improvements for async-io

build.zig

@@ -1180,9 +1180,9 @@ const exercises = [_]Exercise{
     .{
         .main_file = "093_async9.zig",
         .output =
-        \\Main thread continues...
-        \\Computing on a separate thread!
-        \\Main thread done waiting.
+        \\Computing concurrently!
+        \\Main continues...
+        \\Main done waiting.
         \\Result: 123
         , // pay attention to the comma
     },

exercises/093_async9.zig

@@ -5,22 +5,29 @@
 // The difference:
 //
 //   io.async():
-//     * The function MAY run on another thread, or it may run
-//       immediately on the current thread (synchronously).
-//     * Never fails — if no thread is available, it just runs
-//       the function right away.
+//     * The function MAY run on a separate unit of concurrency,
+//       or it may run immediately on the caller (synchronously).
+//     * Never fails — if no concurrency is available, it just
+//       runs the function right away.
 //     * More portable, works with all Io backends.
 //
 //   io.concurrent():
-//     * GUARANTEES a separate unit of concurrency (a real thread
-//       in the Threaded backend).
+//     * GUARANTEES a separate unit of concurrency.
 //     * Can fail with error.ConcurrencyUnavailable if resources
 //       are exhausted or the backend doesn't support it.
-//     * Use when you NEED true parallelism.
+//     * Use when you NEED the task to run independently of the
+//       caller.
+//
+// What is a "unit of concurrency"? That depends on the backend!
+// The Threaded backend uses OS threads. But the Evented backends
+// (Uring, Kqueue, Dispatch) use M:N green threads / fibers,
+// which can provide concurrency even on a SINGLE OS thread.
+// Your code doesn't need to know the difference.
 //
 // Because concurrent() can fail, you must handle the error:
 //
 //     var future = try io.concurrent(myFn, .{args});
+//     defer _ = future.cancel(io);
 //     const result = future.await(io);
 //
 // Notice the 'try' — that's the key difference in usage!
@@ -33,25 +40,30 @@ const print = std.debug.print;
 pub fn main(init: std.process.Init) !void {
     const io = init.io;
 
-    // Launch with a guaranteed separate thread.
-    // Which Io method guarantees true concurrency?
+    // Launch with a guaranteed separate unit of concurrency.
+    // Which Io method guarantees this?
     // (Hint: unlike io.async, this one can fail!)
     var future = try io.???(compute, .{io});
+    defer _ = future.cancel(io);
 
-    print("Main thread continues...\n", .{});
-
-    // Wait 100 millisecond so the output order is deterministic.
+    // Note: All breaks in this excercise (using sleep)
+    // are only necessary for a deterministic result.
     io.sleep(std.Io.Duration.fromMilliseconds(100), .awake) catch {};
 
-    print("Main thread done waiting.\n", .{});
+    print("Main continues...\n", .{});
+
+    // Wait 1 second for the output order.
+    io.sleep(std.Io.Duration.fromMilliseconds(200), .awake) catch {};
+
+    print("Main done waiting.\n", .{});
 
     const result = future.await(io);
     print("Result: {}\n", .{result});
 }
 
 fn compute(io: std.Io) u32 {
-    print("Computing on a separate thread!\n", .{});
+    print("Computing concurrently!\n", .{});
     // Simulate some work.
-    io.sleep(std.Io.Duration.fromMilliseconds(200), .awake) catch return 0;
+    io.sleep(std.Io.Duration.fromMilliseconds(400), .awake) catch return 0;
     return 123;
 }

patches/patches/093_async9.patch

@@ -1,11 +1,11 @@
---- exercises/093_async9.zig	2026-04-03 13:44:50.526780809 +0200
-+++ answers/093_async9.zig	2026-04-03 13:44:54.957870294 +0200
-@@ -36,7 +36,7 @@
-     // Launch with a guaranteed separate thread.
-     // Which Io method guarantees true concurrency?
+--- exercises/093_async9.zig	2026-04-06 19:26:11.388025362 +0200
++++ answers/093_async9.zig	2026-04-06 19:18:36.242931688 +0200
+@@ -43,7 +43,7 @@
+     // Launch with a guaranteed separate unit of concurrency.
+     // Which Io method guarantees this?
      // (Hint: unlike io.async, this one can fail!)
 -    var future = try io.???(compute, .{io});
 +    var future = try io.concurrent(compute, .{io});
+     defer _ = future.cancel(io);
  
-     print("Main thread continues...\n", .{});
- 
+     // Note: All breaks in this excercise (using sleep)