1. Operating systems
Carmen Suárez
Abraham Silva
Gaby García

2. Problem unbounded-bufer (producer-
consumer)
Producer and Consumer share a common buffer
But not everything is happiness and perfection...

3. The producer tries to get to the buffer and add
elements but it’s already full

4. The consumer tries to get to the buffer but it’s
empty

5. When the producer and the consumer go to sleep, and don't
receive calls from anyone. A deadlock occurs
Producer Consumer

6. The solution to this problem is:
The use of semaphores or locks. A semaphore will regulate
the production and the consuming of the buffer, giving
access to only one process at a time.
http://bit.ly/rlGtNe

7. Pseudocode
define N 100
typedef int semaphore;
semaphore mutex = 1; void consumer(void){
semaphore empaty = N; int element:
semaphore full = 0; while(true){
void producer(void){ down(full);
int element; down(mutex);
while(true){ element=down_element();
element = produce_element();
down(empaty);
up(mutex);
down(full); up(empaty);
insert_element(element); consum_element(element)-,
up(mutex); }
up(full);
} }
}

8. http://bit.ly/rlGtNe

9. Dining Philosophers

10. First
"possible" solution
#define N 5
void filosofo(int i)
{
while(TRUE){
• think
• take left fork
• take right fork
• eat
• leave left fork
• leave right fork
}
}

11. Deadlock
• All of the philosophers are holding a fork
while waiting for other fork to be released.
• There are no forks available anymore.
• They can’t eat because they need two forks.
• Starvation

12. Eliminate deadlocks
• TryAcquire
• Release all the locks, wait and then
reacquire them in the right order
• Order the locks: Establish a time for locks

13. Solution

14. Suspension algorithms
There are three components of the time used in a process:
• Spinning: It’s the time the thread takes to check whether
the lock is available or not. During this time the process is
not executing.
• Suspending: Is the amount of time the thread is
suspended or sleeping, waiting for the lock to be available
• Resuming: Is the amount of time the thread takes to
restart the execution after acquiring the lock.

15. Optimal algorithm
• The lock is released in less than the suspend and resume
time, the process spins until it acquires the lock.

16. Optimal Algorithm
• The lock is released in more than the suspend and resume
time, the process should suspend immediately.

17. Other worse algorithm than SR
• if it spins for more than suspend and resume time, then suspend. The
worst case is when lock is free after a thread start the suspension. The
SR algorithm will cost the suspend and resume time but will spin less time
than this algorithm.

18. Bibliography
• Martin Rinard OS Notes http://people.csail.mit.edu/rinard/osnotes/
• Sistemas Operativos Modernos, Andrew S. Tanenbaum, 3rd Ed,
Prentice Hall.
• Dining philosophers image
HTTP://UPLOAD.WIKIMEDIA.ORG/WIKIPEDIA/COMMONS/THUMB/6/6A/DINING_ PHIL
OSOPHERS.PNG/200PX-DINING_PHILOSOPHERS.PNG
• Deadlock image
http://www.cs.rpi.edu/academics/courses/fall04/os/c10/deadlock2.gif

19. Gracias por su atencion 
Blog:
http://os-sistemasoperativos.blogspot.com