C++ Thread Pool (C++17)

A minimal thread pool implementation with safe shutdown, exception propagation, and verified concurrency.

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Motivation

What is a thread pool anyway?

A thread pool is a fixed number of pre-created threads that are initialized at the start of the program. These threads sleep in a loop while waiting for tasks. When a task arrives, it is pushed into a shared queue, and one of the sleeping threads is woken up and assigned the task. This process continues until the pool is instructed to shut down.

This design solves several problems. Creating and destroying threads as requests arrive and leave introduces overhead and increases latency. A thread pool ensures there is a cap on the number of threads in the system, which is important because a CPU can only execute a limited number of threads at a time (depending on the number of cores). Having too many threads leads to excessive waiting, more context switching, and higher overhead, causing the CPU to spend more time switching between threads rather than doing useful work. Additionally, having too many threads destroys cache locality, as the CPU frequently evicts cached data and loads new data every time a different thread gets scheduled.

Where are these used in real life?

• Web servers
• Game engines
• Database engines

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Design Goals

• Bounded number of worker threads
• Task-based execution model
• No busy-waiting
• Safe shutdown via RAII
• Exception-safe task execution

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Components of a Thread Pool

1. submit() — Wraps an incoming task using the future mechanism and wakes a sleeping worker thread.
2. Queue — Holds tasks until a worker becomes available.
3. Worker threads — Execute tasks pulled from the queue.
4. Condition variable — Allows worker threads to sleep without wasting CPU resources and wake up immediately when new tasks arrive. It acts as a communication channel between submit() and the worker threads.
5. Mutex — Protects the task queue, ensuring that multiple threads do not push or pop tasks simultaneously and that submitters and workers do not modify the queue concurrently.
6. Shutdown flag — Signals worker threads to stop waiting for new tasks and exit their execution loop during destruction.

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Testing Strategy

Test	Purpose
test_basic	Correct task execution & futures
test_concurrency	Parallel execution proof
test_shutdown	Safe destruction with active tasks
test_exceptions	Exception propagation
stress_test	High-load correctness

Each test is built as a separate executable and focuses on validating a single property of the thread pool.

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Build and Run Instructions

Requirements

• C++17-compatible compiler (tested with g++)
• POSIX-compatible system (Linux)
• make

Build

All builds are handled via the provided Makefile.

To see available targets:

make