Thread Pools: Handing Work to a Fixed Set of Threads

Ten thousand short tasks through four long-lived threads, instead of ten thousand threads.

A thread pool is a fixed set of threads that stay alive and take tasks from a queue, one after another. In Java it is an ExecutorService, the interface for running submitted tasks on a pool: you submit tasks to it, and it decides which of its threads runs each one and when. Executors.newFixedThreadPool(4) creates a pool of four threads, and submit adds a task to its queue.

Ten thousand short tasks, one new Thread each: the program spends more time creating and destroying threads than doing the work, and if the tasks overlap, every one of them holds a thread and its stack at once. A thread runs once and cannot be restarted, so a thread per task really is a new thread for every task.

java

ExecutorService pool = Executors.newFixedThreadPool(4);

for (Path file : files) {
    pool.submit(() -> process(file));
}

What the pool separates
You decide	The pool decides
What work there is	Which thread runs it
When to submit it	When it starts
How many threads exist	Which thread is free

Four threads handle ten thousand tasks, each taking the next one whenever it finishes the last. Nothing is created per task except the lambda, and the number of threads no longer depends on the amount of work.

So a pool is a decision about capacity rather than scheduling: you choose how much of the machine this work may use, and the pool takes care of the rest.

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