How to add command line options to specify the number of threads?

kojix2 · August 12, 2025, 6:17am

Update:

ExecutionContext allows you to use any number of workers.

text/0002-execution-contexts.md

main

---
Feature Name: execution_contexts
Start Date: 2024-02-05
RFC PR: "https://github.com/crystal-lang/rfcs/pull/2"
Issue: "https://github.com/crystal-lang/crystal/issues/15342"
---

# Summary

Reinvent Crystal's concurrency execution model with respect to multi-threading. It's getting more efficient, for example by avoiding blocked fibers while there are sleeping threads. And it empowers developers with different strategies for scheduling fiber, called _Execution Contexts_.

Note: this proposal takes inspiration from Go and Kotlin.

# Motivation (analysis of current situation)

## Terminology

- **Thread**: program whose execution is controlled by the operating system ([wikipedia](https://en.wikipedia.org/wiki/Thread_(computing))).
- **Fiber**: unit of works or “functions whose execution you can pause” ([wikipedia](https://en.wikipedia.org/wiki/Coroutine)), we can run multiple fibers per thread.
- **Scheduler**: manages fibers’ executions inside the program (controlled by Crystal), unlike threads that are scheduled by the operating system (outside of the accessibility of the program).

This file has been truncated. show original

Support for WaitGroup is currently in progress.

github.com/crystal-lang/crystal

Add `ExecutionContext` support to `WaitGroup`

opened 06:14AM - 08 Apr 25 UTC

spuun

kind:feature topic:stdlib:concurrency

With `ExecutionContext` soon being available I think it would be nice to be able… to pass an optional `ExecutionContext` to `WaitGroup` in which the WG till spawn fibers. I'd like to do something like ```crystal ec = ExecutionContext::MultiThreaded.new("workers", 4) WaitGroup.wait(ec) do |wg| items.each { |i| wg.spawn worker(i) } end ``` instead of ```crystal ec = ExecutionContext::MultiThreaded.new("workers", 4) ec_wg = WaitGroup.new(1) ec.spawn do WaitGroup.wait do |wg| items.each { |i| wg.spawn worker(i) } end ec_wg.done end ec_wg.wait ``` I guess it would be nice to be able to pass an `EC` to the constructor as well.

Thread pools will also become available at some point.

github.com/crystal-lang/crystal

Add `Fiber::ExecutionContext::ThreadPool`

master ← ysbaddaden:feature/execution-context-thread-pool

opened 02:01PM - 06 Jun 25 UTC

ysbaddaden

+310 -76

**Introduces a pool of threads for execution contexts.** This changes the run…time behavior of threads: we no longer start a thread to run a specific scheduler run loop and never terminate in practice (except for the isolated context). Each thread now has its own inner loop that switches to a scheduler loop fiber (the scheduler's main fiber) then switches back to its inner loop (the thread's main fiber) to sleep for a while, then eventually terminates. The benefit of the global thread loop is that threads are kept around instead of being created and thrown away. This is for example helpful for #15871 that will allow to move a scheduler to another thread, as well as for applications that regularly start an isolated fiber. They can keep reusing a pending thread instead of having to create one every time. Threads still eventually terminate after some configurable inactive time, except for the main thread because we need to keep the main fiber's stack alive. A future improvement could park MT threads back into the thread pool, instead of keeping them tied to the MT context. They could be reused by any context that needs parallelism, or to boot a new isolated fiber or ST context, instead of sitting around. **NOTE**: the thread pool makes the min/max range in MT contexts kinda moot; it sounds better to settle to a maximum number of schedulers instead. **NOTE**: the more we decouple schedulers from threads, the more I'm convinced that using "thread" in the execution context names doesn't make much sense; I'm strongly leaning to rename `SingleThreaded` to `Concurrent` and `MultiThreaded` to `Parallel`. Extracted from #15871

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How to add command line options to specify the number of threads?

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