zig 0.16 shipped Io.Threaded as the only production backend; Io.Evented is experimental, has no networking, and its fiber machinery crashes under ReleaseSafe. zio fills that gap as an ordinary package — no compiler fork, no stdlib patch.

design#

ordinary package. std.Io is a plain vtable; zio is just another implementation, the same way Io.Threaded is.
hybrid by composition. Zio wraps an inner Io.Threaded and delegates everything to it except sockets + sleep + concurrency, which run on an evented engine. anything unimplemented falls back to a thread — graceful degradation by construction.
ReleaseSafe-first. the engine core is a readiness loop (epoll/kqueue) driving per-connection state machines — no fibers, no stack switching, nothing the safety checker can object to. fibers are a measured upgrade behind a build option, adopted only if the scoreboard says they win.
scoreboard before runtime. Io.Threaded is the permanent reference column. no engine code lands without a benchmark delta.

surfaces#

zio.Engine — the readiness-loop core: register non-blocking fds with handlers, run one engine per core. callback-style.
zio.Zio — the hybrid std.Io implementation (delegating skeleton today; evented ops splice in as the engine grows).

benchmarks#

methodology in docs/benchmarks.md, results in docs/baselines.md. headline (linux, ReleaseSafe, 2,800 conns): ~200× less RSS than thread-per-conn, O(cores) threads instead of O(conns), higher throughput, and a full 2,800-connection reconnect storm re-established in ~115 ms.

zig build bench -Doptimize=ReleaseSafe -- --backend zio --conns 2800 --seconds 5 --storm

status#

bench harness + Io.Threaded baselines
hybrid delegating skeleton (Zio wrapping Io.Threaded)
engine v0: readiness loops (epoll/kqueue), per-conn state, reconnect storms, interest modification, non-blocking connect
fiber gauntlet: ReleaseSafe stack-switch crashes root-caused (LLVM #167783 x30-clobber bug + missing x18 clobber on linux) and fixed in the vendored primitive — 7/7 on all three platforms, Debug and ReleaseSafe (docs/fiber-gauntlet-2026-07-09.md)
fiber scheduler v1 (zio.Sched): blocking-style read/write/connect/ accept/sleep on one os thread; one-shot poller arming, lazy gen-tagged timers, spurious-wakeup-safe parks, fiber recycling. try it: zig build demo -Doptimize=ReleaseSafe -- --fibers 2000 --seconds 10
fiber-backed std.Io vtable (zio.ZioBackend): concurrent/await/cancel spawn fibers; net connect/accept/read/write park on readiness; sleep on sched timers; fiber-aware futexWait/Wake makes Io.Mutex/Condition/ Queue correct across fiber and thread domains; netLookup spills DNS to an inner-Threaded thread. unmodified Io.net code runs on fibers. cancellation reaches fibers parked on anything (io/futex/timer/await) promptly via an all-fibers interrupt registry. adoption: docs/adoption-guide.md
mmap fiber stacks with guard pages (malloc stacks commit ~256KB/fiber — the demo's RSS is all stacks)
io_uring engine + runtime detection with epoll fallback (default docker seccomp blocks io_uring)
first production consumer integration

origins#

zio came out of running ~2,800 production websocket subscriptions on both Io.Threaded and Io.Evented and watching the experimental backend fail in ways that forced a retreat to thread-per-connection. the write-ups that led here (retrospective, stdlib empirics, plan) live with that project's docs; the short version is in docs/benchmarks.md.

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