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,	...	@@ -39,7 +39,6 @@ cpu_count_error: ?std.Thread.CpuCountError,
39	busy_count: usize = 0,	39	busy_count: usize = 0,
40	worker_threads: std.atomic.Value(?*Thread),	40	worker_threads: std.atomic.Value(?*Thread),
41	pid: Pid = .unknown,	41	pid: Pid = .unknown,
42	robust_cancel: RobustCancel,
43		42
44	wsa: if (is_windows) Wsa else struct {} = .{},	43	wsa: if (is_windows) Wsa else struct {} = .{},
45		44
...	@@ -105,8 +104,6 @@ pub const Environ = struct {	...	@@ -105,8 +104,6 @@ pub const Environ = struct {
105	};	104	};
106	};	105	};
107		106
108	pub const RobustCancel = enum { enabled, disabled };
109
110	pub const Pid = if (native_os == .linux) enum(posix.pid_t) {	107	pub const Pid = if (native_os == .linux) enum(posix.pid_t) {
111	unknown = 0,	108	unknown = 0,
112	_,	109	_,
...	@@ -315,7 +312,7 @@ const Group = struct {	...	@@ -315,7 +312,7 @@ const Group = struct {
315	var need_signal: bool = !skip_signals and g.cancelThreads(t);	312	var need_signal: bool = !skip_signals and g.cancelThreads(t);
316	var timeout_ns: u64 = 1 << 10;	313	var timeout_ns: u64 = 1 << 10;
317	while (true) {	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	Thread.futexWaitUncancelable(&num_completed.raw, 0, if (need_signal) timeout_ns else null);	316	Thread.futexWaitUncancelable(&num_completed.raw, 0, if (need_signal) timeout_ns else null);
320	switch (num_completed.load(.acquire)) { // acquire task results	317	switch (num_completed.load(.acquire)) { // acquire task results
321	0 => {},	318	0 => {},
...	@@ -469,7 +466,7 @@ const Future = struct {	...	@@ -469,7 +466,7 @@ const Future = struct {
469	var need_signal: bool = thread != null and thread.?.cancelAwaitable(.fromFuture(future));	466	var need_signal: bool = thread != null and thread.?.cancelAwaitable(.fromFuture(future));
470	var timeout_ns: u64 = 1 << 10;	467	var timeout_ns: u64 = 1 << 10;
471	while (true) {	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	Thread.futexWaitUncancelable(&num_completed.raw, 0, if (need_signal) timeout_ns else null);	470	Thread.futexWaitUncancelable(&num_completed.raw, 0, if (need_signal) timeout_ns else null);
474	switch (num_completed.load(.acquire)) { // acquire task results	471	switch (num_completed.load(.acquire)) { // acquire task results
475	0 => {},	472	0 => {},
...	@@ -1112,17 +1109,6 @@ pub const InitOptions = struct {	...	@@ -1112,17 +1109,6 @@ pub const InitOptions = struct {
1112	/// concurrent tasks. After this number, calls to `Io.concurrent` return	1109	/// concurrent tasks. After this number, calls to `Io.concurrent` return
1113	/// `error.ConcurrencyUnavailable`.	1110	/// `error.ConcurrencyUnavailable`.
1114	concurrent_limit: Io.Limit = .unlimited,	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	/// Affects the following operations:	1112	/// Affects the following operations:
1127	/// * `processExecutablePath` on OpenBSD and Haiku.	1113	/// * `processExecutablePath` on OpenBSD and Haiku.
1128	argv0: Argv0 = .{},	1114	argv0: Argv0 = .{},
...	@@ -1160,7 +1146,6 @@ pub fn init(	...	@@ -1160,7 +1146,6 @@ pub fn init(
1160	.have_signal_handler = false,	1146	.have_signal_handler = false,
1161	.argv0 = options.argv0,	1147	.argv0 = options.argv0,
1162	.environ = options.environ,	1148	.environ = options.environ,
1163	.robust_cancel = options.robust_cancel,
1164	.worker_threads = .init(null),	1149	.worker_threads = .init(null),
1165	};	1150	};
1166		1151
...	@@ -1195,7 +1180,6 @@ pub const init_single_threaded: Threaded = .{	...	@@ -1195,7 +1180,6 @@ pub const init_single_threaded: Threaded = .{
1195	.old_sig_io = undefined,	1180	.old_sig_io = undefined,
1196	.old_sig_pipe = undefined,	1181	.old_sig_pipe = undefined,
1197	.have_signal_handler = false,	1182	.have_signal_handler = false,
1198	.robust_cancel = .disabled,
1199	.argv0 = .{},	1183	.argv0 = .{},
1200	.environ = .{},	1184	.environ = .{},
1201	.worker_threads = .init(null),	1185	.worker_threads = .init(null),