Commit 2c395e326f2db4e2389af729b88bd93e76e4d27c | zig

author	Matthew Lugg <mlugg@mlugg.co.uk> 2025-12-30 20:43:53+00:00
committer	Matthew Lugg <mlugg@mlugg.co.uk> 2026-01-03 15:45:10+00:00
log	2c395e326f2db4e2389af729b88bd93e76e4d27c
tree	d005a7ec304236a88d310f3013e032d817211270
parent	f27134d671450a2aac3d48c82ccfbc5a947849f3
signature	Commit is signed but in an unrecognized format.

std.Io.Threaded: always use robust cancelation

As of this branch, the performance impact of robust cancelation is now
negligible (and in fact entirely unmeasurable in almost all cases), so
there is no good reason to not enable it in all cases. The performance
issues before were primarily down to a typo in the robust cancelation
logic which resulted in every canceled syscall potentially being sent
hundreds of signals in quick succession, because the delay between
signals started out at 1ns instead of 1us!

1 files changed, 2 insertions(+), 18 deletions(-)

lib
- std
  - Io
    Threaded.zig

lib/std/Io/Threaded.zig+2-18

...	...	@@ -39,7 +39,6 @@ cpu_count_error: ?std.Thread.CpuCountError,
39	39	busy_count: usize = 0,
40	40	worker_threads: std.atomic.Value(?*Thread),
41	41	pid: Pid = .unknown,
42		robust_cancel: RobustCancel,
43	42
44	43	wsa: if (is_windows) Wsa else struct {} = .{},
45	44
...	...	@@ -105,8 +104,6 @@ pub const Environ = struct {
105	104	};
106	105	};
107	106
108		pub const RobustCancel = enum { enabled, disabled };
109
110	107	pub const Pid = if (native_os == .linux) enum(posix.pid_t) {
111	108	unknown = 0,
112	109	_,
...	...	@@ -315,7 +312,7 @@ const Group = struct {
315	312	var need_signal: bool = !skip_signals and g.cancelThreads(t);
316	313	var timeout_ns: u64 = 1 << 10;
317	314	while (true) {
318		need_signal = need_signal and g.signalAllCanceledSyscalls(t) and t.robust_cancel == .enabled;
	315	need_signal = need_signal and g.signalAllCanceledSyscalls(t);
319	316	Thread.futexWaitUncancelable(&num_completed.raw, 0, if (need_signal) timeout_ns else null);
320	317	switch (num_completed.load(.acquire)) { // acquire task results
321	318	0 => {},
...	...	@@ -469,7 +466,7 @@ const Future = struct {
469	466	var need_signal: bool = thread != null and thread.?.cancelAwaitable(.fromFuture(future));
470	467	var timeout_ns: u64 = 1 << 10;
471	468	while (true) {
472		need_signal = need_signal and thread.?.signalCanceledSyscall(t, .fromFuture(future)) and t.robust_cancel == .enabled;
	469	need_signal = need_signal and thread.?.signalCanceledSyscall(t, .fromFuture(future));
473	470	Thread.futexWaitUncancelable(&num_completed.raw, 0, if (need_signal) timeout_ns else null);
474	471	switch (num_completed.load(.acquire)) { // acquire task results
475	472	0 => {},
...	...	@@ -1112,17 +1109,6 @@ pub const InitOptions = struct {
1112	1109	/// concurrent tasks. After this number, calls to `Io.concurrent` return
1113	1110	/// `error.ConcurrencyUnavailable`.
1114	1111	concurrent_limit: Io.Limit = .unlimited,
1115		/// When a cancel request is made, blocking syscalls can be unblocked by
1116		/// issuing a signal. However, if the signal arrives after the check and before
1117		/// the syscall instruction, it is missed.
1118		///
1119		/// This option solves the race condition by retrying the signal delivery
1120		/// until it is acknowledged, with an exponential backoff.
1121		///
1122		/// Unfortunately, trying again until the cancellation request is acknowledged
1123		/// has been observed to be relatively slow, and usually strong cancellation
1124		/// guarantees are not needed, so this defaults to off.
1125		robust_cancel: RobustCancel = .disabled,
1126	1112	/// Affects the following operations:
1127	1113	/// * `processExecutablePath` on OpenBSD and Haiku.
1128	1114	argv0: Argv0 = .{},
...	...	@@ -1160,7 +1146,6 @@ pub fn init(
1160	1146	.have_signal_handler = false,
1161	1147	.argv0 = options.argv0,
1162	1148	.environ = options.environ,
1163		.robust_cancel = options.robust_cancel,
1164	1149	.worker_threads = .init(null),
1165	1150	};
1166	1151
...	...	@@ -1195,7 +1180,6 @@ pub const init_single_threaded: Threaded = .{
1195	1180	.old_sig_io = undefined,
1196	1181	.old_sig_pipe = undefined,
1197	1182	.have_signal_handler = false,
1198		.robust_cancel = .disabled,
1199	1183	.argv0 = .{},
1200	1184	.environ = .{},
1201	1185	.worker_threads = .init(null),