Semaphores
Semaphores
Concept
A semaphore is a kernel-managed integer variable used to coordinate access to shared resources among concurrent processes or threads. Two indivisible operations manipulate its value:
| Operation | Conceptual Action |
|---|---|
| wait (P) | Decrease the semaphore value. If it becomes negative, the caller is suspended. |
| signal (V) | Increase the semaphore value. If the new value is non-positive, one suspended caller is resumed. |
Atomicity of each operation prevents race conditions on the semaphore itself.
Implementation Styles
| Style | Core Idea | When a Process Must Wait | CPU Cost While Waiting | Typical Use Case |
|---|---|---|---|---|
| Spinlock (busy-waiting) | Caller repeatedly tests the semaphore in a tight loop. | Spins inside user code or a short kernel loop. | Consumes CPU cycles. | Ultra-short critical sections on multiprocessors where a context-switch would cost more than the spin. |
| Blocking (queue-based) | Caller is placed on a kernel queue linked to the semaphore. | Kernel deschedules the caller (WAITING state). | No CPU cycles consumed; context switch cost instead. | Longer critical sections or uniprocessor systems where CPU time must be preserved. |
Spinlocks favor latency; blocking semaphores favor overall CPU efficiency.
Binary vs Counting Semaphores
| Type | Initial Range | Meaning of Value | Main Purpose | Analogy |
|---|---|---|---|---|
| Binary Semaphore | 0 or 1 | 1 ⇒ resource free, 0 ⇒ resource busy | Acts like a mutex lock for a single shared resource or critical section. | Light switch: on or off. |
| Counting Semaphore | Any non-negative integer | Value = number of identical resource units available | Controls access to a pool of interchangeable resources (buffer slots, database connections, etc.). | Parking lot counter: slots left. |
Binary Semaphore Characteristics
- Ensures mutual exclusion for one resource.
- Only the first caller after a
signalproceeds; others must wait.
Counting Semaphore Characteristics
- Allows up to N concurrent accesses where N is the initial value.
- Becomes binary when initialized to 1.
Functional Roles
| Responsibility | How a Semaphore Fulfills It |
|---|---|
| Mutual Exclusion | A binary semaphore guards entry and exit of a critical section. |
| Resource Counting | A counting semaphore tracks how many identical resources remain; callers wait when none are free. |
| Producer–Consumer Synchronization | Two counting semaphores (empty, full) plus one binary semaphore (mutex) coordinate buffer space and mutual exclusion. |
| Ordering / Signaling | A semaphore initialized to 0 can force one thread to pause until another issues a signal, establishing sequence constraints. |
Semaphores thus provide a foundational, hardware-independent mechanism for coordinating concurrent activities in operating systems and multithreaded programs.
Last updated on