Download - 6.5 Semaphore Can only be accessed via two indivisible (atomic) operations wait (S) { while S
6.5 Semaphore
Can only be accessed via two indivisible (atomic) operations
wait (S) { while S <= 0 ; // no-op S--;}
signal (S) { S++;}
6.5 Semaphore
Binary semaphore –integer value can range only between 0 and 1; can be simpler to implement
Counting semaphore –integer value can range over an unrestricted domain
6.5 Semaphore
The main disadvantage of the semaphore is that it requires busy waiting, which wastes CPU cycle that some other process might be able to use productively
This type of semaphore is also called a spinlock because the process “spins” while waiting for the lock
6.5 Semaphore
To overcome the busy waiting problem, we create two more operations:
block–place the process invoking the operation on the appropriate waiting queue.
wakeup –remove one of processes in the waiting queue and place it in the ready queue.
6.6 Classical Problems of Synchronization
Bounded-Buffer Problem Readers and Writers Problem Dining-Philosophers Problem
6.6 Classical Problems of Synchronization
N buffers, each can hold one item Semaphore mutex initialized to the
value 1 Semaphore full initialized to the
value 0 Semaphore empty initialized to the
value N.
Bounded Buffer Problem
Bounded Buffer Problem
Readers-Writers Problem
A data set is shared among a number of concurrent processes
Readers –only read the data set; they do not perform any updates
Writers –can both read and write.
First readers-writers problem: requires that no reader will be kept waiting unless a writer has already obtained permission to use the shared object
Readers-Writers Problem
Shared Data Data set Semaphore mutex initialized to 1. Semaphore wrt initialized to 1. Integer read count initialized to 0.
Readers Readers-Writers Problem
Readers Readers-Writers Problem
Dining-Philosophers Problem The philosophers share a circular table
surrounded by five chairs, each belonging to one philosopher
In the center of table is a bowl of rice, and the table is laid with 5 single chopsticks
From time to time, a philosopher gets hungry and tries to pick up the two chopsticks that are closest to her
When a hungry philosopher has both her chopsticks at the same time, she eats without releasing her chopsticks
When she is finished eating, she puts down both her chopsticks and starts thinking
Dining-Philosophers Problem
Dining-Philosophers Problem
Dining-Philosophers Problem
Methods to avoid deadlock: Allow at most four philosophers to
be sitting simultaneously Allow a philosopher to pick up her
chopsticks only if both chopsticks are available (pick them up is a critical section)
Problems with Semaphores
signal (mutex) //violate mutual exclusive critical sectionwait (mutex)
wait (mutex) //deadlock occurs critical sectionwait (mutex)
Omitting of wait (mutex) or signal (mutex) (or both)
Monitors
A high-level abstraction that provides a convenient and effective mechanism for process synchronization
Only one process may be active within the monitor at a time
Syntax of Monitor
monitor monitor-name{ // shared variable declarations procedure P1 (…) { …. } … procedure Pn(…) {……} Initialization code ( ….) { …} …}
Monitor
Condition Variables However, the monitor construct, as defined so
far, is not powerful enough We need to define one or more variables of
type condition: condition x, y;
Two operations on a condition variable:
x.wait() –a process that invokes the operation is suspended.
x.signal() –resumes one of processes (if any) that invoked x.wait()
Monitor with Condition Variables
Syntax of Monitor
monitor monitor-name{ // shared variable declarations procedure P1 (…) { …. } … procedure Pn(…) {……} Initialization code ( ….) { …} …}
Solution to Dining Philosophers
Solution to Dining Philosophers
Solution to Dining Philosophers
Each philosopher I invokes the operations pickup() and putdown() in the following sequence:
dp.pickup(i) EATdp.putdown(i)
Monitor Implementation Using Semaphores
Monitor Implementation Using Semaphores
Monitor Implementation Using Semaphores