Go-style stackful coroutines for Python. Write blocking code — fiber(fn), plain recv/send, no async/await — and run a million of them across every core in one process. Hand-rolled asm context switch + C work-stealing scheduler + netpoll, built for free-threaded Python 3.14t (GIL off).

import threading, runloom
from urllib.request import urlopen
runloom.monkey.patch()

def crawl(url):
    # urlopen() looks blocking -- but monkey.patch() parks the goroutine on the
    # socket instead of the OS thread, so all 64 fetches overlap on real cores.
    body = urlopen(url, timeout=10).read()
    print(threading.get_native_id(), len(body))

def main():
    for _ in range(64):
        runloom.fiber(crawl, "http://example.com")

runloom.run(8, main)   # 8 hub threads -> real cores on 3.14t (GIL off)

Same box (64c, free-threaded CPython 3.13t), 8 hubs / GOMAXPROCS=8, warm steady-state. Go ≈ 2.1 M spawn/s here.

The short story: on spawn, scheduling, and throughput, runloom trades blows with Go and beats it on raw spawn — a stackful coroutine runtime on CPython matching a compiled language even with a Python handler (596 k vs 603 k req/s at saturation; a C handler beats Go). The one honest gap left is memory: a suspended fiber carries a CPython eval frame, ~3.3× Go's per-fiber RSS. Full cross-runtime numbers + cold spawn-vs-N curves: benchmark report · perf summary.

runloom.optimize("throughput")   # runloom.fiber -> max spawn rate (fiber_fast)
runloom.optimize("memory")       # runloom.fiber -> small right-sized stacks (default)

import runloom      # scheduler + channels, plus monkey/time/context/sync/aio

Prebuilt wheels (no compiler needed) for CPython 3.11–3.14 on Linux (x86_64/aarch64), macOS (arm64/x86_64), Windows (AMD64); source build elsewhere. No runtime dependencies.

• Hand-rolled asm context switch (x86_64 SysV, aarch64) — ~80 ns/swap, no syscall; Windows Fibers / POSIX ucontext fallback.
• M:N work-stealing scheduler (3.13t) — Chase-Lev deque per hub, per-hub MPSC submission, woken goroutines routed back to their origin hub.
• Per-goroutine PyThreadState snapshot — cframe, datastack, exc_info, contextvars, recursion; a million yielded goroutines share their hub threads with no frame-chain cliff.
• netpoll — epoll / kqueue / IOCP / WSAPoll / select; goroutines park transparently on fd readiness, lost-wake-free 3-state park-commit.
• Go-style channels — Chan(capacity), select, for v in ch.
• Stall isolation + recovery — one unanticipated blocking call stalls only its hub, and the runtime detects + recovers it (default on, 3.13t).
• monkey.patch() makes blocking stdlib (socket, time, threading, …) cooperative, so existing blocking code runs unchanged.

Already have async def code? The runloom.aio bridge runs it on the single-threaded scheduler (runloom.aio.run(main()) ≈ asyncio.run) — a zero-rewrite port path, not a multi-core speedup (use the sync API with run(n>1, main) for that).

• The multi-core win needs free-threaded CPython 3.13t (3.11+ for the frame snapshot at all). On a GIL build runloom still runs — cheap spawn, the goroutine model, netpoll — but single-core like asyncio.
• runloom doesn't make Python faster per core. CPython's ~80 k pure-Python ops/s/core is a constant it can't raise; it lets one process hit that on every core at once with a blocking model. The scheduler itself is Go-class.
• Higher memory per goroutine than Go (~3.3× for an empty fiber — the CPython eval frame; a C handler closes most of it).
• Preemption fires only at Python bytecode boundaries — a goroutine inside a tight pure-C call (e.g. numpy) holds its hub until it returns (same as Go + cgo).
• Linux x86_64 / 3.13t is the primary, heavily-validated target (2 M-conn runs, fuzzing, sanitizers, formal models); other backends are maintained in-step but less deeply exercised.

Full guide in docs/: Quickstart · Asyncio bridge · Sync API · Channels · M:N parallelism · Cookbook · API reference

Build from source (contributors): pip install -e . from a clone (needs a C compiler; scripts/install.sh / scripts\install.bat bootstrap one). To hack on runloom against free-threaded CPython, use a 3.13t interpreter.