@samitouri / QOS-React-2 / commits / c80b336d23

Implement requestPaint in the actual scheduler (#31784)

When implementing passive effects we did a pretty massive oversight. While the passive effect is scheduled into its own scheduler task, the scheduler doesn't always yield to the browser if it has time left. That means that if you have a fast commit phase, it might try to squeeze in the passive effects in the same frame but those then might end being very heavy. We had `requestPaint()` for this but that was only implemented for the `isInputPending` experiment. It wasn't thought we needed it for the regular scheduler because it yields "every frame" anyway - but it doesn't yield every task. While the `isInputPending` experiment showed that it wasn't actually any significant impact, and it was better to keep shorter yield time anyway. Which is why we deleted the code. Whatever small win it did see in some cases might have been actually due to this issue rather than anything to do with `isInputPending` at all. As you can see in https://github.com/facebook/react/pull/31782 we do have this implemented in the mock scheduler and a lot of behavior that we assert assumes that this works. So this just implements yielding after `requestPaint` is called. Before: <img width="1023" alt="Screenshot 2024-12-14 at 3 40 24 PM" src="https://github.com/user-attachments/assets/d60f4bb2-c8f8-4f91-a402-9ac25b278450" /> After: <img width="1108" alt="Screenshot 2024-12-14 at 3 41 25 PM" src="https://github.com/user-attachments/assets/170cdb90-a049-436f-9501-be3fb9bc04ca" /> Notice how in after the native task is split into two. It might not always actually paint and the native scheduler might make the same mistake and think it has enough time left but it's at least less likely to. We do have another way to do this. When we yield a continuation we also yield to the native browser. This is to enable the Suspense Optimization (currently disabled) to work. We could do the same for passive effects and, in fact, I have a branch that does but because that requires a lot more tests to be fixed it's a lot more invasive of a change. The nice thing about this approach is that this is not even running in tests at all and the tests we do have assert that this is the behavior already. 😬

Sebastian Markbåge committed Dec 14, 2024 at 16:17 UTC c80b336d23aa472b5e5910268138ac0447d6ae19

2 files changed +12 -3

packages/scheduler/src/__tests__/Scheduler-test.js

+2 -2

@@ -183,7 +183,7 @@ describe('SchedulerBrowser', () => {

183 it('task with continuation', () => {

184 scheduleCallback(NormalPriority, () => {

185 runtime.log('Task');

186 - // Request paint so that we yield at the end of the frame interval

186 + // Request paint so that we yield immediately

187 requestPaint();

188 while (!Scheduler.unstable_shouldYield()) {

189 runtime.advanceTime(1);

@@ -199,7 +199,7 @@ describe('SchedulerBrowser', () => {

199 runtime.assertLog([

200 'Message Event',

201 'Task',

202 - gate(flags => (flags.www ? 'Yield at 10ms' : 'Yield at 5ms')),

202 + 'Yield at 0ms',

203 'Post Message',

204 ]);

205

packages/scheduler/src/forks/Scheduler.js

+10 -1

@@ -93,6 +93,8 @@ var isPerformingWork = false;

93 var isHostCallbackScheduled = false;

94 var isHostTimeoutScheduled = false;

96 +var needsPaint = false;

97 +

98 // Capture local references to native APIs, in case a polyfill overrides them.

99 const localSetTimeout = typeof setTimeout === 'function' ? setTimeout : null;

100 const localClearTimeout =

@@ -456,6 +458,10 @@ let frameInterval = frameYieldMs;

458 let startTime = -1;

459

460 function shouldYieldToHost(): boolean {

461 + if (needsPaint) {

462 + // Yield now.

463 + return true;

464 + }

465 const timeElapsed = getCurrentTime() - startTime;

466 if (timeElapsed < frameInterval) {

467 // The main thread has only been blocked for a really short amount of time;

@@ -466,7 +472,9 @@ function shouldYieldToHost(): boolean {

472 return true;

473 }

474

469 -function requestPaint() {}

475 +function requestPaint() {

476 + needsPaint = true;

477 +}

478

479 function forceFrameRate(fps: number) {

480 if (fps < 0 || fps > 125) {

@@ -486,6 +494,7 @@ function forceFrameRate(fps: number) {

494 }

495

496 const performWorkUntilDeadline = () => {

497 + needsPaint = false;

498 if (isMessageLoopRunning) {

499 const currentTime = getCurrentTime();

500 // Keep track of the start time so we can measure how long the main thread