Threads Notes & Formulas

1. Definition

A thread is the smallest unit of CPU execution within a process. Multiple threads share the same process address space but execute independently.

2. Thread vs Process (Very Important)

| | Thread | Process |
|---|---|---|
| Address space | Shared with other threads | Separate |
| Resources | Shared (files, heap, code) | Separate |
| Communication | Easy - shared memory | Requires IPC |
| Creation overhead | Low (lightweight) | High (heavyweight) |
| Context switch | Faster | Slower |
| Fault isolation | One thread crash can kill all | Process isolation |

Per-thread: Program counter, registers, stack, thread ID, state
Shared across threads: Code, data, heap, open files, signals

3. Benefits of Threads

• Responsiveness: UI thread stays responsive while worker threads process
• Resource sharing: Cheaper than IPC for communication
• Economy: Faster to create and context-switch than processes
• Scalability: Parallel execution on multi-core processors

4. Thread Models (Very Important)

Many-to-One (Green threads)

• Many user threads mapped to one kernel thread
• Thread library manages scheduling in user space
• Blocking system call blocks entire process
• No true parallelism on multiprocessors

One-to-One

• Each user thread maps to one kernel thread
• True parallelism on multi-core
• Overhead: creating many threads creates many kernel threads (limit imposed)
• Used by: Linux, Windows

Many-to-Many

• Many user threads multiplexed onto fewer or equal kernel threads
• Best of both worlds - no blocking, controlled parallelism
• Complex to implement

Two-level model: Many-to-many + allows one-to-one binding for critical threads

5. POSIX Threads (pthreads)

• pthread_create(): create thread
• pthread_join(): wait for thread to finish (like wait() for processes)
• pthread_exit(): terminate calling thread
• pthread_mutex_lock/unlock(): mutual exclusion

6. Thread Issues

fork() with threads: Does child get all threads or just one?

• POSIX: fork() duplicates only calling thread in child - other threads not copied

exec() with threads: Replaces entire process (all threads)

Thread cancellation:

• Asynchronous: terminate immediately - resources may not be released
• Deferred (default): thread checks cancellation point periodically - safe cleanup

Thread Local Storage (TLS): Each thread has its own copy of certain data - useful for thread-specific state

7. GATE Trick

Threads share heap and global data - race conditions possible. Each thread has its OWN stack and program counter. One-to-one (Linux) enables parallelism; many-to-one cannot run in parallel. Thread creation is faster than process creation - key advantage for server applications. After fork() in multi-threaded program: only calling thread exists in child - be careful with mutexes held by other threads (may cause deadlock in child).