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Reduce memory overhead of tracking #3833

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merged 8 commits into from
Mar 7, 2024
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Reduce memory overhead of tracking #3833

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7897edd
20% legroom only
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Merge branch 'main' into tomas/tracking-memory-overhead-reduction
realyze Mar 4, 2024
1d4893a
Better initial deps detection
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remove ceil
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9 changes: 6 additions & 3 deletions packages/mobx/src/core/derivation.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -166,10 +166,13 @@ export function checkIfStateReadsAreAllowed(observable: IObservable) {
*/
export function trackDerivedFunction<T>(derivation: IDerivation, f: () => T, context: any) {
const prevAllowStateReads = allowStateReadsStart(true)
// pre allocate array allocation + room for variation in deps
// array will be trimmed by bindDependencies
changeDependenciesStateTo0(derivation)
derivation.newObserving_ = new Array(derivation.observing_.length + 100)
// Preallocate array; will be trimmed by bindDependencies.
derivation.newObserving_ = new Array(
derivation.observing_.length
? Math.ceil(derivation.observing_.length * 1.2) // 20% extra room for variation in deps
: 100 // Establishing initial dependencies, preallocate constant space.
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@urugator urugator Feb 23, 2024 •
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Is derivation.observing_.length a good indicator of "initial"? Ideally we would like to avoid prealocating anything for non-initial runs if there are no observable deps, eg:

const Cmp = observer(() => <div>no-observable-deps</div>)

Maybe there is an opportunity to bail-out even earlier is this case, dunno.

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Hmmm, good question! I think we wouldn't re-run trackDerivedFunction in case there are no dependencies, right? (Because nothing "inside" would trigger a rerun and external calls would get the cached value.)

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@urugator urugator Mar 4, 2024 •
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No, deps can still change based on something not observable like prop/state.
The main point is that zero deps isn't special case - we assume the same thing - the number of deps don't change significantly between runs. Therefore we preallocate array of the same size - in this case an array of zero length. Probably not much else we can do. But we need to somehow differentiate between initial run (preallocate constant space) and non-initial run (preallocate zero). Probably we can just change the condition to: derivation.runId_ !== 0

)
derivation.unboundDepsCount_ = 0
derivation.runId_ = ++globalState.runId
const prevTracking = globalState.trackingDerivation
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