Add support for optional out-of-band garbage collection.

ext/io/event/selector/epoll.c

@@ -870,6 +870,8 @@ void * select_internal(void *_arguments) {
 
 static
 void select_internal_without_gvl(struct select_arguments *arguments) {
+	IO_Event_Selector_before_waiting(arguments->selector);
+
 	arguments->selector->blocked = 1;
 	rb_thread_call_without_gvl(select_internal, (void *)arguments, RUBY_UBF_IO, 0);
 	arguments->selector->blocked = 0;

ext/io/event/selector/kqueue.c

@@ -842,6 +842,8 @@ void * select_internal(void *_arguments) {
 
 static
 void select_internal_without_gvl(struct select_arguments *arguments) {
+	IO_Event_Selector_before_waiting(arguments->selector);
+
 	arguments->selector->blocked = 1;
 
 	rb_thread_call_without_gvl(select_internal, (void *)arguments, RUBY_UBF_IO, 0);

ext/io/event/selector/selector.c

@@ -78,6 +78,8 @@ static VALUE IO_Event_Selector_nonblock(VALUE class, VALUE io)
 	return rb_ensure(rb_yield, io, IO_Event_Selector_nonblock_ensure, (VALUE)&arguments);
 }
 
+static float IO_Event_Selector_GARBAGE_COLLECTION = -1;
+
 void Init_IO_Event_Selector(VALUE IO_Event_Selector) {
 #ifndef HAVE_RB_IO_DESCRIPTOR
 	id_fileno = rb_intern("fileno");
@@ -89,6 +91,11 @@ void Init_IO_Event_Selector(VALUE IO_Event_Selector) {
 	rb_gc_register_mark_object(rb_Process_Status);
 #endif
 
+	char *garbage_collection_env = getenv("IO_EVENT_SELECTOR_GARBAGE_COLLECTION");
+	if (garbage_collection_env) {
+		IO_Event_Selector_GARBAGE_COLLECTION = atof(garbage_collection_env);
+	}
+
 	rb_define_singleton_method(IO_Event_Selector, "nonblock", IO_Event_Selector_nonblock, 1);
 }
 
@@ -98,6 +105,9 @@ void IO_Event_Selector_initialize(struct IO_Event_Selector *backend, VALUE self,
 
 	backend->waiting = NULL;
 	backend->ready = NULL;
+
+	backend->garbage_collection = IO_Event_Selector_GARBAGE_COLLECTION;
+	backend->last_garbage_collection = (struct timespec){0, 0};
 }
 
 VALUE IO_Event_Selector_loop_resume(struct IO_Event_Selector *backend, VALUE fiber, int argc, VALUE *argv) {

ext/io/event/selector/selector.h

@@ -81,10 +81,27 @@ struct IO_Event_Selector {
 	struct IO_Event_Selector_Queue *waiting;
 	// Process from ready (back/tail of queue).
 	struct IO_Event_Selector_Queue *ready;
+
+	// Minimum interval in seconds between out-of-band garbage collections, or < 0 to disable:
+	float garbage_collection;
+	struct timespec last_garbage_collection;
 };
 
 void IO_Event_Selector_initialize(struct IO_Event_Selector *backend, VALUE self, VALUE loop);
 
+static inline IO_Event_Selector_before_waiting(struct IO_Event_Selector *backend) {
+	// Experimental support for out-of-band garbage collection. This allows the selector to perform garbage collection at regular intervals while waiting for events, which can help reduce overall latency.
+	if (backend->garbage_collection >= 0) {
+		struct timespec now;
+		IO_Event_Time_current(&now);
+
+		if (IO_Event_Time_delta(&backend->last_garbage_collection, &now) >= backend->garbage_collection) {
+			rb_gc();
+			backend->last_garbage_collection = now;
+		}
+	}
+}
+
 static inline
 void IO_Event_Selector_mark(struct IO_Event_Selector *backend) {
 	rb_gc_mark_movable(backend->self);

ext/io/event/selector/uring.c

@@ -1053,6 +1053,8 @@ static
 int select_internal_without_gvl(struct select_arguments *arguments) {
 	io_uring_submit_flush(arguments->selector);
 
+	IO_Event_Selector_before_waiting(arguments->selector);
+
 	arguments->selector->blocked = 1;
 	rb_thread_call_without_gvl(select_internal, (void *)arguments, RUBY_UBF_IO, 0);
 	arguments->selector->blocked = 0;

releases.md

@@ -1,5 +1,9 @@
 # Releases
 
+## Unreleased
+
+  - **Experimental** Add support for out-of-band garbage collection. When enabled via the `IO_EVENT_SELECTOR_GARBAGE_COLLECTION` environment variable, the selector will run garbage collection at a minimum interval (in seconds) while the event loop is blocking, reducing GC pressure during active I/O. A negative value (default) disables the feature; `0` runs GC on every block.
+
 ## v1.14.4
 
   - Allow `epoll_pwait2` to be disabled via `--disable-epoll_pwait2`.