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Parallel transitions: Assign different lanes to consecutive transitions #20672

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merged 2 commits into from
Feb 8, 2021
Merged

Parallel transitions: Assign different lanes to consecutive transitions #20672

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cb328f5
Land enableTransitionEntanglement changes
acdlite Feb 8, 2021
261cf29
Assign different lanes to consecutive transitions
acdlite Jan 27, 2021
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146 changes: 60 additions & 86 deletions packages/react-reconciler/src/ReactFiberLane.new.js
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Original file line number Diff line number Diff line change
Expand Up @@ -36,10 +36,7 @@ export type Lane = number;
export type LaneMap<T> = Array<T>;

import invariant from 'shared/invariant';
import {
enableCache,
enableTransitionEntanglement,
} from 'shared/ReactFeatureFlags';
import {enableCache} from 'shared/ReactFeatureFlags';

import {
ImmediatePriority as ImmediateSchedulerPriority,
Expand Down Expand Up @@ -95,6 +92,7 @@ export const DefaultLanes: Lanes = /* */ 0b0000000000000000000

const TransitionHydrationLane: Lane = /* */ 0b0000000000000000001000000000000;
const TransitionLanes: Lanes = /* */ 0b0000000001111111110000000000000;
const SomeTransitionLane: Lane = /* */ 0b0000000000000000010000000000000;

const RetryLanes: Lanes = /* */ 0b0000011110000000000000000000000;

Expand All @@ -113,6 +111,9 @@ export const NoTimestamp = -1;

let currentUpdateLanePriority: LanePriority = NoLanePriority;

let nextTransitionLane: Lane = SomeTransitionLane;
let nextRetryLane: Lane = SomeRetryLane;

export function getCurrentUpdateLanePriority(): LanePriority {
return currentUpdateLanePriority;
}
Expand Down Expand Up @@ -309,15 +310,6 @@ export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
return NoLanes;
}

if (enableTransitionEntanglement) {
// We don't need to do anything extra here, because we apply per-lane
// transition entanglement in the entanglement loop below.
} else {
// If there are higher priority lanes, we'll include them even if they
// are suspended.
nextLanes = pendingLanes & getEqualOrHigherPriorityLanes(nextLanes);
}

// If we're already in the middle of a render, switching lanes will interrupt
// it and we'll lose our progress. We should only do this if the new lanes are
// higher priority.
Expand Down Expand Up @@ -350,6 +342,11 @@ export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
// entanglement is usually "best effort": we'll try our best to render the
// lanes in the same batch, but it's not worth throwing out partially
// completed work in order to do it.
// TODO: Reconsider this. The counter-argument is that the partial work
// represents an intermediate state, which we don't want to show to the user.
// And by spending extra time finishing it, we're increasing the amount of
// time it takes to show the final state, which is what they are actually
// waiting for.
//
// For those exceptions where entanglement is semantically important, like
// useMutableSource, we should ensure that there is no partial work at the
Expand Down Expand Up @@ -559,34 +556,23 @@ export function findUpdateLane(
);
}

// To ensure consistency across multiple updates in the same event, this should
// be pure function, so that it always returns the same lane for given inputs.
export function findTransitionLane(wipLanes: Lanes, pendingLanes: Lanes): Lane {
// First look for lanes that are completely unclaimed, i.e. have no
// pending work.
let lane = pickArbitraryLane(TransitionLanes & ~pendingLanes);
if (lane === NoLane) {
// If all lanes have pending work, look for a lane that isn't currently
// being worked on.
lane = pickArbitraryLane(TransitionLanes & ~wipLanes);
if (lane === NoLane) {
// If everything is being worked on, pick any lane. This has the
// effect of interrupting the current work-in-progress.
lane = pickArbitraryLane(TransitionLanes);
}
export function claimNextTransitionLane(): Lane {
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This is the essential part

// Cycle through the lanes, assigning each new transition to the next lane.
// In most cases, this means every transition gets its own lane, until we
// run out of lanes and cycle back to the beginning.
const lane = nextTransitionLane;
nextTransitionLane <<= 1;
if ((nextTransitionLane & TransitionLanes) === 0) {
nextTransitionLane = SomeTransitionLane;
}
return lane;
}

// To ensure consistency across multiple updates in the same event, this should
// be pure function, so that it always returns the same lane for given inputs.
export function findRetryLane(wipLanes: Lanes): Lane {
// This is a fork of `findUpdateLane` designed specifically for Suspense
// "retries" — a special update that attempts to flip a Suspense boundary
// from its placeholder state to its primary/resolved state.
let lane = pickArbitraryLane(RetryLanes & ~wipLanes);
if (lane === NoLane) {
lane = pickArbitraryLane(RetryLanes);
export function claimNextRetryLane(): Lane {
const lane = nextRetryLane;
nextRetryLane <<= 1;
if ((nextRetryLane & RetryLanes) === 0) {
nextRetryLane = SomeRetryLane;
}
return lane;
}
Expand All @@ -595,16 +581,6 @@ function getHighestPriorityLane(lanes: Lanes) {
return lanes & -lanes;
}

function getLowestPriorityLane(lanes: Lanes): Lane {
// This finds the most significant non-zero bit.
const index = 31 - clz32(lanes);
return index < 0 ? NoLanes : 1 << index;
}

function getEqualOrHigherPriorityLanes(lanes: Lanes | Lane): Lanes {
return (getLowestPriorityLane(lanes) << 1) - 1;
}

export function pickArbitraryLane(lanes: Lanes): Lane {
// This wrapper function gets inlined. Only exists so to communicate that it
// doesn't matter which bit is selected; you can pick any bit without
Expand Down Expand Up @@ -676,39 +652,21 @@ export function markRootUpdated(
) {
root.pendingLanes |= updateLane;

// TODO: Theoretically, any update to any lane can unblock any other lane. But
// it's not practical to try every single possible combination. We need a
// heuristic to decide which lanes to attempt to render, and in which batches.
// For now, we use the same heuristic as in the old ExpirationTimes model:
// retry any lane at equal or lower priority, but don't try updates at higher
// priority without also including the lower priority updates. This works well
// when considering updates across different priority levels, but isn't
// sufficient for updates within the same priority, since we want to treat
// those updates as parallel.

// Unsuspend any update at equal or lower priority.
const higherPriorityLanes = updateLane - 1; // Turns 0b1000 into 0b0111

if (enableTransitionEntanglement) {
// If there are any suspended transitions, it's possible this new update
// could unblock them. Clear the suspended lanes so that we can try rendering
// them again.
//
// TODO: We really only need to unsuspend only lanes that are in the
// `subtreeLanes` of the updated fiber, or the update lanes of the return
// path. This would exclude suspended updates in an unrelated sibling tree,
// since there's no way for this update to unblock it.
//
// We don't do this if the incoming update is idle, because we never process
// idle updates until after all the regular updates have finished; there's no
// way it could unblock a transition.
if ((updateLane & IdleLanes) === NoLanes) {
root.suspendedLanes = NoLanes;
root.pingedLanes = NoLanes;
}
} else {
root.suspendedLanes &= higherPriorityLanes;
root.pingedLanes &= higherPriorityLanes;
// If there are any suspended transitions, it's possible this new update
// could unblock them. Clear the suspended lanes so that we can try rendering
// them again.
//
// TODO: We really only need to unsuspend only lanes that are in the
// `subtreeLanes` of the updated fiber, or the update lanes of the return
// path. This would exclude suspended updates in an unrelated sibling tree,
// since there's no way for this update to unblock it.
//
// We don't do this if the incoming update is idle, because we never process
// idle updates until after all the regular updates have finished; there's no
// way it could unblock a transition.
if ((updateLane & IdleLanes) === NoLanes) {
root.suspendedLanes = NoLanes;
root.pingedLanes = NoLanes;
}

const eventTimes = root.eventTimes;
Expand Down Expand Up @@ -801,16 +759,32 @@ export function markRootFinished(root: FiberRoot, remainingLanes: Lanes) {
}

export function markRootEntangled(root: FiberRoot, entangledLanes: Lanes) {
root.entangledLanes |= entangledLanes;
// In addition to entangling each of the given lanes with each other, we also
// have to consider _transitive_ entanglements. For each lane that is already
// entangled with *any* of the given lanes, that lane is now transitively
// entangled with *all* the given lanes.
//
// Translated: If C is entangled with A, then entangling A with B also
// entangles C with B.
//
// If this is hard to grasp, it might help to intentionally break this
// function and look at the tests that fail in ReactTransition-test.js. Try
// commenting out one of the conditions below.

const rootEntangledLanes = (root.entangledLanes |= entangledLanes);
const entanglements = root.entanglements;
let lanes = entangledLanes;
while (lanes > 0) {
let lanes = rootEntangledLanes;
while (lanes) {
const index = pickArbitraryLaneIndex(lanes);
const lane = 1 << index;

entanglements[index] |= entangledLanes;

if (
// Is this one of the newly entangled lanes?
(lane & entangledLanes) |
// Is this lane transitively entangled with the newly entangled lanes?
(entanglements[index] & entangledLanes)
) {
entanglements[index] |= entangledLanes;
}
lanes &= ~lane;
}
}
Expand Down
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