[Merged by Bors] - Start running systems while prepare_systems is running

crates/bevy_ecs/Cargo.toml

@@ -21,6 +21,7 @@ bevy_utils = { path = "../bevy_utils", version = "0.8.0-dev" }
 bevy_ecs_macros = { path = "macros", version = "0.8.0-dev" }
 
 async-channel = "1.4"
+event-listener = "2.5"
 thread_local = "1.1.4"
 fixedbitset = "0.4"
 fxhash = "0.2"

crates/bevy_ecs/src/schedule/executor_parallel.rs

@@ -8,16 +8,15 @@ use async_channel::{Receiver, Sender};
 use bevy_tasks::{ComputeTaskPool, Scope, TaskPool};
 #[cfg(feature = "trace")]
 use bevy_utils::tracing::Instrument;
+use event_listener::Event;
 use fixedbitset::FixedBitSet;
 
 #[cfg(test)]
 use SchedulingEvent::*;
 
 struct SystemSchedulingMetadata {
     /// Used to signal the system's task to start the system.
-    start_sender: Sender<()>,
-    /// Receives the signal to start the system.
-    start_receiver: Receiver<()>,
+    start: Event,
     /// Indices of systems that depend on this one, used to decrement their
     /// dependency counters when this system finishes.
     dependants: Vec<usize>,
@@ -56,7 +55,11 @@ pub struct ParallelExecutor {
 
 impl Default for ParallelExecutor {
     fn default() -> Self {
-        let (finish_sender, finish_receiver) = async_channel::unbounded();
+        // Using a bounded channel here as it avoids allocations when signaling
+        // and generally remains hotter in memory. It'll take 128 systems completing
+        // before the parallel executor runs before this overflows. If it overflows
+        // all systems will just suspend until the parallel executor runs.
+        let (finish_sender, finish_receiver) = async_channel::bounded(128);
         Self {
             system_metadata: Default::default(),
             finish_sender,
@@ -84,10 +87,8 @@ impl ParallelSystemExecutor for ParallelExecutor {
         for container in systems.iter() {
             let dependencies_total = container.dependencies().len();
             let system = container.system();
-            let (start_sender, start_receiver) = async_channel::bounded(1);
             self.system_metadata.push(SystemSchedulingMetadata {
-                start_sender,
-                start_receiver,
+                start: Event::new(),
                 dependants: vec![],
                 dependencies_total,
                 dependencies_now: 0,
@@ -125,10 +126,13 @@ impl ParallelSystemExecutor for ParallelExecutor {
 
         ComputeTaskPool::init(TaskPool::default).scope(|scope| {
             self.prepare_systems(scope, systems, world);
+            if self.should_run.count_ones(..) == 0 {
+                return;
+            }
             let parallel_executor = async {
                 // All systems have been ran if there are no queued or running systems.
                 while 0 != self.queued.count_ones(..) + self.running.count_ones(..) {
-                    self.process_queued_systems().await;
+                    self.process_queued_systems();
                     // Avoid deadlocking if no systems were actually started.
                     if self.running.count_ones(..) != 0 {
                         // Wait until at least one system has finished.
@@ -166,33 +170,95 @@ impl ParallelExecutor {
         systems: &'scope mut [ParallelSystemContainer],
         world: &'scope World,
     ) {
+        // These are used as a part of a unit test.
+        #[cfg(test)]
+        let mut started_systems = 0;
         #[cfg(feature = "trace")]
         let _span = bevy_utils::tracing::info_span!("prepare_systems").entered();
         self.should_run.clear();
         for (index, (system_data, system)) in
             self.system_metadata.iter_mut().zip(systems).enumerate()
         {
+            let should_run = system.should_run();
+            let can_start = should_run
+                && system_data.dependencies_total == 0
+                && Self::can_start_now(
+                    self.non_send_running,
+                    system_data,
+                    &self.active_archetype_component_access,
+                );
+
+            // Queue the system if it has no dependencies, otherwise reset its dependency counter.
+            if system_data.dependencies_total == 0 {
+                if !can_start {
+                    self.queued.insert(index);
+                }
+            } else {
+                system_data.dependencies_now = system_data.dependencies_total;
+            }
+
+            if !should_run {
+                continue;
+            }
+
             // Spawn the system task.
-            if system.should_run() {
-                self.should_run.set(index, true);
-                let start_receiver = system_data.start_receiver.clone();
-                let finish_sender = self.finish_sender.clone();
-                let system = system.system_mut();
-                #[cfg(feature = "trace")] // NB: outside the task to get the TLS current span
-                let system_span = bevy_utils::tracing::info_span!("system", name = &*system.name());
+            self.should_run.insert(index);
+            let finish_sender = self.finish_sender.clone();
+            let system = system.system_mut();
+            #[cfg(feature = "trace")] // NB: outside the task to get the TLS current span
+            let system_span = bevy_utils::tracing::info_span!("system", name = &*system.name());
+            #[cfg(feature = "trace")]
+            let overhead_span =
+                bevy_utils::tracing::info_span!("system overhead", name = &*system.name());
+
+            let mut run = move || {
                 #[cfg(feature = "trace")]
-                let overhead_span =
-                    bevy_utils::tracing::info_span!("system overhead", name = &*system.name());
+                let _system_guard = system_span.enter();
+                unsafe { system.run_unsafe((), world) };
+            };
+
+            if can_start {
                 let task = async move {
-                    start_receiver
-                        .recv()
+                    run();
+                    // This will never panic:
+                    //  - The channel is never closed or dropped.
+                    //  - Overflowing the bounded size will just suspend until
+                    //    there is capacity.
+                    finish_sender
+                        .send(index)
                         .await
                         .unwrap_or_else(|error| unreachable!("{}", error));
-                    #[cfg(feature = "trace")]
-                    let system_guard = system_span.enter();
-                    unsafe { system.run_unsafe((), world) };
-                    #[cfg(feature = "trace")]
-                    drop(system_guard);
+                };
+
+                #[cfg(feature = "trace")]
+                let task = task.instrument(overhead_span);
+                if system_data.is_send {
+                    scope.spawn(task);
+                } else {
+                    scope.spawn_local(task);
+                }
+
+                #[cfg(test)]
+                {
+                    started_systems += 1;
+                }
+
+                self.running.insert(index);
+                if !system_data.is_send {
+                    self.non_send_running = true;
+                }
+                // Add this system's access information to the active access information.
+                self.active_archetype_component_access
+                    .extend(&system_data.archetype_component_access);
+            } else {
+                let start_listener = system_data.start.listen();
+                let task = async move {
+                    start_listener.await;
+                    run();
+                    // This will never panic:
+                    //  - The channel is never closed or dropped.
+                    //  - Overflowing the bounded size will just suspend until
+                    //    there is capacity.
                     finish_sender
                         .send(index)
                         .await
@@ -207,29 +273,33 @@ impl ParallelExecutor {
                     scope.spawn_local(task);
                 }
             }
-            // Queue the system if it has no dependencies, otherwise reset its dependency counter.
-            if system_data.dependencies_total == 0 {
-                self.queued.insert(index);
-            } else {
-                system_data.dependencies_now = system_data.dependencies_total;
-            }
+        }
+        #[cfg(test)]
+        if started_systems != 0 {
+            self.emit_event(StartedSystems(started_systems));
         }
     }
 
     /// Determines if the system with given index has no conflicts with already running systems.
-    fn can_start_now(&self, index: usize) -> bool {
-        let system_data = &self.system_metadata[index];
+    #[inline]
+    fn can_start_now(
+        non_send_running: bool,
+        system_data: &SystemSchedulingMetadata,
+        active_archetype_component_access: &Access<ArchetypeComponentId>,
+    ) -> bool {
         // Non-send systems are considered conflicting with each other.
-        (!self.non_send_running || system_data.is_send)
+        (!non_send_running || system_data.is_send)
             && system_data
                 .archetype_component_access
-                .is_compatible(&self.active_archetype_component_access)
+                .is_compatible(active_archetype_component_access)
     }
 
     /// Starts all non-conflicting queued systems, moves them from `queued` to `running`,
     /// adds their access information to active access information;
     /// processes queued systems that shouldn't run this iteration as completed immediately.
-    async fn process_queued_systems(&mut self) {
+    fn process_queued_systems(&mut self) {
+        // These are used as a part of a unit test as seen in `process_queued_systems`.
+        // Removing them will cause the test to fail.
         #[cfg(test)]
         let mut started_systems = 0;
         for index in self.queued.ones() {
@@ -238,17 +308,17 @@ impl ParallelExecutor {
             let system_metadata = &self.system_metadata[index];
             if !self.should_run[index] {
                 self.dependants_scratch.extend(&system_metadata.dependants);
-            } else if self.can_start_now(index) {
+            } else if Self::can_start_now(
+                self.non_send_running,
+                system_metadata,
+                &self.active_archetype_component_access,
+            ) {
                 #[cfg(test)]
                 {
                     started_systems += 1;
                 }
-                system_metadata
-                    .start_sender
-                    .send(())
-                    .await
-                    .unwrap_or_else(|error| unreachable!("{}", error));
-                self.running.set(index, true);
+                system_metadata.start.notify_additional_relaxed(1);
+                self.running.insert(index);
                 if !system_metadata.is_send {
                     self.non_send_running = true;
                 }