diff --git a/lib/std/Io.zig b/lib/std/Io.zig
index ffec231b4f..effb0e9383 100644
--- a/lib/std/Io.zig
+++ b/lib/std/Io.zig
@@ -558,6 +558,7 @@ test {
 const Io = @This();
 
 pub const EventLoop = @import("Io/EventLoop.zig");
+pub const ThreadPool = @import("Io/ThreadPool.zig");
 
 userdata: ?*anyopaque,
 vtable: *const VTable,
diff --git a/lib/std/Io/ThreadPool.zig b/lib/std/Io/ThreadPool.zig
new file mode 100644
index 0000000000..f356a87054
--- /dev/null
+++ b/lib/std/Io/ThreadPool.zig
@@ -0,0 +1,852 @@
+const builtin = @import("builtin");
+const std = @import("../std.zig");
+const Allocator = std.mem.Allocator;
+const assert = std.debug.assert;
+const WaitGroup = std.Thread.WaitGroup;
+const Io = std.Io;
+const Pool = @This();
+
+/// Must be a thread-safe allocator.
+allocator: std.mem.Allocator,
+mutex: std.Thread.Mutex = .{},
+cond: std.Thread.Condition = .{},
+run_queue: std.SinglyLinkedList = .{},
+is_running: bool = true,
+threads: std.ArrayListUnmanaged(std.Thread),
+ids: if (builtin.single_threaded) struct {
+    inline fn deinit(_: @This(), _: std.mem.Allocator) void {}
+    fn getIndex(_: @This(), _: std.Thread.Id) usize {
+        return 0;
+    }
+} else std.AutoArrayHashMapUnmanaged(std.Thread.Id, void),
+stack_size: usize,
+
+threadlocal var current_closure: ?*AsyncClosure = null;
+
+pub const Runnable = struct {
+    runFn: RunProto,
+    node: std.SinglyLinkedList.Node = .{},
+};
+
+pub const RunProto = *const fn (*Runnable, id: ?usize) void;
+
+pub const Options = struct {
+    allocator: std.mem.Allocator,
+    n_jobs: ?usize = null,
+    track_ids: bool = false,
+    stack_size: usize = std.Thread.SpawnConfig.default_stack_size,
+};
+
+pub fn init(pool: *Pool, options: Options) !void {
+    const gpa = options.allocator;
+    const thread_count = options.n_jobs orelse @max(1, std.Thread.getCpuCount() catch 1);
+    const threads = try gpa.alloc(std.Thread, thread_count);
+    errdefer gpa.free(threads);
+
+    pool.* = .{
+        .allocator = gpa,
+        .threads = .initBuffer(threads),
+        .ids = .{},
+        .stack_size = options.stack_size,
+    };
+
+    if (builtin.single_threaded) return;
+
+    if (options.track_ids) {
+        try pool.ids.ensureTotalCapacity(gpa, 1 + thread_count);
+        pool.ids.putAssumeCapacityNoClobber(std.Thread.getCurrentId(), {});
+    }
+}
+
+pub fn deinit(pool: *Pool) void {
+    const gpa = pool.allocator;
+    pool.join();
+    pool.threads.deinit(gpa);
+    pool.ids.deinit(gpa);
+    pool.* = undefined;
+}
+
+fn join(pool: *Pool) void {
+    if (builtin.single_threaded) return;
+
+    {
+        pool.mutex.lock();
+        defer pool.mutex.unlock();
+
+        // ensure future worker threads exit the dequeue loop
+        pool.is_running = false;
+    }
+
+    // wake up any sleeping threads (this can be done outside the mutex)
+    // then wait for all the threads we know are spawned to complete.
+    pool.cond.broadcast();
+    for (pool.threads.items) |thread| thread.join();
+}
+
+/// Runs `func` in the thread pool, calling `WaitGroup.start` beforehand, and
+/// `WaitGroup.finish` after it returns.
+///
+/// In the case that queuing the function call fails to allocate memory, or the
+/// target is single-threaded, the function is called directly.
+pub fn spawnWg(pool: *Pool, wait_group: *WaitGroup, comptime func: anytype, args: anytype) void {
+    wait_group.start();
+
+    if (builtin.single_threaded) {
+        @call(.auto, func, args);
+        wait_group.finish();
+        return;
+    }
+
+    const Args = @TypeOf(args);
+    const Closure = struct {
+        arguments: Args,
+        pool: *Pool,
+        runnable: Runnable = .{ .runFn = runFn },
+        wait_group: *WaitGroup,
+
+        fn runFn(runnable: *Runnable, _: ?usize) void {
+            const closure: *@This() = @alignCast(@fieldParentPtr("runnable", runnable));
+            @call(.auto, func, closure.arguments);
+            closure.wait_group.finish();
+            closure.pool.allocator.destroy(closure);
+        }
+    };
+
+    pool.mutex.lock();
+
+    const gpa = pool.allocator;
+    const closure = gpa.create(Closure) catch {
+        pool.mutex.unlock();
+        @call(.auto, func, args);
+        wait_group.finish();
+        return;
+    };
+    closure.* = .{
+        .arguments = args,
+        .pool = pool,
+        .wait_group = wait_group,
+    };
+
+    pool.run_queue.prepend(&closure.runnable.node);
+
+    if (pool.threads.items.len < pool.threads.capacity) {
+        pool.threads.addOneAssumeCapacity().* = std.Thread.spawn(.{
+            .stack_size = pool.stack_size,
+            .allocator = gpa,
+        }, worker, .{pool}) catch t: {
+            pool.threads.items.len -= 1;
+            break :t undefined;
+        };
+    }
+
+    pool.mutex.unlock();
+    pool.cond.signal();
+}
+
+/// Runs `func` in the thread pool, calling `WaitGroup.start` beforehand, and
+/// `WaitGroup.finish` after it returns.
+///
+/// The first argument passed to `func` is a dense `usize` thread id, the rest
+/// of the arguments are passed from `args`. Requires the pool to have been
+/// initialized with `.track_ids = true`.
+///
+/// In the case that queuing the function call fails to allocate memory, or the
+/// target is single-threaded, the function is called directly.
+pub fn spawnWgId(pool: *Pool, wait_group: *WaitGroup, comptime func: anytype, args: anytype) void {
+    wait_group.start();
+
+    if (builtin.single_threaded) {
+        @call(.auto, func, .{0} ++ args);
+        wait_group.finish();
+        return;
+    }
+
+    const Args = @TypeOf(args);
+    const Closure = struct {
+        arguments: Args,
+        pool: *Pool,
+        runnable: Runnable = .{ .runFn = runFn },
+        wait_group: *WaitGroup,
+
+        fn runFn(runnable: *Runnable, id: ?usize) void {
+            const closure: *@This() = @alignCast(@fieldParentPtr("runnable", runnable));
+            @call(.auto, func, .{id.?} ++ closure.arguments);
+            closure.wait_group.finish();
+            closure.pool.allocator.destroy(closure);
+        }
+    };
+
+    pool.mutex.lock();
+
+    const gpa = pool.allocator;
+    const closure = gpa.create(Closure) catch {
+        const id: ?usize = pool.ids.getIndex(std.Thread.getCurrentId());
+        pool.mutex.unlock();
+        @call(.auto, func, .{id.?} ++ args);
+        wait_group.finish();
+        return;
+    };
+    closure.* = .{
+        .arguments = args,
+        .pool = pool,
+        .wait_group = wait_group,
+    };
+
+    pool.run_queue.prepend(&closure.runnable.node);
+
+    if (pool.threads.items.len < pool.threads.capacity) {
+        pool.threads.addOneAssumeCapacity().* = std.Thread.spawn(.{
+            .stack_size = pool.stack_size,
+            .allocator = gpa,
+        }, worker, .{pool}) catch t: {
+            pool.threads.items.len -= 1;
+            break :t undefined;
+        };
+    }
+
+    pool.mutex.unlock();
+    pool.cond.signal();
+}
+
+pub fn spawn(pool: *Pool, comptime func: anytype, args: anytype) void {
+    if (builtin.single_threaded) {
+        @call(.auto, func, args);
+        return;
+    }
+
+    const Args = @TypeOf(args);
+    const Closure = struct {
+        arguments: Args,
+        pool: *Pool,
+        runnable: Runnable = .{ .runFn = runFn },
+
+        fn runFn(runnable: *Runnable, _: ?usize) void {
+            const closure: *@This() = @alignCast(@fieldParentPtr("runnable", runnable));
+            @call(.auto, func, closure.arguments);
+            closure.pool.allocator.destroy(closure);
+        }
+    };
+
+    pool.mutex.lock();
+
+    const gpa = pool.allocator;
+    const closure = gpa.create(Closure) catch {
+        pool.mutex.unlock();
+        @call(.auto, func, args);
+        return;
+    };
+    closure.* = .{
+        .arguments = args,
+        .pool = pool,
+    };
+
+    pool.run_queue.prepend(&closure.runnable.node);
+
+    if (pool.threads.items.len < pool.threads.capacity) {
+        pool.threads.addOneAssumeCapacity().* = std.Thread.spawn(.{
+            .stack_size = pool.stack_size,
+            .allocator = gpa,
+        }, worker, .{pool}) catch t: {
+            pool.threads.items.len -= 1;
+            break :t undefined;
+        };
+    }
+
+    pool.mutex.unlock();
+    pool.cond.signal();
+}
+
+test spawn {
+    const TestFn = struct {
+        fn checkRun(completed: *bool) void {
+            completed.* = true;
+        }
+    };
+
+    var completed: bool = false;
+
+    {
+        var pool: Pool = undefined;
+        try pool.init(.{
+            .allocator = std.testing.allocator,
+        });
+        defer pool.deinit();
+        pool.spawn(TestFn.checkRun, .{&completed});
+    }
+
+    try std.testing.expectEqual(true, completed);
+}
+
+fn worker(pool: *Pool) void {
+    pool.mutex.lock();
+    defer pool.mutex.unlock();
+
+    const id: ?usize = if (pool.ids.count() > 0) @intCast(pool.ids.count()) else null;
+    if (id) |_| pool.ids.putAssumeCapacityNoClobber(std.Thread.getCurrentId(), {});
+
+    while (true) {
+        while (pool.run_queue.popFirst()) |run_node| {
+            // Temporarily unlock the mutex in order to execute the run_node
+            pool.mutex.unlock();
+            defer pool.mutex.lock();
+
+            const runnable: *Runnable = @fieldParentPtr("node", run_node);
+            runnable.runFn(runnable, id);
+        }
+
+        // Stop executing instead of waiting if the thread pool is no longer running.
+        if (pool.is_running) {
+            pool.cond.wait(&pool.mutex);
+        } else {
+            break;
+        }
+    }
+}
+
+pub fn waitAndWork(pool: *Pool, wait_group: *WaitGroup) void {
+    var id: ?usize = null;
+
+    while (!wait_group.isDone()) {
+        pool.mutex.lock();
+        if (pool.run_queue.popFirst()) |run_node| {
+            id = id orelse pool.ids.getIndex(std.Thread.getCurrentId());
+            pool.mutex.unlock();
+            const runnable: *Runnable = @fieldParentPtr("node", run_node);
+            runnable.runFn(runnable, id);
+            continue;
+        }
+
+        pool.mutex.unlock();
+        wait_group.wait();
+        return;
+    }
+}
+
+pub fn getIdCount(pool: *Pool) usize {
+    return @intCast(1 + pool.threads.items.len);
+}
+
+pub fn io(pool: *Pool) Io {
+    return .{
+        .userdata = pool,
+        .vtable = &.{
+            .async = async,
+            .await = await,
+            .go = go,
+            .cancel = cancel,
+            .cancelRequested = cancelRequested,
+            .select = select,
+
+            .mutexLock = mutexLock,
+            .mutexUnlock = mutexUnlock,
+
+            .conditionWait = conditionWait,
+            .conditionWake = conditionWake,
+
+            .createFile = createFile,
+            .openFile = openFile,
+            .closeFile = closeFile,
+            .pread = pread,
+            .pwrite = pwrite,
+
+            .now = now,
+            .sleep = sleep,
+        },
+    };
+}
+
+const AsyncClosure = struct {
+    func: *const fn (context: *anyopaque, result: *anyopaque) void,
+    runnable: Runnable = .{ .runFn = runFn },
+    reset_event: std.Thread.ResetEvent,
+    select_condition: ?*std.Thread.ResetEvent,
+    cancel_tid: std.Thread.Id,
+    context_offset: usize,
+    result_offset: usize,
+
+    const done_reset_event: *std.Thread.ResetEvent = @ptrFromInt(@alignOf(std.Thread.ResetEvent));
+
+    const canceling_tid: std.Thread.Id = switch (@typeInfo(std.Thread.Id)) {
+        .int => |int_info| switch (int_info.signedness) {
+            .signed => -1,
+            .unsigned => std.math.maxInt(std.Thread.Id),
+        },
+        .pointer => @ptrFromInt(std.math.maxInt(usize)),
+        else => @compileError("unsupported std.Thread.Id: " ++ @typeName(std.Thread.Id)),
+    };
+
+    fn runFn(runnable: *Pool.Runnable, _: ?usize) void {
+        const closure: *AsyncClosure = @alignCast(@fieldParentPtr("runnable", runnable));
+        const tid = std.Thread.getCurrentId();
+        if (@cmpxchgStrong(
+            std.Thread.Id,
+            &closure.cancel_tid,
+            0,
+            tid,
+            .acq_rel,
+            .acquire,
+        )) |cancel_tid| {
+            assert(cancel_tid == canceling_tid);
+            return;
+        }
+        current_closure = closure;
+        closure.func(closure.contextPointer(), closure.resultPointer());
+        current_closure = null;
+        if (@cmpxchgStrong(
+            std.Thread.Id,
+            &closure.cancel_tid,
+            tid,
+            0,
+            .acq_rel,
+            .acquire,
+        )) |cancel_tid| assert(cancel_tid == canceling_tid);
+
+        if (@atomicRmw(
+            ?*std.Thread.ResetEvent,
+            &closure.select_condition,
+            .Xchg,
+            done_reset_event,
+            .release,
+        )) |select_reset| {
+            assert(select_reset != done_reset_event);
+            select_reset.set();
+        }
+        closure.reset_event.set();
+    }
+
+    fn contextOffset(context_alignment: std.mem.Alignment) usize {
+        return context_alignment.forward(@sizeOf(AsyncClosure));
+    }
+
+    fn resultOffset(
+        context_alignment: std.mem.Alignment,
+        context_len: usize,
+        result_alignment: std.mem.Alignment,
+    ) usize {
+        return result_alignment.forward(contextOffset(context_alignment) + context_len);
+    }
+
+    fn resultPointer(closure: *AsyncClosure) [*]u8 {
+        const base: [*]u8 = @ptrCast(closure);
+        return base + closure.result_offset;
+    }
+
+    fn contextPointer(closure: *AsyncClosure) [*]u8 {
+        const base: [*]u8 = @ptrCast(closure);
+        return base + closure.context_offset;
+    }
+
+    fn waitAndFree(closure: *AsyncClosure, gpa: Allocator, result: []u8) void {
+        closure.reset_event.wait();
+        const base: [*]align(@alignOf(AsyncClosure)) u8 = @ptrCast(closure);
+        @memcpy(result, closure.resultPointer()[0..result.len]);
+        gpa.free(base[0 .. closure.result_offset + result.len]);
+    }
+};
+
+fn async(
+    userdata: ?*anyopaque,
+    result: []u8,
+    result_alignment: std.mem.Alignment,
+    context: []const u8,
+    context_alignment: std.mem.Alignment,
+    start: *const fn (context: *const anyopaque, result: *anyopaque) void,
+) ?*Io.AnyFuture {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    pool.mutex.lock();
+
+    const gpa = pool.allocator;
+    const context_offset = context_alignment.forward(@sizeOf(AsyncClosure));
+    const result_offset = result_alignment.forward(context_offset + context.len);
+    const n = result_offset + result.len;
+    const closure: *AsyncClosure = @alignCast(@ptrCast(gpa.alignedAlloc(u8, .of(AsyncClosure), n) catch {
+        pool.mutex.unlock();
+        start(context.ptr, result.ptr);
+        return null;
+    }));
+    closure.* = .{
+        .func = start,
+        .context_offset = context_offset,
+        .result_offset = result_offset,
+        .reset_event = .{},
+        .cancel_tid = 0,
+        .select_condition = null,
+    };
+    @memcpy(closure.contextPointer()[0..context.len], context);
+    pool.run_queue.prepend(&closure.runnable.node);
+
+    if (pool.threads.items.len < pool.threads.capacity) {
+        pool.threads.addOneAssumeCapacity().* = std.Thread.spawn(.{
+            .stack_size = pool.stack_size,
+            .allocator = gpa,
+        }, worker, .{pool}) catch t: {
+            pool.threads.items.len -= 1;
+            break :t undefined;
+        };
+    }
+
+    pool.mutex.unlock();
+    pool.cond.signal();
+
+    return @ptrCast(closure);
+}
+
+const DetachedClosure = struct {
+    pool: *Pool,
+    func: *const fn (context: *anyopaque) void,
+    run_node: Pool.RunQueue.Node = .{ .data = .{ .runFn = runFn } },
+    context_alignment: std.mem.Alignment,
+    context_len: usize,
+
+    fn runFn(runnable: *Pool.Runnable, _: ?usize) void {
+        const run_node: *Pool.RunQueue.Node = @fieldParentPtr("data", runnable);
+        const closure: *DetachedClosure = @alignCast(@fieldParentPtr("run_node", run_node));
+        closure.func(closure.contextPointer());
+        const gpa = closure.pool.allocator;
+        const base: [*]align(@alignOf(DetachedClosure)) u8 = @ptrCast(closure);
+        gpa.free(base[0..contextEnd(closure.context_alignment, closure.context_len)]);
+    }
+
+    fn contextOffset(context_alignment: std.mem.Alignment) usize {
+        return context_alignment.forward(@sizeOf(DetachedClosure));
+    }
+
+    fn contextEnd(context_alignment: std.mem.Alignment, context_len: usize) usize {
+        return contextOffset(context_alignment) + context_len;
+    }
+
+    fn contextPointer(closure: *DetachedClosure) [*]u8 {
+        const base: [*]u8 = @ptrCast(closure);
+        return base + contextOffset(closure.context_alignment);
+    }
+};
+
+fn go(
+    userdata: ?*anyopaque,
+    context: []const u8,
+    context_alignment: std.mem.Alignment,
+    start: *const fn (context: *const anyopaque) void,
+) void {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    pool.mutex.lock();
+
+    const gpa = pool.allocator;
+    const n = DetachedClosure.contextEnd(context_alignment, context.len);
+    const closure: *DetachedClosure = @alignCast(@ptrCast(gpa.alignedAlloc(u8, .of(DetachedClosure), n) catch {
+        pool.mutex.unlock();
+        start(context.ptr);
+        return;
+    }));
+    closure.* = .{
+        .pool = pool,
+        .func = start,
+        .context_alignment = context_alignment,
+        .context_len = context.len,
+    };
+    @memcpy(closure.contextPointer()[0..context.len], context);
+    pool.run_queue.prepend(&closure.run_node);
+
+    if (pool.threads.items.len < pool.threads.capacity) {
+        pool.threads.addOneAssumeCapacity().* = std.Thread.spawn(.{
+            .stack_size = pool.stack_size,
+            .allocator = gpa,
+        }, worker, .{pool}) catch t: {
+            pool.threads.items.len -= 1;
+            break :t undefined;
+        };
+    }
+
+    pool.mutex.unlock();
+    pool.cond.signal();
+}
+
+fn await(
+    userdata: ?*anyopaque,
+    any_future: *std.Io.AnyFuture,
+    result: []u8,
+    result_alignment: std.mem.Alignment,
+) void {
+    _ = result_alignment;
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    const closure: *AsyncClosure = @ptrCast(@alignCast(any_future));
+    closure.waitAndFree(pool.allocator, result);
+}
+
+fn cancel(
+    userdata: ?*anyopaque,
+    any_future: *Io.AnyFuture,
+    result: []u8,
+    result_alignment: std.mem.Alignment,
+) void {
+    _ = result_alignment;
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    const closure: *AsyncClosure = @ptrCast(@alignCast(any_future));
+    switch (@atomicRmw(
+        std.Thread.Id,
+        &closure.cancel_tid,
+        .Xchg,
+        AsyncClosure.canceling_tid,
+        .acq_rel,
+    )) {
+        0, AsyncClosure.canceling_tid => {},
+        else => |cancel_tid| switch (builtin.os.tag) {
+            .linux => _ = std.os.linux.tgkill(
+                std.os.linux.getpid(),
+                @bitCast(cancel_tid),
+                std.posix.SIG.IO,
+            ),
+            else => {},
+        },
+    }
+    closure.waitAndFree(pool.allocator, result);
+}
+
+fn cancelRequested(userdata: ?*anyopaque) bool {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    _ = pool;
+    const closure = current_closure orelse return false;
+    return @atomicLoad(std.Thread.Id, &closure.cancel_tid, .acquire) == AsyncClosure.canceling_tid;
+}
+
+fn checkCancel(pool: *Pool) error{Canceled}!void {
+    if (cancelRequested(pool)) return error.Canceled;
+}
+
+fn mutexLock(userdata: ?*anyopaque, prev_state: Io.Mutex.State, mutex: *Io.Mutex) error{Canceled}!void {
+    _ = userdata;
+    if (prev_state == .contended) {
+        std.Thread.Futex.wait(@ptrCast(&mutex.state), @intFromEnum(Io.Mutex.State.contended));
+    }
+    while (@atomicRmw(
+        Io.Mutex.State,
+        &mutex.state,
+        .Xchg,
+        .contended,
+        .acquire,
+    ) != .unlocked) {
+        std.Thread.Futex.wait(@ptrCast(&mutex.state), @intFromEnum(Io.Mutex.State.contended));
+    }
+}
+fn mutexUnlock(userdata: ?*anyopaque, prev_state: Io.Mutex.State, mutex: *Io.Mutex) void {
+    _ = userdata;
+    _ = prev_state;
+    if (@atomicRmw(Io.Mutex.State, &mutex.state, .Xchg, .unlocked, .release) == .contended) {
+        std.Thread.Futex.wake(@ptrCast(&mutex.state), 1);
+    }
+}
+
+fn conditionWait(userdata: ?*anyopaque, cond: *Io.Condition, mutex: *Io.Mutex) Io.Cancelable!void {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    comptime assert(@TypeOf(cond.state) == u64);
+    const ints: *[2]std.atomic.Value(u32) = @ptrCast(&cond.state);
+    const cond_state = &ints[0];
+    const cond_epoch = &ints[1];
+    const one_waiter = 1;
+    const waiter_mask = 0xffff;
+    const one_signal = 1 << 16;
+    const signal_mask = 0xffff << 16;
+    // Observe the epoch, then check the state again to see if we should wake up.
+    // The epoch must be observed before we check the state or we could potentially miss a wake() and deadlock:
+    //
+    // - T1: s = LOAD(&state)
+    // - T2: UPDATE(&s, signal)
+    // - T2: UPDATE(&epoch, 1) + FUTEX_WAKE(&epoch)
+    // - T1: e = LOAD(&epoch) (was reordered after the state load)
+    // - T1: s & signals == 0 -> FUTEX_WAIT(&epoch, e) (missed the state update + the epoch change)
+    //
+    // Acquire barrier to ensure the epoch load happens before the state load.
+    var epoch = cond_epoch.load(.acquire);
+    var state = cond_state.fetchAdd(one_waiter, .monotonic);
+    assert(state & waiter_mask != waiter_mask);
+    state += one_waiter;
+
+    mutex.unlock(pool.io());
+    defer mutex.lock(pool.io()) catch @panic("TODO");
+
+    var futex_deadline = std.Thread.Futex.Deadline.init(null);
+
+    while (true) {
+        futex_deadline.wait(cond_epoch, epoch) catch |err| switch (err) {
+            error.Timeout => unreachable,
+        };
+
+        epoch = cond_epoch.load(.acquire);
+        state = cond_state.load(.monotonic);
+
+        // Try to wake up by consuming a signal and decremented the waiter we added previously.
+        // Acquire barrier ensures code before the wake() which added the signal happens before we decrement it and return.
+        while (state & signal_mask != 0) {
+            const new_state = state - one_waiter - one_signal;
+            state = cond_state.cmpxchgWeak(state, new_state, .acquire, .monotonic) orelse return;
+        }
+    }
+}
+
+fn conditionWake(userdata: ?*anyopaque, cond: *Io.Condition, wake: Io.Condition.Wake) void {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    _ = pool;
+    comptime assert(@TypeOf(cond.state) == u64);
+    const ints: *[2]std.atomic.Value(u32) = @ptrCast(&cond.state);
+    const cond_state = &ints[0];
+    const cond_epoch = &ints[1];
+    const one_waiter = 1;
+    const waiter_mask = 0xffff;
+    const one_signal = 1 << 16;
+    const signal_mask = 0xffff << 16;
+    var state = cond_state.load(.monotonic);
+    while (true) {
+        const waiters = (state & waiter_mask) / one_waiter;
+        const signals = (state & signal_mask) / one_signal;
+
+        // Reserves which waiters to wake up by incrementing the signals count.
+        // Therefore, the signals count is always less than or equal to the waiters count.
+        // We don't need to Futex.wake if there's nothing to wake up or if other wake() threads have reserved to wake up the current waiters.
+        const wakeable = waiters - signals;
+        if (wakeable == 0) {
+            return;
+        }
+
+        const to_wake = switch (wake) {
+            .one => 1,
+            .all => wakeable,
+        };
+
+        // Reserve the amount of waiters to wake by incrementing the signals count.
+        // Release barrier ensures code before the wake() happens before the signal it posted and consumed by the wait() threads.
+        const new_state = state + (one_signal * to_wake);
+        state = cond_state.cmpxchgWeak(state, new_state, .release, .monotonic) orelse {
+            // Wake up the waiting threads we reserved above by changing the epoch value.
+            // NOTE: a waiting thread could miss a wake up if *exactly* ((1<<32)-1) wake()s happen between it observing the epoch and sleeping on it.
+            // This is very unlikely due to how many precise amount of Futex.wake() calls that would be between the waiting thread's potential preemption.
+            //
+            // Release barrier ensures the signal being added to the state happens before the epoch is changed.
+            // If not, the waiting thread could potentially deadlock from missing both the state and epoch change:
+            //
+            // - T2: UPDATE(&epoch, 1) (reordered before the state change)
+            // - T1: e = LOAD(&epoch)
+            // - T1: s = LOAD(&state)
+            // - T2: UPDATE(&state, signal) + FUTEX_WAKE(&epoch)
+            // - T1: s & signals == 0 -> FUTEX_WAIT(&epoch, e) (missed both epoch change and state change)
+            _ = cond_epoch.fetchAdd(1, .release);
+            std.Thread.Futex.wake(cond_epoch, to_wake);
+            return;
+        };
+    }
+}
+
+fn createFile(
+    userdata: ?*anyopaque,
+    dir: Io.Dir,
+    sub_path: []const u8,
+    flags: Io.File.CreateFlags,
+) Io.File.OpenError!Io.File {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    try pool.checkCancel();
+    const fs_dir: std.fs.Dir = .{ .fd = dir.handle };
+    const fs_file = try fs_dir.createFile(sub_path, flags);
+    return .{ .handle = fs_file.handle };
+}
+
+fn openFile(
+    userdata: ?*anyopaque,
+    dir: Io.Dir,
+    sub_path: []const u8,
+    flags: Io.File.OpenFlags,
+) Io.File.OpenError!Io.File {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    try pool.checkCancel();
+    const fs_dir: std.fs.Dir = .{ .fd = dir.handle };
+    const fs_file = try fs_dir.openFile(sub_path, flags);
+    return .{ .handle = fs_file.handle };
+}
+
+fn closeFile(userdata: ?*anyopaque, file: Io.File) void {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    _ = pool;
+    const fs_file: std.fs.File = .{ .handle = file.handle };
+    return fs_file.close();
+}
+
+fn pread(userdata: ?*anyopaque, file: Io.File, buffer: []u8, offset: std.posix.off_t) Io.File.PReadError!usize {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    try pool.checkCancel();
+    const fs_file: std.fs.File = .{ .handle = file.handle };
+    return switch (offset) {
+        -1 => fs_file.read(buffer),
+        else => fs_file.pread(buffer, @bitCast(offset)),
+    };
+}
+
+fn pwrite(userdata: ?*anyopaque, file: Io.File, buffer: []const u8, offset: std.posix.off_t) Io.File.PWriteError!usize {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    try pool.checkCancel();
+    const fs_file: std.fs.File = .{ .handle = file.handle };
+    return switch (offset) {
+        -1 => fs_file.write(buffer),
+        else => fs_file.pwrite(buffer, @bitCast(offset)),
+    };
+}
+
+fn now(userdata: ?*anyopaque, clockid: std.posix.clockid_t) Io.ClockGetTimeError!Io.Timestamp {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    try pool.checkCancel();
+    const timespec = try std.posix.clock_gettime(clockid);
+    return @enumFromInt(@as(i128, timespec.sec) * std.time.ns_per_s + timespec.nsec);
+}
+
+fn sleep(userdata: ?*anyopaque, clockid: std.posix.clockid_t, deadline: Io.Deadline) Io.SleepError!void {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    const deadline_nanoseconds: i96 = switch (deadline) {
+        .duration => |duration| duration.nanoseconds,
+        .timestamp => |timestamp| @intFromEnum(timestamp),
+    };
+    var timespec: std.posix.timespec = .{
+        .sec = @intCast(@divFloor(deadline_nanoseconds, std.time.ns_per_s)),
+        .nsec = @intCast(@mod(deadline_nanoseconds, std.time.ns_per_s)),
+    };
+    while (true) {
+        try pool.checkCancel();
+        switch (std.os.linux.E.init(std.os.linux.clock_nanosleep(clockid, .{ .ABSTIME = switch (deadline) {
+            .duration => false,
+            .timestamp => true,
+        } }, &timespec, &timespec))) {
+            .SUCCESS => return,
+            .FAULT => unreachable,
+            .INTR => {},
+            .INVAL => return error.UnsupportedClock,
+            else => |err| return std.posix.unexpectedErrno(err),
+        }
+    }
+}
+
+fn select(userdata: ?*anyopaque, futures: []const *Io.AnyFuture) usize {
+    const pool: *Pool = @alignCast(@ptrCast(userdata));
+    _ = pool;
+
+    var reset_event: std.Thread.ResetEvent = .{};
+
+    for (futures, 0..) |future, i| {
+        const closure: *AsyncClosure = @ptrCast(@alignCast(future));
+        if (@atomicRmw(?*std.Thread.ResetEvent, &closure.select_condition, .Xchg, &reset_event, .seq_cst) == AsyncClosure.done_reset_event) {
+            for (futures[0..i]) |cleanup_future| {
+                const cleanup_closure: *AsyncClosure = @ptrCast(@alignCast(cleanup_future));
+                if (@atomicRmw(?*std.Thread.ResetEvent, &cleanup_closure.select_condition, .Xchg, null, .seq_cst) == AsyncClosure.done_reset_event) {
+                    cleanup_closure.reset_event.wait(); // Ensure no reference to our stack-allocated reset_event.
+                }
+            }
+            return i;
+        }
+    }
+
+    reset_event.wait();
+
+    var result: ?usize = null;
+    for (futures, 0..) |future, i| {
+        const closure: *AsyncClosure = @ptrCast(@alignCast(future));
+        if (@atomicRmw(?*std.Thread.ResetEvent, &closure.select_condition, .Xchg, null, .seq_cst) == AsyncClosure.done_reset_event) {
+            closure.reset_event.wait(); // Ensure no reference to our stack-allocated reset_event.
+            if (result == null) result = i; // In case multiple are ready, return first.
+        }
+    }
+    return result.?;
+}
diff --git a/lib/std/Thread/Pool.zig b/lib/std/Thread/Pool.zig
index b9d60c3568..e836665d70 100644
--- a/lib/std/Thread/Pool.zig
+++ b/lib/std/Thread/Pool.zig
@@ -1,34 +1,27 @@
-const builtin = @import("builtin");
 const std = @import("std");
-const Allocator = std.mem.Allocator;
-const assert = std.debug.assert;
-const WaitGroup = @import("WaitGroup.zig");
-const Io = std.Io;
+const builtin = @import("builtin");
 const Pool = @This();
+const WaitGroup = @import("WaitGroup.zig");
 
-/// Must be a thread-safe allocator.
-allocator: std.mem.Allocator,
 mutex: std.Thread.Mutex = .{},
 cond: std.Thread.Condition = .{},
 run_queue: std.SinglyLinkedList = .{},
 is_running: bool = true,
-threads: std.ArrayListUnmanaged(std.Thread),
+allocator: std.mem.Allocator,
+threads: if (builtin.single_threaded) [0]std.Thread else []std.Thread,
 ids: if (builtin.single_threaded) struct {
     inline fn deinit(_: @This(), _: std.mem.Allocator) void {}
     fn getIndex(_: @This(), _: std.Thread.Id) usize {
         return 0;
     }
 } else std.AutoArrayHashMapUnmanaged(std.Thread.Id, void),
-stack_size: usize,
 
-threadlocal var current_closure: ?*AsyncClosure = null;
-
-pub const Runnable = struct {
+const Runnable = struct {
     runFn: RunProto,
     node: std.SinglyLinkedList.Node = .{},
 };
 
-pub const RunProto = *const fn (*Runnable, id: ?usize) void;
+const RunProto = *const fn (*Runnable, id: ?usize) void;
 
 pub const Options = struct {
     allocator: std.mem.Allocator,
@@ -38,36 +31,48 @@ pub const Options = struct {
 };
 
 pub fn init(pool: *Pool, options: Options) !void {
-    const gpa = options.allocator;
-    const thread_count = options.n_jobs orelse @max(1, std.Thread.getCpuCount() catch 1);
-    const threads = try gpa.alloc(std.Thread, thread_count);
-    errdefer gpa.free(threads);
+    const allocator = options.allocator;
 
     pool.* = .{
-        .allocator = gpa,
-        .threads = .initBuffer(threads),
+        .allocator = allocator,
+        .threads = if (builtin.single_threaded) .{} else &.{},
         .ids = .{},
-        .stack_size = options.stack_size,
     };
 
-    if (builtin.single_threaded) return;
+    if (builtin.single_threaded) {
+        return;
+    }
 
+    const thread_count = options.n_jobs orelse @max(1, std.Thread.getCpuCount() catch 1);
     if (options.track_ids) {
-        try pool.ids.ensureTotalCapacity(gpa, 1 + thread_count);
+        try pool.ids.ensureTotalCapacity(allocator, 1 + thread_count);
         pool.ids.putAssumeCapacityNoClobber(std.Thread.getCurrentId(), {});
     }
+
+    // kill and join any threads we spawned and free memory on error.
+    pool.threads = try allocator.alloc(std.Thread, thread_count);
+    var spawned: usize = 0;
+    errdefer pool.join(spawned);
+
+    for (pool.threads) |*thread| {
+        thread.* = try std.Thread.spawn(.{
+            .stack_size = options.stack_size,
+            .allocator = allocator,
+        }, worker, .{pool});
+        spawned += 1;
+    }
 }
 
 pub fn deinit(pool: *Pool) void {
-    const gpa = pool.allocator;
-    pool.join();
-    pool.threads.deinit(gpa);
-    pool.ids.deinit(gpa);
+    pool.join(pool.threads.len); // kill and join all threads.
+    pool.ids.deinit(pool.allocator);
     pool.* = undefined;
 }
 
-fn join(pool: *Pool) void {
-    if (builtin.single_threaded) return;
+fn join(pool: *Pool, spawned: usize) void {
+    if (builtin.single_threaded) {
+        return;
+    }
 
     {
         pool.mutex.lock();
@@ -80,7 +85,11 @@ fn join(pool: *Pool) void {
     // wake up any sleeping threads (this can be done outside the mutex)
     // then wait for all the threads we know are spawned to complete.
     pool.cond.broadcast();
-    for (pool.threads.items) |thread| thread.join();
+    for (pool.threads[0..spawned]) |thread| {
+        thread.join();
+    }
+
+    pool.allocator.free(pool.threads);
 }
 
 /// Runs `func` in the thread pool, calling `WaitGroup.start` beforehand, and
@@ -108,38 +117,36 @@ pub fn spawnWg(pool: *Pool, wait_group: *WaitGroup, comptime func: anytype, args
             const closure: *@This() = @alignCast(@fieldParentPtr("runnable", runnable));
             @call(.auto, func, closure.arguments);
             closure.wait_group.finish();
+
+            // The thread pool's allocator is protected by the mutex.
+            const mutex = &closure.pool.mutex;
+            mutex.lock();
+            defer mutex.unlock();
+
             closure.pool.allocator.destroy(closure);
         }
     };
 
-    pool.mutex.lock();
+    {
+        pool.mutex.lock();
 
-    const gpa = pool.allocator;
-    const closure = gpa.create(Closure) catch {
-        pool.mutex.unlock();
-        @call(.auto, func, args);
-        wait_group.finish();
-        return;
-    };
-    closure.* = .{
-        .arguments = args,
-        .pool = pool,
-        .wait_group = wait_group,
-    };
-
-    pool.run_queue.prepend(&closure.runnable.node);
-
-    if (pool.threads.items.len < pool.threads.capacity) {
-        pool.threads.addOneAssumeCapacity().* = std.Thread.spawn(.{
-            .stack_size = pool.stack_size,
-            .allocator = gpa,
-        }, worker, .{pool}) catch t: {
-            pool.threads.items.len -= 1;
-            break :t undefined;
+        const closure = pool.allocator.create(Closure) catch {
+            pool.mutex.unlock();
+            @call(.auto, func, args);
+            wait_group.finish();
+            return;
         };
+        closure.* = .{
+            .arguments = args,
+            .pool = pool,
+            .wait_group = wait_group,
+        };
+
+        pool.run_queue.prepend(&closure.runnable.node);
+        pool.mutex.unlock();
     }
 
-    pool.mutex.unlock();
+    // Notify waiting threads outside the lock to try and keep the critical section small.
     pool.cond.signal();
 }
 
@@ -172,43 +179,41 @@ pub fn spawnWgId(pool: *Pool, wait_group: *WaitGroup, comptime func: anytype, ar
             const closure: *@This() = @alignCast(@fieldParentPtr("runnable", runnable));
             @call(.auto, func, .{id.?} ++ closure.arguments);
             closure.wait_group.finish();
+
+            // The thread pool's allocator is protected by the mutex.
+            const mutex = &closure.pool.mutex;
+            mutex.lock();
+            defer mutex.unlock();
+
             closure.pool.allocator.destroy(closure);
         }
     };
 
-    pool.mutex.lock();
+    {
+        pool.mutex.lock();
 
-    const gpa = pool.allocator;
-    const closure = gpa.create(Closure) catch {
-        const id: ?usize = pool.ids.getIndex(std.Thread.getCurrentId());
-        pool.mutex.unlock();
-        @call(.auto, func, .{id.?} ++ args);
-        wait_group.finish();
-        return;
-    };
-    closure.* = .{
-        .arguments = args,
-        .pool = pool,
-        .wait_group = wait_group,
-    };
-
-    pool.run_queue.prepend(&closure.runnable.node);
-
-    if (pool.threads.items.len < pool.threads.capacity) {
-        pool.threads.addOneAssumeCapacity().* = std.Thread.spawn(.{
-            .stack_size = pool.stack_size,
-            .allocator = gpa,
-        }, worker, .{pool}) catch t: {
-            pool.threads.items.len -= 1;
-            break :t undefined;
+        const closure = pool.allocator.create(Closure) catch {
+            const id: ?usize = pool.ids.getIndex(std.Thread.getCurrentId());
+            pool.mutex.unlock();
+            @call(.auto, func, .{id.?} ++ args);
+            wait_group.finish();
+            return;
         };
+        closure.* = .{
+            .arguments = args,
+            .pool = pool,
+            .wait_group = wait_group,
+        };
+
+        pool.run_queue.prepend(&closure.runnable.node);
+        pool.mutex.unlock();
     }
 
-    pool.mutex.unlock();
+    // Notify waiting threads outside the lock to try and keep the critical section small.
     pool.cond.signal();
 }
 
-pub fn spawn(pool: *Pool, comptime func: anytype, args: anytype) void {
+pub fn spawn(pool: *Pool, comptime func: anytype, args: anytype) !void {
     if (builtin.single_threaded) {
         @call(.auto, func, args);
         return;
@@ -223,36 +228,30 @@ pub fn spawn(pool: *Pool, comptime func: anytype, args: anytype) void {
         fn runFn(runnable: *Runnable, _: ?usize) void {
             const closure: *@This() = @alignCast(@fieldParentPtr("runnable", runnable));
             @call(.auto, func, closure.arguments);
+
+            // The thread pool's allocator is protected by the mutex.
+            const mutex = &closure.pool.mutex;
+            mutex.lock();
+            defer mutex.unlock();
+
             closure.pool.allocator.destroy(closure);
         }
     };
 
-    pool.mutex.lock();
+    {
+        pool.mutex.lock();
+        defer pool.mutex.unlock();
 
-    const gpa = pool.allocator;
-    const closure = gpa.create(Closure) catch {
-        pool.mutex.unlock();
-        @call(.auto, func, args);
-        return;
-    };
-    closure.* = .{
-        .arguments = args,
-        .pool = pool,
-    };
-
-    pool.run_queue.prepend(&closure.runnable.node);
-
-    if (pool.threads.items.len < pool.threads.capacity) {
-        pool.threads.addOneAssumeCapacity().* = std.Thread.spawn(.{
-            .stack_size = pool.stack_size,
-            .allocator = gpa,
-        }, worker, .{pool}) catch t: {
-            pool.threads.items.len -= 1;
-            break :t undefined;
+        const closure = try pool.allocator.create(Closure);
+        closure.* = .{
+            .arguments = args,
+            .pool = pool,
         };
+
+        pool.run_queue.prepend(&closure.runnable.node);
     }
 
-    pool.mutex.unlock();
+    // Notify waiting threads outside the lock to try and keep the critical section small.
     pool.cond.signal();
 }
 
@@ -271,7 +270,7 @@ test spawn {
             .allocator = std.testing.allocator,
         });
         defer pool.deinit();
-        pool.spawn(TestFn.checkRun, .{&completed});
+        try pool.spawn(TestFn.checkRun, .{&completed});
     }
 
     try std.testing.expectEqual(true, completed);
@@ -323,530 +322,5 @@ pub fn waitAndWork(pool: *Pool, wait_group: *WaitGroup) void {
 }
 
 pub fn getIdCount(pool: *Pool) usize {
-    return @intCast(1 + pool.threads.items.len);
-}
-
-pub fn io(pool: *Pool) Io {
-    return .{
-        .userdata = pool,
-        .vtable = &.{
-            .async = async,
-            .await = await,
-            .go = go,
-            .cancel = cancel,
-            .cancelRequested = cancelRequested,
-            .select = select,
-
-            .mutexLock = mutexLock,
-            .mutexUnlock = mutexUnlock,
-
-            .conditionWait = conditionWait,
-            .conditionWake = conditionWake,
-
-            .createFile = createFile,
-            .openFile = openFile,
-            .closeFile = closeFile,
-            .pread = pread,
-            .pwrite = pwrite,
-
-            .now = now,
-            .sleep = sleep,
-        },
-    };
-}
-
-const AsyncClosure = struct {
-    func: *const fn (context: *anyopaque, result: *anyopaque) void,
-    runnable: Runnable = .{ .runFn = runFn },
-    reset_event: std.Thread.ResetEvent,
-    select_condition: ?*std.Thread.ResetEvent,
-    cancel_tid: std.Thread.Id,
-    context_offset: usize,
-    result_offset: usize,
-
-    const done_reset_event: *std.Thread.ResetEvent = @ptrFromInt(@alignOf(std.Thread.ResetEvent));
-
-    const canceling_tid: std.Thread.Id = switch (@typeInfo(std.Thread.Id)) {
-        .int => |int_info| switch (int_info.signedness) {
-            .signed => -1,
-            .unsigned => std.math.maxInt(std.Thread.Id),
-        },
-        .pointer => @ptrFromInt(std.math.maxInt(usize)),
-        else => @compileError("unsupported std.Thread.Id: " ++ @typeName(std.Thread.Id)),
-    };
-
-    fn runFn(runnable: *std.Thread.Pool.Runnable, _: ?usize) void {
-        const closure: *AsyncClosure = @alignCast(@fieldParentPtr("runnable", runnable));
-        const tid = std.Thread.getCurrentId();
-        if (@cmpxchgStrong(
-            std.Thread.Id,
-            &closure.cancel_tid,
-            0,
-            tid,
-            .acq_rel,
-            .acquire,
-        )) |cancel_tid| {
-            assert(cancel_tid == canceling_tid);
-            return;
-        }
-        current_closure = closure;
-        closure.func(closure.contextPointer(), closure.resultPointer());
-        current_closure = null;
-        if (@cmpxchgStrong(
-            std.Thread.Id,
-            &closure.cancel_tid,
-            tid,
-            0,
-            .acq_rel,
-            .acquire,
-        )) |cancel_tid| assert(cancel_tid == canceling_tid);
-
-        if (@atomicRmw(
-            ?*std.Thread.ResetEvent,
-            &closure.select_condition,
-            .Xchg,
-            done_reset_event,
-            .release,
-        )) |select_reset| {
-            assert(select_reset != done_reset_event);
-            select_reset.set();
-        }
-        closure.reset_event.set();
-    }
-
-    fn contextOffset(context_alignment: std.mem.Alignment) usize {
-        return context_alignment.forward(@sizeOf(AsyncClosure));
-    }
-
-    fn resultOffset(
-        context_alignment: std.mem.Alignment,
-        context_len: usize,
-        result_alignment: std.mem.Alignment,
-    ) usize {
-        return result_alignment.forward(contextOffset(context_alignment) + context_len);
-    }
-
-    fn resultPointer(closure: *AsyncClosure) [*]u8 {
-        const base: [*]u8 = @ptrCast(closure);
-        return base + closure.result_offset;
-    }
-
-    fn contextPointer(closure: *AsyncClosure) [*]u8 {
-        const base: [*]u8 = @ptrCast(closure);
-        return base + closure.context_offset;
-    }
-
-    fn waitAndFree(closure: *AsyncClosure, gpa: Allocator, result: []u8) void {
-        closure.reset_event.wait();
-        const base: [*]align(@alignOf(AsyncClosure)) u8 = @ptrCast(closure);
-        @memcpy(result, closure.resultPointer()[0..result.len]);
-        gpa.free(base[0 .. closure.result_offset + result.len]);
-    }
-};
-
-fn async(
-    userdata: ?*anyopaque,
-    result: []u8,
-    result_alignment: std.mem.Alignment,
-    context: []const u8,
-    context_alignment: std.mem.Alignment,
-    start: *const fn (context: *const anyopaque, result: *anyopaque) void,
-) ?*Io.AnyFuture {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    pool.mutex.lock();
-
-    const gpa = pool.allocator;
-    const context_offset = context_alignment.forward(@sizeOf(AsyncClosure));
-    const result_offset = result_alignment.forward(context_offset + context.len);
-    const n = result_offset + result.len;
-    const closure: *AsyncClosure = @alignCast(@ptrCast(gpa.alignedAlloc(u8, @alignOf(AsyncClosure), n) catch {
-        pool.mutex.unlock();
-        start(context.ptr, result.ptr);
-        return null;
-    }));
-    closure.* = .{
-        .func = start,
-        .context_offset = context_offset,
-        .result_offset = result_offset,
-        .reset_event = .{},
-        .cancel_tid = 0,
-        .select_condition = null,
-    };
-    @memcpy(closure.contextPointer()[0..context.len], context);
-    pool.run_queue.prepend(&closure.runnable.node);
-
-    if (pool.threads.items.len < pool.threads.capacity) {
-        pool.threads.addOneAssumeCapacity().* = std.Thread.spawn(.{
-            .stack_size = pool.stack_size,
-            .allocator = gpa,
-        }, worker, .{pool}) catch t: {
-            pool.threads.items.len -= 1;
-            break :t undefined;
-        };
-    }
-
-    pool.mutex.unlock();
-    pool.cond.signal();
-
-    return @ptrCast(closure);
-}
-
-const DetachedClosure = struct {
-    pool: *Pool,
-    func: *const fn (context: *anyopaque) void,
-    run_node: std.Thread.Pool.RunQueue.Node = .{ .data = .{ .runFn = runFn } },
-    context_alignment: std.mem.Alignment,
-    context_len: usize,
-
-    fn runFn(runnable: *std.Thread.Pool.Runnable, _: ?usize) void {
-        const run_node: *std.Thread.Pool.RunQueue.Node = @fieldParentPtr("data", runnable);
-        const closure: *DetachedClosure = @alignCast(@fieldParentPtr("run_node", run_node));
-        closure.func(closure.contextPointer());
-        const gpa = closure.pool.allocator;
-        const base: [*]align(@alignOf(DetachedClosure)) u8 = @ptrCast(closure);
-        gpa.free(base[0..contextEnd(closure.context_alignment, closure.context_len)]);
-    }
-
-    fn contextOffset(context_alignment: std.mem.Alignment) usize {
-        return context_alignment.forward(@sizeOf(DetachedClosure));
-    }
-
-    fn contextEnd(context_alignment: std.mem.Alignment, context_len: usize) usize {
-        return contextOffset(context_alignment) + context_len;
-    }
-
-    fn contextPointer(closure: *DetachedClosure) [*]u8 {
-        const base: [*]u8 = @ptrCast(closure);
-        return base + contextOffset(closure.context_alignment);
-    }
-};
-
-fn go(
-    userdata: ?*anyopaque,
-    context: []const u8,
-    context_alignment: std.mem.Alignment,
-    start: *const fn (context: *const anyopaque) void,
-) void {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    pool.mutex.lock();
-
-    const gpa = pool.allocator;
-    const n = DetachedClosure.contextEnd(context_alignment, context.len);
-    const closure: *DetachedClosure = @alignCast(@ptrCast(gpa.alignedAlloc(u8, @alignOf(DetachedClosure), n) catch {
-        pool.mutex.unlock();
-        start(context.ptr);
-        return;
-    }));
-    closure.* = .{
-        .pool = pool,
-        .func = start,
-        .context_alignment = context_alignment,
-        .context_len = context.len,
-    };
-    @memcpy(closure.contextPointer()[0..context.len], context);
-    pool.run_queue.prepend(&closure.run_node);
-
-    if (pool.threads.items.len < pool.threads.capacity) {
-        pool.threads.addOneAssumeCapacity().* = std.Thread.spawn(.{
-            .stack_size = pool.stack_size,
-            .allocator = gpa,
-        }, worker, .{pool}) catch t: {
-            pool.threads.items.len -= 1;
-            break :t undefined;
-        };
-    }
-
-    pool.mutex.unlock();
-    pool.cond.signal();
-}
-
-fn await(
-    userdata: ?*anyopaque,
-    any_future: *std.Io.AnyFuture,
-    result: []u8,
-    result_alignment: std.mem.Alignment,
-) void {
-    _ = result_alignment;
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    const closure: *AsyncClosure = @ptrCast(@alignCast(any_future));
-    closure.waitAndFree(pool.allocator, result);
-}
-
-fn cancel(
-    userdata: ?*anyopaque,
-    any_future: *Io.AnyFuture,
-    result: []u8,
-    result_alignment: std.mem.Alignment,
-) void {
-    _ = result_alignment;
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    const closure: *AsyncClosure = @ptrCast(@alignCast(any_future));
-    switch (@atomicRmw(
-        std.Thread.Id,
-        &closure.cancel_tid,
-        .Xchg,
-        AsyncClosure.canceling_tid,
-        .acq_rel,
-    )) {
-        0, AsyncClosure.canceling_tid => {},
-        else => |cancel_tid| switch (builtin.os.tag) {
-            .linux => _ = std.os.linux.tgkill(
-                std.os.linux.getpid(),
-                @bitCast(cancel_tid),
-                std.posix.SIG.IO,
-            ),
-            else => {},
-        },
-    }
-    closure.waitAndFree(pool.allocator, result);
-}
-
-fn cancelRequested(userdata: ?*anyopaque) bool {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    _ = pool;
-    const closure = current_closure orelse return false;
-    return @atomicLoad(std.Thread.Id, &closure.cancel_tid, .acquire) == AsyncClosure.canceling_tid;
-}
-
-fn checkCancel(pool: *Pool) error{Canceled}!void {
-    if (cancelRequested(pool)) return error.Canceled;
-}
-
-fn mutexLock(userdata: ?*anyopaque, prev_state: Io.Mutex.State, mutex: *Io.Mutex) error{Canceled}!void {
-    _ = userdata;
-    if (prev_state == .contended) {
-        std.Thread.Futex.wait(@ptrCast(&mutex.state), @intFromEnum(Io.Mutex.State.contended));
-    }
-    while (@atomicRmw(
-        Io.Mutex.State,
-        &mutex.state,
-        .Xchg,
-        .contended,
-        .acquire,
-    ) != .unlocked) {
-        std.Thread.Futex.wait(@ptrCast(&mutex.state), @intFromEnum(Io.Mutex.State.contended));
-    }
-}
-fn mutexUnlock(userdata: ?*anyopaque, prev_state: Io.Mutex.State, mutex: *Io.Mutex) void {
-    _ = userdata;
-    _ = prev_state;
-    if (@atomicRmw(Io.Mutex.State, &mutex.state, .Xchg, .unlocked, .release) == .contended) {
-        std.Thread.Futex.wake(@ptrCast(&mutex.state), 1);
-    }
-}
-
-fn conditionWait(userdata: ?*anyopaque, cond: *Io.Condition, mutex: *Io.Mutex) Io.Cancelable!void {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    comptime assert(@TypeOf(cond.state) == u64);
-    const ints: *[2]std.atomic.Value(u32) = @ptrCast(&cond.state);
-    const cond_state = &ints[0];
-    const cond_epoch = &ints[1];
-    const one_waiter = 1;
-    const waiter_mask = 0xffff;
-    const one_signal = 1 << 16;
-    const signal_mask = 0xffff << 16;
-    // Observe the epoch, then check the state again to see if we should wake up.
-    // The epoch must be observed before we check the state or we could potentially miss a wake() and deadlock:
-    //
-    // - T1: s = LOAD(&state)
-    // - T2: UPDATE(&s, signal)
-    // - T2: UPDATE(&epoch, 1) + FUTEX_WAKE(&epoch)
-    // - T1: e = LOAD(&epoch) (was reordered after the state load)
-    // - T1: s & signals == 0 -> FUTEX_WAIT(&epoch, e) (missed the state update + the epoch change)
-    //
-    // Acquire barrier to ensure the epoch load happens before the state load.
-    var epoch = cond_epoch.load(.acquire);
-    var state = cond_state.fetchAdd(one_waiter, .monotonic);
-    assert(state & waiter_mask != waiter_mask);
-    state += one_waiter;
-
-    mutex.unlock(pool.io());
-    defer mutex.lock(pool.io()) catch @panic("TODO");
-
-    var futex_deadline = std.Thread.Futex.Deadline.init(null);
-
-    while (true) {
-        futex_deadline.wait(cond_epoch, epoch) catch |err| switch (err) {
-            error.Timeout => unreachable,
-        };
-
-        epoch = cond_epoch.load(.acquire);
-        state = cond_state.load(.monotonic);
-
-        // Try to wake up by consuming a signal and decremented the waiter we added previously.
-        // Acquire barrier ensures code before the wake() which added the signal happens before we decrement it and return.
-        while (state & signal_mask != 0) {
-            const new_state = state - one_waiter - one_signal;
-            state = cond_state.cmpxchgWeak(state, new_state, .acquire, .monotonic) orelse return;
-        }
-    }
-}
-
-fn conditionWake(userdata: ?*anyopaque, cond: *Io.Condition, wake: Io.Condition.Wake) void {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    _ = pool;
-    comptime assert(@TypeOf(cond.state) == u64);
-    const ints: *[2]std.atomic.Value(u32) = @ptrCast(&cond.state);
-    const cond_state = &ints[0];
-    const cond_epoch = &ints[1];
-    const one_waiter = 1;
-    const waiter_mask = 0xffff;
-    const one_signal = 1 << 16;
-    const signal_mask = 0xffff << 16;
-    var state = cond_state.load(.monotonic);
-    while (true) {
-        const waiters = (state & waiter_mask) / one_waiter;
-        const signals = (state & signal_mask) / one_signal;
-
-        // Reserves which waiters to wake up by incrementing the signals count.
-        // Therefore, the signals count is always less than or equal to the waiters count.
-        // We don't need to Futex.wake if there's nothing to wake up or if other wake() threads have reserved to wake up the current waiters.
-        const wakeable = waiters - signals;
-        if (wakeable == 0) {
-            return;
-        }
-
-        const to_wake = switch (wake) {
-            .one => 1,
-            .all => wakeable,
-        };
-
-        // Reserve the amount of waiters to wake by incrementing the signals count.
-        // Release barrier ensures code before the wake() happens before the signal it posted and consumed by the wait() threads.
-        const new_state = state + (one_signal * to_wake);
-        state = cond_state.cmpxchgWeak(state, new_state, .release, .monotonic) orelse {
-            // Wake up the waiting threads we reserved above by changing the epoch value.
-            // NOTE: a waiting thread could miss a wake up if *exactly* ((1<<32)-1) wake()s happen between it observing the epoch and sleeping on it.
-            // This is very unlikely due to how many precise amount of Futex.wake() calls that would be between the waiting thread's potential preemption.
-            //
-            // Release barrier ensures the signal being added to the state happens before the epoch is changed.
-            // If not, the waiting thread could potentially deadlock from missing both the state and epoch change:
-            //
-            // - T2: UPDATE(&epoch, 1) (reordered before the state change)
-            // - T1: e = LOAD(&epoch)
-            // - T1: s = LOAD(&state)
-            // - T2: UPDATE(&state, signal) + FUTEX_WAKE(&epoch)
-            // - T1: s & signals == 0 -> FUTEX_WAIT(&epoch, e) (missed both epoch change and state change)
-            _ = cond_epoch.fetchAdd(1, .release);
-            std.Thread.Futex.wake(cond_epoch, to_wake);
-            return;
-        };
-    }
-}
-
-fn createFile(
-    userdata: ?*anyopaque,
-    dir: Io.Dir,
-    sub_path: []const u8,
-    flags: Io.File.CreateFlags,
-) Io.File.OpenError!Io.File {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    try pool.checkCancel();
-    const fs_dir: std.fs.Dir = .{ .fd = dir.handle };
-    const fs_file = try fs_dir.createFile(sub_path, flags);
-    return .{ .handle = fs_file.handle };
-}
-
-fn openFile(
-    userdata: ?*anyopaque,
-    dir: Io.Dir,
-    sub_path: []const u8,
-    flags: Io.File.OpenFlags,
-) Io.File.OpenError!Io.File {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    try pool.checkCancel();
-    const fs_dir: std.fs.Dir = .{ .fd = dir.handle };
-    const fs_file = try fs_dir.openFile(sub_path, flags);
-    return .{ .handle = fs_file.handle };
-}
-
-fn closeFile(userdata: ?*anyopaque, file: Io.File) void {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    _ = pool;
-    const fs_file: std.fs.File = .{ .handle = file.handle };
-    return fs_file.close();
-}
-
-fn pread(userdata: ?*anyopaque, file: Io.File, buffer: []u8, offset: std.posix.off_t) Io.File.PReadError!usize {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    try pool.checkCancel();
-    const fs_file: std.fs.File = .{ .handle = file.handle };
-    return switch (offset) {
-        -1 => fs_file.read(buffer),
-        else => fs_file.pread(buffer, @bitCast(offset)),
-    };
-}
-
-fn pwrite(userdata: ?*anyopaque, file: Io.File, buffer: []const u8, offset: std.posix.off_t) Io.File.PWriteError!usize {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    try pool.checkCancel();
-    const fs_file: std.fs.File = .{ .handle = file.handle };
-    return switch (offset) {
-        -1 => fs_file.write(buffer),
-        else => fs_file.pwrite(buffer, @bitCast(offset)),
-    };
-}
-
-fn now(userdata: ?*anyopaque, clockid: std.posix.clockid_t) Io.ClockGetTimeError!Io.Timestamp {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    try pool.checkCancel();
-    const timespec = try std.posix.clock_gettime(clockid);
-    return @enumFromInt(@as(i128, timespec.sec) * std.time.ns_per_s + timespec.nsec);
-}
-
-fn sleep(userdata: ?*anyopaque, clockid: std.posix.clockid_t, deadline: Io.Deadline) Io.SleepError!void {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    const deadline_nanoseconds: i96 = switch (deadline) {
-        .duration => |duration| duration.nanoseconds,
-        .timestamp => |timestamp| @intFromEnum(timestamp),
-    };
-    var timespec: std.posix.timespec = .{
-        .sec = @intCast(@divFloor(deadline_nanoseconds, std.time.ns_per_s)),
-        .nsec = @intCast(@mod(deadline_nanoseconds, std.time.ns_per_s)),
-    };
-    while (true) {
-        try pool.checkCancel();
-        switch (std.os.linux.E.init(std.os.linux.clock_nanosleep(clockid, .{ .ABSTIME = switch (deadline) {
-            .duration => false,
-            .timestamp => true,
-        } }, &timespec, &timespec))) {
-            .SUCCESS => return,
-            .FAULT => unreachable,
-            .INTR => {},
-            .INVAL => return error.UnsupportedClock,
-            else => |err| return std.posix.unexpectedErrno(err),
-        }
-    }
-}
-
-fn select(userdata: ?*anyopaque, futures: []const *Io.AnyFuture) usize {
-    const pool: *std.Thread.Pool = @alignCast(@ptrCast(userdata));
-    _ = pool;
-
-    var reset_event: std.Thread.ResetEvent = .{};
-
-    for (futures, 0..) |future, i| {
-        const closure: *AsyncClosure = @ptrCast(@alignCast(future));
-        if (@atomicRmw(?*std.Thread.ResetEvent, &closure.select_condition, .Xchg, &reset_event, .seq_cst) == AsyncClosure.done_reset_event) {
-            for (futures[0..i]) |cleanup_future| {
-                const cleanup_closure: *AsyncClosure = @ptrCast(@alignCast(cleanup_future));
-                if (@atomicRmw(?*std.Thread.ResetEvent, &cleanup_closure.select_condition, .Xchg, null, .seq_cst) == AsyncClosure.done_reset_event) {
-                    cleanup_closure.reset_event.wait(); // Ensure no reference to our stack-allocated reset_event.
-                }
-            }
-            return i;
-        }
-    }
-
-    reset_event.wait();
-
-    var result: ?usize = null;
-    for (futures, 0..) |future, i| {
-        const closure: *AsyncClosure = @ptrCast(@alignCast(future));
-        if (@atomicRmw(?*std.Thread.ResetEvent, &closure.select_condition, .Xchg, null, .seq_cst) == AsyncClosure.done_reset_event) {
-            closure.reset_event.wait(); // Ensure no reference to our stack-allocated reset_event.
-            if (result == null) result = i; // In case multiple are ready, return first.
-        }
-    }
-    return result.?;
+    return @intCast(1 + pool.threads.len);
 }