110
Operating Systems [CS-604] Lecture No. 23
Operating Systems
Lecture No. 23
Reading Material
๎๎Chapter 7 of the textbook
๎๎Lecture 23 on Virtual TV
Summary
๎๎Busy waiting
๎๎New definition of semaphore
๎๎Process synchronization
๎๎Problems with the use of semaphore: deadlock, starvation, and violation of mutual
exclusion
Semaphores
The main disadvantage of the semaphore discussed in the previous section is that they all
require busy waiting. While a process is in its critical section, any other process that tries
to enter its critical section must loop continuously in the entry code. This continual
looping is clearly a problem in a real multiprogramming system, where a single CPU is
shared among many processes. Busy waiting wastes CPU cycles that some other process
may be able to use productively. This type of semaphore is also called a spinlock
(because the process spins while waiting for the lock). Spinlocks are useful in
multiprocessor systems. The advantage of a spinlock is that no context switch is required
when a process must wait on a lock, and a context switch may take considerable time.
This, when locks are expected to be held for short times, spinlocks are useful.
To overcome the need for busy waiting, we can modify the definition of semaphore
and the wait and signal operations on it. When a process executes the wait operation and
finds that the semaphore value is not positive, it must wait. However, rather than busy
waiting, the process can block itself. The block operation places a process into a waiting
queue associated with the semaphore, and the state of the process is switched to the
waiting state. Then, control is transferred to the CPU scheduler, which selects another
process to execute.
A process that is blocked, waiting on a semaphore S, should be restarted when some
other process executes a signal operation. The process is restarted by a wakeup operation,
which changes the process from the waiting state to the ready state. The process is then
placed in the ready queue. (The CPU may or may not be switched from the running
process to the newly ready process, depending on the CPU scheduling algorithm.)
Such an implementation of a semaphore is as follows:
typedef struct {
int value;
struct process *L;
} semaphore;
docsity.com
