zig

commit d60760d61e19018aa354fbb2f241444db88ca51b (tree)
parent 29e418cbfb3d5e5800ac2ad267b21cf2b8942c2c
Author: Andrew Kelley <andrew@ziglang.org>
Date:   Fri, 28 Nov 2025 18:07:42 -0800

std.Io.Threaded: tune requestCancel

On a heavily loaded Linux 6.17.5, I observed a maximum of 20 attempts
not acknowledged before the timeout (including exponential backoff) was
sufficient, despite the heavy load.

The time wasted here sleeping is mitigated by the fact that, later on,
the system will likely wait for the canceled task, causing it to
indefinitely yield until the canceled task finishes, and the task must
acknowledge the cancel before it proceeds to that point.

Diffstat:
M
lib/std/Io/Threaded.zig
 | 
27
++++++++++++++++++++++-----

1 file changed, 22 insertions(+), 5 deletions(-)
diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig
@@ -201,8 +201,22 @@ const Closure = struct {
         // We can send a signal to interrupt the syscall, but if it arrives before
         // the syscall instruction, it will be missed. Therefore, this code tries
         // again until the cancellation request is acknowledged.
-        const max_attempts = 3;
-        for (0..max_attempts) |_| {
+
+        // 1 << 10 ns is about 1 microsecond, approximately syscall overhead.
+        // 1 << 20 ns is about 1 millisecond.
+        // 1 << 30 ns is about 1 second.
+        //
+        // On a heavily loaded Linux 6.17.5, I observed a maximum of 20
+        // attempts not acknowledged before the timeout (including exponential
+        // backoff) was sufficient, despite the heavy load.
+        //
+        // The time wasted here sleeping is mitigated by the fact that, later
+        // on, the system will likely wait for the canceled task, causing it
+        // to indefinitely yield until the canceled task finishes, and the
+        // task must acknowledge the cancel before it proceeds to that point.
+        const max_attempts = 22;
+
+        for (0..max_attempts) |attempt_index| {
             if (std.Thread.use_pthreads) {
                 const rc = std.c.pthread_kill(signal_id, .IO);
                 if (is_debug) assert(rc == 0);
@@ -219,11 +233,14 @@ const Closure = struct {
                 return;
             }
 
-            // TODO make this a nanosleep with 1 << attempt duration
-            std.Thread.yield() catch {};
+            var timespec: posix.timespec = .{
+                .sec = 0,
+                .nsec = @as(isize, 1) << @intCast(attempt_index),
+            };
+            _ = posix.system.nanosleep(&timespec, &timespec);
 
             switch (@atomicRmw(CancelStatus, &closure.cancel_status, .Xchg, .requested, .monotonic).unpack()) {
-                .requested => continue,
+                .requested => continue, // Retry needed in case other thread hasn't yet entered the syscall.
                 .none, .acknowledged => return,
                 .signal_id => |new_signal_id| signal_id = new_signal_id,
             }