Threads
- Definition: Threads are lightweight processes that run concurrently within a single program. They share the same memory space and can execute independently.
- Concurrency vs. Parallelism:
- Threads provide concurrency, meaning they allow interleaved execution of tasks. The operating system scheduler switches between threads preemptively based on its algorithm.
- On multi-core processors, threads can run in parallel, as each thread can be scheduled onto a separate processor.
- Preemptive Scheduling:
- Threads are preemptively scheduled by the operating system. The kernel allocates time to each thread, and if a thread isn’t finished within that time, it’s preempted (switched to another thread).
- Kernel Threads:
- Threads are associated with kernel threads (also called user threads). These kernel threads share memory space within their owning process.
- The operating system scheduler runs each thread for a certain amount of time, switching between them.
- Use Cases:
- Threads are suitable for parallelizing tasks that can run independently, such as I/O-bound or CPU-bound operations.
Coroutines
- Definition: Coroutines are also lightweight, but they are cooperatively multitasked. Unlike threads, coroutines yield control explicitly.
- Cooperative Switching:
- Coroutine switches are cooperative, meaning the programmer (and possibly the programming language/runtime) controls when a switch occurs.
- The kernel is not involved in coroutine switches.
- Single Thread Execution:
- Coroutines run within a single thread until they yield or finish. They don’t rely on multiple kernel threads.
- Use Cases:
- Coroutines are useful for managing asynchronous tasks, event-driven programming, and avoiding thread overhead.
- They allow pausing and resuming functions at specific points, typically within a single thread.
Here’s a table summarizing the key differences:
| Feature | Threads | Coroutines |
|---|---|---|
| Origin | Operating system | User-level abstractions |
| Resource usage | High | Low |
| Execution | Independent, concurrent | Cooperative, controlled |
| Scheduling | Preemptive by OS | Cooperative, programmer-controlled |
| Stack space | Dedicated stack per thread | Shared or smaller footprint |
In summary, threads are pre-emptively scheduled by the operating system, while coroutine switches are cooperative and controlled by the programmer. Threads are better for parallelism, while coroutines excel in managing asynchronous tasks and avoiding thread-related overhead.