In computer science, the sleeping barber problem is a classic inter-process communication and synchronization problem between multiple operating system processes. The problem is analogous to that of keeping a barber working when there are customers, resting when there are none, and doing so in an orderly manner.
The Sleeping Barber Problem is often attributed to Edsger Dijkstra (1965), one of the pioneers in computer science.
Problem Statement
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| Sleeping Barber problem |
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| Flow chart for Sleeping barber problem |
The analogy is based upon a hypothetical barber shop with one barber. The barber has one barber chair and a waiting room with a number of chairs in it. When the barber finishes cutting a customer's hair, he dismisses the customer and then goes to the waiting room to see if there are other customers waiting. If there are, he brings one of them back to the chair and cuts his hair. If there are no other customers waiting, he returns to his chair and sleeps in it.
Each customer, when he arrives, looks to see what the barber is doing. If the barber is sleeping, then the customer wakes him up and sits in the chair. If the barber is cutting hair, then the customer goes to the waiting room. If there is a free chair in the waiting room, the customer sits in it and waits his turn. If there is no free chair, then the customer leaves.
Based on a naïve analysis, the above description should ensure that the shop functions correctly, with the barber cutting the hair of anyone who arrives until there are no more customers, and then sleeping until the next customer arrives. In practice, there are a number of problems that can occur that are illustrative of general scheduling problems.
Boundary Cases
à A customer may arrive and observe that the barber is cutting hair, so he goes to the waiting room. While he is on his way, the barber finishes the haircut he is doing and goes to check the waiting room. Since there is no one there (the customer not having arrived yet), he goes back to his chair and sleeps. The barber is now waiting for a customer and the customer is waiting for the barber.
à Two customers may arrive at the same time when there happens to be a single seat in the waiting room. They observe that the barber is cutting hair, go to the waiting room, and both attempt to occupy the single chair.
Implementation
semaphore.sem_wait() locks the semaphore.sem_post() unlocks it.
Semaphore Customers = 0;
Semaphore Barber = 0;
Mutex accessSeats = 1;
int NumberOfFreeSeats = N;
Barber {
while(1) {
/* waits for a customer (sleeps). */
sem_wait(Customers);
/* mutex to protect the number of available seats.*/
sem_wait(accessSeats);
/* a chair gets free.*/
NumberOfFreeSeats++;
/* bring customer for haircut.*/
sem_post(Barber);
/* release the mutex on the chair.*/
sem_post(accessSeats);
/* barber is cutting hair.*/
}
}
Customer {
while(1) {
/* protects seats so only 1 thread tries to sit in a chair if that's the case.*/
sem_wait(accessSeats);
if(NumberOfFreeSeats > 0) {
/* sitting down.*/
NumberOfFreeSeats--;
/* notify the barber. */
sem_post(Customers);
/* release the lock */
sem_post(accessSeats);
/* wait in the waiting room if barber is busy. */
sem_wait(Barber);
// customer is having hair cut
} else {
/* release the lock */
sem_post(accessSeats);
// customer leaves
}
}
}
A multiple sleeping barbers problem has the additional complexity of coordinating several barbers among the waiting customers.
References:
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