstick[(i+1)% 5])
think
} while (1);
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9. Possibility of Deadlock
If all philosophers become hungry at the same time and pick up their left chopstick, a deadlock occurs.
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Possible Solutions
Allow at most four philosophers to be sitting simultaneously at the table.
Allow a philosopher to pick up his/her chopsticks only if both chopsticks are available (to do this she must pick them up in a critical section)
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Possible Solutions
Use an asymmetric solution; that is, an odd philosopher picks up first her left chopstick, whereas an even philosopher picks up her right chopstick and then her left chopstick.
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12. Possibility of Starvation
Two philosophers who are fast eaters and fast thinkers, and lock the chopsticks before others every time.
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13. High-level Synchronization Constructs
Shift the responsibility of enforcing mutual exclusion from the programmer (where it resides when semaphores are used) to the compiler.
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Critical Regions
A critical region is a section of code that is always executed under mutual exclusion.
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Critical Regions
They consist of two parts:
Variables that must be accessed under mutual exclusion.
A new language statement that identifies a critical region in which the variables are accessed.
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The region Statement
A shared variable v of type T, is declared as:
v: shared T;
region v when B do S;
Variable v is accessed only inside S when B is true and when S is being executed, no other process can access variable v.
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The region Statement
When a process tries to execute the region statement, the Boolean expression B is evaluated. If B is true, statement S is executed. If it is false, the process is delayed until B becomes true and no other process is in the region associated with v.
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18. Bounded Buffer Example
Shared data
struct buffer {
int pool[n];
int count, in, out; }
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Producer Process
Producer process inserts nextp into the shared buffer
region buffer when (count < n) { pool[in] = nextp; in:= (in+1) % n; count++; }
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Consumer Process
Consumer process removes an item from the buffer and puts it in nextc
region buffer when (count > 0) { nextc = pool[out]; out = (out+1) % n; count--; }
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The region Statement
What happens when two region statements are executed simultaneously?
region v when B do S1;
region v when B do S2;
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Monitors
High-level synchronization construct that allows the safe sharing of an abstract data type among concurrent cooperating processes.
Only one process at a time is active within a monitor.
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23. Schematic View of a Monitor
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Monitors
monitor monitor-name {
shared variable declarations
procedure P1 (…) { }
procedure P2 (…) { } …
initialization code }
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25. Monitors with Condition Variables
Additional synchronization constructs are needed to model some synchronization problems. They can be modeled with condition variables.
condition x, y;
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26. Monitors with Condition Variables
Only two operations can be performed on condition variables, wait and signal.
x.wait: Process invoking this operation is suspended until another process invokes x.signal.
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27. Monitors with Condition Variables
x.signal: Resumes exactly one suspended process. If no process is suspended, this is a null operation, i.e., the state of x is unaffected.
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28. Monitor with Condition Variables
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29. Dining Philosophers Example
monitor dp {
enum {thinking, hungry, eating} state[5];
condition self[5];
void pickup(int i) // Following slides
void putdown(int i) // Following slides
void test(int i) // Following slides
void init() {
for (int i = 0; i < 5; i++)
state[i] = thinking; }
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Dining Philosophers
void pickup(int i) {
state[i] = hungry;
test(i);
if (state[i] != eating)
self[i].wait(); }
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Dining Philosophers
void putdown(int i) {
state[i] = thinking;
// test left and right // neighbors
test((i+4) % 5);
test((i+1) % 5); }
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Dining Philosophers
void test(int i) {
if ((state[(i+4)%5]!= eating)
&& (state[i] == hungry) &&
(state[(i+1)%5]!= eating)) {
state[i] = eating;
self[i].signal(); }
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Recap of Lecture
Dining philosophers problem
High-level language constructs
Critical region
Monitor
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34. Lecture 25 Syed Mansoor Sarwar
Operating Systems
Lecture 25
Syed Mansoor Sarwar