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stream: cache minimum cursor count in share #63262

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8a32f1c
stream: cache minimum cursor count in share
trivikr May 12, 2026
389bd73
chore: remove 128 consumers from share benchmark
trivikr May 12, 2026
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32 changes: 32 additions & 0 deletions benchmark/streams/iter-throughput-share.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,32 @@
'use strict';

const common = require('../common.js');

const bench = common.createBenchmark(main, {
consumers: [2, 8, 32],
batches: [1e4],
backpressure: ['block'],
n: [5],
}, {
flags: ['--experimental-stream-iter'],
});

async function main({ consumers, batches, backpressure, n }) {
const { share, array } = require('stream/iter');
const chunk = Buffer.alloc(1024);
const totalOps = batches * consumers * n;

async function* source() {
for (let i = 0; i < batches; i++) {
yield [chunk];
}
}

bench.start();
for (let i = 0; i < n; i++) {
const shared = share(source(), { highWaterMark: 64, backpressure });
const readers = Array.from({ length: consumers }, () => array(shared.pull()));
await Promise.all(readers);
}
bench.end(totalOps);
}
3 changes: 2 additions & 1 deletion lib/internal/streams/iter/broadcast.js
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Original file line number Diff line number Diff line change
Expand Up @@ -343,8 +343,9 @@
// Private methods

#recomputeMinCursor() {
this.#cachedMinCursor = getMinCursor(
const [minCursor] = getMinCursor(

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@Renegade334 Renegade334 May 12, 2026

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Suggested change
const [minCursor] = getMinCursor(
const { 0: minCursor } = getMinCursor(

this.#consumers, this.#bufferStart + this.#buffer.length);
this.#cachedMinCursor = minCursor;
this.#minCursorDirty = false;
}

Expand Down
142 changes: 114 additions & 28 deletions lib/internal/streams/iter/share.js
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Original file line number Diff line number Diff line change
Expand Up @@ -77,6 +77,8 @@
#cancelled = false;
#pulling = false;
#pullWaiters = [];
#cachedMinCursor = 0;
#cachedMinCursorConsumers = 0;

constructor(source, options) {
this.#source = source;
Expand Down Expand Up @@ -114,6 +116,14 @@
};

this.#consumers.add(state);
if (this.#consumers.size === 1) {
this.#cachedMinCursor = state.cursor;
this.#cachedMinCursorConsumers = 1;
} else if (state.cursor === this.#cachedMinCursor) {
this.#cachedMinCursorConsumers++;
} else {
this.#recomputeMinCursor();
}
const self = this;

return {
Expand All @@ -139,22 +149,26 @@

if (self.#cancelled) {
state.detached = true;
self.#consumers.delete(state);
self.#deleteConsumer(state);
return { __proto__: null, done: true, value: undefined };
}

// Check if data is available in buffer
const bufferIndex = state.cursor - self.#bufferStart;
if (bufferIndex < self.#buffer.length) {
const chunk = self.#buffer.get(bufferIndex);
const cursor = state.cursor;
state.cursor++;
self.#tryTrimBuffer();
if (cursor === self.#cachedMinCursor &&
--self.#cachedMinCursorConsumers === 0) {
self.#tryTrimBuffer();
}
return { __proto__: null, done: false, value: chunk };
}

if (self.#sourceExhausted) {
state.detached = true;
self.#consumers.delete(state);
self.#deleteConsumer(state);
if (self.#sourceError) throw self.#sourceError;
return { __proto__: null, done: true, value: undefined };
}
Expand All @@ -163,7 +177,7 @@
const canPull = await self.#waitForBufferSpace();
if (!canPull) {
state.detached = true;
self.#consumers.delete(state);
self.#deleteConsumer(state);
if (self.#sourceError) throw self.#sourceError;
return { __proto__: null, done: true, value: undefined };
}
Expand All @@ -176,17 +190,19 @@
state.detached = true;
state.resolve = null;
state.reject = null;
self.#consumers.delete(state);
self.#tryTrimBuffer();
if (self.#deleteConsumer(state)) {
self.#tryTrimBuffer();
}
return { __proto__: null, done: true, value: undefined };
},

async throw() {
state.detached = true;
state.resolve = null;
state.reject = null;
self.#consumers.delete(state);
self.#tryTrimBuffer();
if (self.#deleteConsumer(state)) {
self.#tryTrimBuffer();
}
return { __proto__: null, done: true, value: undefined };
},
};
Expand Down Expand Up @@ -254,9 +270,11 @@
this.#bufferStart++;
for (const consumer of this.#consumers) {
if (consumer.cursor < this.#bufferStart) {
this.#deleteConsumerFromMin(consumer);
consumer.cursor = this.#bufferStart;
}
}
this.#recomputeMinCursor();
return true;
case 'drop-newest':
return true;
Expand Down Expand Up @@ -324,18 +342,41 @@
}

#tryTrimBuffer() {
const minCursor = getMinCursor(
this.#consumers, this.#bufferStart + this.#buffer.length);
const trimCount = minCursor - this.#bufferStart;
if (this.#cachedMinCursorConsumers === 0) {
this.#recomputeMinCursor();
}
const trimCount = this.#cachedMinCursor - this.#bufferStart;
if (trimCount > 0) {
this.#buffer.trimFront(trimCount);
this.#bufferStart = minCursor;
this.#bufferStart = this.#cachedMinCursor;
for (let i = 0; i < this.#pullWaiters.length; i++) {
this.#pullWaiters[i]();
}
this.#pullWaiters = [];
}
}

#recomputeMinCursor() {
const [minCursor, minCursorConsumers] = getMinCursor(

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Suggested change
const [minCursor, minCursorConsumers] = getMinCursor(
const { 0: minCursor, 1: minCursorConsumers } = getMinCursor(

this.#consumers, this.#bufferStart + this.#buffer.length);
this.#cachedMinCursor = minCursor;
this.#cachedMinCursorConsumers = minCursorConsumers;
}

#deleteConsumerFromMin(consumer) {
if (consumer.cursor === this.#cachedMinCursor) {
this.#cachedMinCursorConsumers--;
}
}

#deleteConsumer(consumer) {
if (this.#consumers.delete(consumer)) {
const wasAtMin = consumer.cursor === this.#cachedMinCursor;
this.#deleteConsumerFromMin(consumer);
return wasAtMin && this.#cachedMinCursorConsumers === 0;
}
return false;
}
Comment on lines +366 to +379
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@Renegade334 Renegade334 May 12, 2026

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Deduplication:

Suggested change
#deleteConsumerFromMin(consumer) {
if (consumer.cursor === this.#cachedMinCursor) {
this.#cachedMinCursorConsumers--;
}
}
#deleteConsumer(consumer) {
if (this.#consumers.delete(consumer)) {
const wasAtMin = consumer.cursor === this.#cachedMinCursor;
this.#deleteConsumerFromMin(consumer);
return wasAtMin && this.#cachedMinCursorConsumers === 0;
}
return false;
}
#deleteConsumerFromMin(consumer) {
if (consumer.cursor === this.#cachedMinCursor) {
this.#cachedMinCursorConsumers--;
return true;
}
return false;
}
#deleteConsumer(consumer) {
if (this.#consumers.delete(consumer) &&
this.#deleteConsumerFromMin(consumer) &&
this.#cachedMinCursorConsumers === 0) {
return true;
}
return false;
}

}

// =============================================================================
Expand All @@ -352,6 +393,8 @@
#sourceExhausted = false;
#sourceError = null;
#cancelled = false;
#cachedMinCursor = 0;
#cachedMinCursorConsumers = 0;

constructor(source, options) {
this.#source = source;
Expand Down Expand Up @@ -383,6 +426,14 @@
};

this.#consumers.add(state);
if (this.#consumers.size === 1) {
this.#cachedMinCursor = state.cursor;
this.#cachedMinCursorConsumers = 1;
} else if (state.cursor === this.#cachedMinCursor) {
this.#cachedMinCursorConsumers++;
} else {
this.#recomputeMinCursor();
}
const self = this;

return {
Expand All @@ -396,26 +447,30 @@
}
if (self.#sourceError) {
state.detached = true;
self.#consumers.delete(state);
self.#deleteConsumer(state);
throw self.#sourceError;
}
if (self.#cancelled) {
state.detached = true;
self.#consumers.delete(state);
self.#deleteConsumer(state);
return { __proto__: null, done: true, value: undefined };
}

const bufferIndex = state.cursor - self.#bufferStart;
if (bufferIndex < self.#buffer.length) {
const chunk = self.#buffer.get(bufferIndex);
const cursor = state.cursor;
state.cursor++;
self.#tryTrimBuffer();
if (cursor === self.#cachedMinCursor &&
--self.#cachedMinCursorConsumers === 0) {
self.#tryTrimBuffer();
}
return { __proto__: null, done: false, value: chunk };
}

if (self.#sourceExhausted) {
state.detached = true;
self.#consumers.delete(state);
self.#deleteConsumer(state);
return { __proto__: null, done: true, value: undefined };
}

Expand All @@ -436,13 +491,15 @@
self.#bufferStart++;
for (const consumer of self.#consumers) {
if (consumer.cursor < self.#bufferStart) {
self.#deleteConsumerFromMin(consumer);
consumer.cursor = self.#bufferStart;
}
}
self.#recomputeMinCursor();
break;
case 'drop-newest':
state.detached = true;
self.#consumers.delete(state);
self.#deleteConsumer(state);
return { __proto__: null, done: true, value: undefined };
}
}
Expand All @@ -451,21 +508,25 @@

if (self.#sourceError) {
state.detached = true;
self.#consumers.delete(state);
self.#deleteConsumer(state);
throw self.#sourceError;
}

const newBufferIndex = state.cursor - self.#bufferStart;
if (newBufferIndex < self.#buffer.length) {
const chunk = self.#buffer.get(newBufferIndex);
const cursor = state.cursor;
state.cursor++;
self.#tryTrimBuffer();
if (cursor === self.#cachedMinCursor &&
--self.#cachedMinCursorConsumers === 0) {
self.#tryTrimBuffer();
}
return { __proto__: null, done: false, value: chunk };
}

if (self.#sourceExhausted) {
state.detached = true;
self.#consumers.delete(state);
self.#deleteConsumer(state);
return { __proto__: null, done: true, value: undefined };
}

Expand All @@ -474,15 +535,17 @@

return() {
state.detached = true;
self.#consumers.delete(state);
self.#tryTrimBuffer();
if (self.#deleteConsumer(state)) {
self.#tryTrimBuffer();
}
return { __proto__: null, done: true, value: undefined };
},

throw() {
state.detached = true;
self.#consumers.delete(state);
self.#tryTrimBuffer();
if (self.#deleteConsumer(state)) {
self.#tryTrimBuffer();
}
return { __proto__: null, done: true, value: undefined };
},
};
Expand Down Expand Up @@ -532,13 +595,36 @@
}

#tryTrimBuffer() {
const minCursor = getMinCursor(
this.#consumers, this.#bufferStart + this.#buffer.length);
const trimCount = minCursor - this.#bufferStart;
if (this.#cachedMinCursorConsumers === 0) {
this.#recomputeMinCursor();
}
const trimCount = this.#cachedMinCursor - this.#bufferStart;
if (trimCount > 0) {
this.#buffer.trimFront(trimCount);
this.#bufferStart = minCursor;
this.#bufferStart = this.#cachedMinCursor;
}
}

#recomputeMinCursor() {
const [minCursor, minCursorConsumers] = getMinCursor(

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@Renegade334 Renegade334 May 12, 2026

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Suggested change
const [minCursor, minCursorConsumers] = getMinCursor(
const { 0: minCursor, 1: minCursorConsumers } = getMinCursor(

this.#consumers, this.#bufferStart + this.#buffer.length);
this.#cachedMinCursor = minCursor;
this.#cachedMinCursorConsumers = minCursorConsumers;
}

#deleteConsumerFromMin(consumer) {
if (consumer.cursor === this.#cachedMinCursor) {
this.#cachedMinCursorConsumers--;
}
}

#deleteConsumer(consumer) {
if (this.#consumers.delete(consumer)) {
const wasAtMin = consumer.cursor === this.#cachedMinCursor;
this.#deleteConsumerFromMin(consumer);
return wasAtMin && this.#cachedMinCursorConsumers === 0;
}
return false;
}
}

Expand Down
20 changes: 12 additions & 8 deletions lib/internal/streams/iter/utils.js
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Expand Up @@ -70,20 +70,24 @@ function onSignalAbort(signal, handler) {
}

/**
* Compute the minimum cursor across a set of consumers.
* Returns fallback if the set is empty.
* Compute the minimum cursor across a set of consumers and count how many
* consumers are at that cursor.
* @param {Set} consumers - Set of objects with a `cursor` property
* @param {number} fallback - Value to return when set is empty
* @returns {number}
* @param {number} fallback - Cursor to return when set is empty
* @returns {[number, number]}
*/
function getMinCursor(consumers, fallback) {
let min = Infinity;
let minCursor = fallback;
let minCursorConsumers = 0;
for (const consumer of consumers) {
if (consumer.cursor < min) {
min = consumer.cursor;
if (consumer.cursor < minCursor) {
minCursor = consumer.cursor;
minCursorConsumers = 1;
} else if (consumer.cursor === minCursor) {
minCursorConsumers++;
}
}
return min === Infinity ? fallback : min;
return [minCursor, minCursorConsumers];
}

/**
Expand Down
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