C discrete event SIM w stackful coroutines runs 45x faster than SimPy
Hi all, I have built Cimba, a multithreaded discrete event simulation library in C. Cimba uses POSIX pthread multithreading for parallel execution of multiple simulation trials, while coroutines provide concurrency inside each simulated trial universe. The simulated processes are based on asymmetric stackful coroutines with the context switching hand-coded in assembly. The stackful coroutines make it natural to express agentic behavior by conceptually placing oneself "inside" that process and describing what it does. A process can run in an infinite loop or just act as a one-shot customer…
In plain words
Cimba is a discrete event simulation library written in C that combines POSIX multithreading for parallel trial execution with stackful coroutines for concurrency within each simulation. Processes use asymmetric coroutines with assembly-level context switching, allowing developers to describe agent behavior intuitively from within the process perspective. Processes can run continuously or execute once, yielding and resuming from any call stack depth to act as both active agents and passive objects.
written from the facts on this page · September 2026
From the sources
In the maker’s words, at launch
Hi all, I have built Cimba, a multithreaded discrete event simulation library in C. Cimba uses POSIX pthread multithreading for parallel execution of multiple simulation trials, while coroutines provide concurrency inside each simulated trial universe. The simulated processes are based on asymmetric stackful coroutines with the context switching hand-coded in assembly. The stackful coroutines make it natural to express agentic behavior by conceptually placing oneself "inside" that process and describing what it does. A process can run in an infinite loop or just act as a one-shot customer passing through the system, yielding and resuming execution from any level of its call stack, acting both as an active agent and a passive object as needed. This is inspired by my own experience programming in Simula67, many moons ago, where I found the coroutines more important than the deservedly famous object-orientation. Cimba turned out to run really fast. In a simple benchmark, 100 trials of an M/M/1 queue run for one million time units each, it ran 45 times faster than an equivalent model built in SimPy + Python multiprocessing. The running time was reduced by 97.8 % vs the SimPy model. Cimba even processed more simulated events per second on a single CPU core than SimPy could do on all 64 cores. The speed is not only due to the efficient coroutines. Other parts are also designed for speed, such as a hash-heap event queue (binary heap plus Fibonacci hash map), fast random number generators and distributions, memory pools for frequently used object types, and so on. The initial implementation supports the AMD64/x86-64 architecture for Linux and Windows. I plan to target Apple Silicon next, then probably ARM. I believe this may interest the HN community. I would appreciate your views on both the API and the code. Any thoughts on future target architectures to consider? Docs: https://cimba.readthedocs.io/en/latest/ Repo: https://github.com/ambonvik/cimba
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