[Fiber] Yield every other frame for Transition/Retry work (#31828)

This flag first moves the `shouldYield()` logic into React itself. We
need this for `postTask` compatibility anyway since this logic is no
longer a concern of the scheduler. This means that there can also be no
global `requestPaint()` that asks for painting earlier. So this is best
rolled out with `enableAlwaysYieldScheduler` (and ideally
`enableYieldingBeforePassive`) instead of `enableRequestPaint`.

Once in React we can change the yield timing heuristics. This uses the
previous 5ms for Idle work to keep everything responsive while doing
background work. However, for Transitions and Retries we have seen that
same thread animations (like loading states animating, or constant
animations like cool Three.js stuff) can take CPU time away from the
Transition that causes moving into new content to slow down. Therefore
we only yield every 25ms.

The purpose of this yield is not to avoid the overhead of yielding,
which is very low, but rather to intentionally block any frequently
occurring other main thread work like animations from starving our work.
If we could we could just tell everyone else to throttle their stuff for
ideal scheduling but that's not quite realistic. In other words, the
purpose of this is to reduce the frame rate of animations to 30 fps and
we achieve this by not yielding. We still do yield to allow the
animations to not just stall. This seems like a good balance.

The 5ms of Idle is because we don't really need to yield less often
since the overhead is low. We keep it low to allow 120 fps animations to
run if necessary and our work may not be the only work within a frame so
we need to yield early enough to leave enough time left.

Similarly we choose 25ms rather than say 35ms to ensure that we push
long enough to guarantee to half the frame rate but low enough that
there's plenty of time left for a rAF to power each animation every
other frame. It's also low enough that if something else interrupts the
work like a new interaction, we can still be responsive to that within
50ms or so. We also need to yield in case there's I/O work that needs to
get bounced through the main thread.

This flag is currently off everywhere since we have so many other
scheduling flags but that means there's some urgency to roll those out
fully so we can test this one. There's also some tests to update since
this doesn't go through the Mock scheduler anymore for yields.

Author: sebmarkbage

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -41,6 +41,7 @@ import {
   enableSiblingPrerendering,
   enableComponentPerformanceTrack,
   enableYieldingBeforePassive,
+  enableThrottledScheduling,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import is from 'shared/objectIs';
@@ -2610,8 +2611,10 @@ function renderRootConcurrent(root: FiberRoot, lanes: Lanes) {
         // can't trust the result of `shouldYield`, because the host I/O is
         // likely mocked.
         workLoopSync();
+      } else if (enableThrottledScheduling) {
+        workLoopConcurrent(includesNonIdleWork(lanes));
       } else {
-        workLoopConcurrent();
+        workLoopConcurrentByScheduler();
       }
       break;
     } catch (thrownValue) {
@@ -2650,10 +2653,27 @@ function renderRootConcurrent(root: FiberRoot, lanes: Lanes) {
 }
 
 /** @noinline */
-function workLoopConcurrent() {
+function workLoopConcurrent(nonIdle: boolean) {
+  // We yield every other "frame" when rendering Transition or Retries. Those are blocking
+  // revealing new content. The purpose of this yield is not to avoid the overhead of yielding,
+  // which is very low, but rather to intentionally block any frequently occuring other main
+  // thread work like animations from starving our work. In other words, the purpose of this
+  // is to reduce the framerate of animations to 30 frames per second.
+  // For Idle work we yield every 5ms to keep animations going smooth.
+  if (workInProgress !== null) {
+    const yieldAfter = now() + (nonIdle ? 25 : 5);
+    do {
+      // $FlowFixMe[incompatible-call] flow doesn't know that now() is side-effect free
+      performUnitOfWork(workInProgress);
+    } while (workInProgress !== null && now() < yieldAfter);
+  }
+}
+
+/** @noinline */
+function workLoopConcurrentByScheduler() {
   // Perform work until Scheduler asks us to yield
   while (workInProgress !== null && !shouldYield()) {
-    // $FlowFixMe[incompatible-call] found when upgrading Flow
+    // $FlowFixMe[incompatible-call] flow doesn't know that shouldYield() is side-effect free
     performUnitOfWork(workInProgress);
   }
 }

packages/shared/ReactFeatureFlags.js

@@ -81,6 +81,9 @@ export const enableLegacyFBSupport = false;
 // Fix gated tests that fail with this flag enabled before turning it back on.
 export const enableYieldingBeforePassive = false;
 
+// Experiment to intentionally yield less to block high framerate animations.
+export const enableThrottledScheduling = false;
+
 export const enableLegacyCache = __EXPERIMENTAL__;
 
 export const enableAsyncIterableChildren = __EXPERIMENTAL__;

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -81,6 +81,7 @@ export const syncLaneExpirationMs = 250;
 export const transitionLaneExpirationMs = 5000;
 export const enableHydrationLaneScheduling = true;
 export const enableYieldingBeforePassive = false;
+export const enableThrottledScheduling = false;
 
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): FeatureFlagsType): ExportsType);

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -73,6 +73,8 @@ export const enableHydrationLaneScheduling = true;
 
 export const enableYieldingBeforePassive = false;
 
+export const enableThrottledScheduling = false;
+
 // Profiling Only
 export const enableProfilerTimer = __PROFILE__;
 export const enableProfilerCommitHooks = __PROFILE__;

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -72,6 +72,8 @@ export const enableUseResourceEffectHook = false;
 
 export const enableYieldingBeforePassive = true;
 
+export const enableThrottledScheduling = false;
+
 // TODO: This must be in sync with the main ReactFeatureFlags file because
 // the Test Renderer's value must be the same as the one used by the
 // react package.

packages/shared/forks/ReactFeatureFlags.test-renderer.native-fb.js

@@ -69,6 +69,7 @@ export const enableSiblingPrerendering = true;
 export const enableUseResourceEffectHook = true;
 export const enableHydrationLaneScheduling = true;
 export const enableYieldingBeforePassive = false;
+export const enableThrottledScheduling = false;
 
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): FeatureFlagsType): ExportsType);

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -84,5 +84,7 @@ export const enableHydrationLaneScheduling = true;
 
 export const enableYieldingBeforePassive = false;
 
+export const enableThrottledScheduling = false;
+
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): FeatureFlagsType): ExportsType);

packages/shared/forks/ReactFeatureFlags.www.js

@@ -58,6 +58,8 @@ export const enableLegacyFBSupport = true;
 
 export const enableYieldingBeforePassive = false;
 
+export const enableThrottledScheduling = false;
+
 export const enableHydrationLaneScheduling = true;
 
 export const enableComponentPerformanceTrack = false;