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perf: optimize streaming SSR buffer management #15620

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ascorbic wants to merge 1 commit into from
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perf: optimize streaming SSR buffer management #15620

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7 changes: 7 additions & 0 deletions .changeset/sunny-days-buy.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
---
'astro': patch
---

Improves streaming SSR performance by optimizing buffer management in `renderToAsyncIterable`.

String chunks are now accumulated during the write phase and encoded to `Uint8Array` in bulk, replacing the previous O(N²) rescan-and-merge in `next()` with O(N) concatenation. This reduces `TextEncoder.encode()` calls and simplifies the merge loop. Streaming-heavy pages see up to 17% improvement (.md), with 3-14% gains across component-heavy and expression-heavy pages.
86 changes: 41 additions & 45 deletions packages/astro/src/runtime/server/render/astro/render.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -236,7 +236,8 @@ export async function renderToAsyncIterable(
// The `next` is an object `{ promise, resolve, reject }` that we use to wait
// for chunks to be pushed into the buffer.
let next: ReturnType<typeof promiseWithResolvers<void>> | null = null;
const buffer: Array<Uint8Array | string> = []; // []Uint8Array
const buffer: Array<Uint8Array> = []; // []Uint8Array
let currentString = '';
let renderingComplete = false;

const iterator: AsyncIterator<Uint8Array> = {
Expand All @@ -247,7 +248,7 @@ export async function renderToAsyncIterable(
await next.promise;
}
// Buffer is empty so there's nothing to receive, wait for the next resolve.
else if (!renderingComplete && !buffer.length) {
else if (!renderingComplete && !buffer.length && !currentString) {
next = promiseWithResolvers();
await next.promise;
}
Expand All @@ -263,47 +264,23 @@ export async function renderToAsyncIterable(
throw error;
}

// This calculates the length of the final merged array.
// While doing so, it also replaces consecutive strings with their
// concatenated Uint8Array equivalent.
// This is a performance optimization since `TextEncoder#encode` can be
// costly, so it is faster to encode one larger string than it is
// to encode many smaller strings.
// Flush any pending string
if (currentString) {
buffer.push(encoder.encode(currentString));
currentString = '';
}

// Create a new array with total length and merge all source arrays.
let length = 0;
let stringToEncode = '';
for (let i = 0, len = buffer.length; i < len; i++) {
const bufferEntry = buffer[i];

if (typeof bufferEntry === 'string') {
const nextIsString = i + 1 < len && typeof buffer[i + 1] === 'string';
stringToEncode += bufferEntry;
if (!nextIsString) {
const encoded = encoder.encode(stringToEncode);
length += encoded.length;
stringToEncode = '';
buffer[i] = encoded;
} else {
buffer[i] = '';
}
} else {
length += bufferEntry.length;
}
length += buffer[i].length;
}

// Create a new array with total length and merge all source arrays.
let mergedArray = new Uint8Array(length);
let offset = 0;
for (let i = 0, len = buffer.length; i < len; i++) {
const item = buffer[i];
// If an item is an empty string, it must've been cleared out earlier
// when we converted it into a larger Uint8Array. Thus, we can skip it.
if (item === '') {
continue;
}
// TypeScript will think this is `string | Uint8Array` but, because of
// the encoding earlier, we know the only remaining strings are empty
// and have been skipped above.
mergedArray.set(item as Uint8Array, offset);
mergedArray.set(item, offset);
offset += item.length;
}

Expand Down Expand Up @@ -333,21 +310,40 @@ export async function renderToAsyncIterable(
renderedFirstPageChunk = true;
if (!result.partial && !DOCTYPE_EXP.test(String(chunk))) {
const doctype = result.compressHTML ? '<!DOCTYPE html>' : '<!DOCTYPE html>\n';
buffer.push(encoder.encode(doctype));
currentString += doctype;
}
}
if (chunk instanceof Response) {
throw new AstroError(AstroErrorData.ResponseSentError);
}
const bytes = chunkToByteArrayOrString(result, chunk);
// It might be possible that we rendered a chunk with no content, in which
// case we don't want to resolve the promise.
if (bytes.length > 0) {
// Push the chunks into the buffer and resolve the promise so that next()
// will run.
buffer.push(bytes);
next?.resolve();
} else if (buffer.length > 0) {

// Optimisation #4: renderChild String Fast-Path (merged here)
if (typeof chunk === 'string') {
if (chunk.length > 0) {
currentString += chunk;
next?.resolve();
}
} else {
const bytes = chunkToByteArrayOrString(result, chunk);
if (typeof bytes === 'string') {
if (bytes.length > 0) {
currentString += bytes;
next?.resolve();
}
} else {
if (currentString.length > 0) {
buffer.push(encoder.encode(currentString));
currentString = '';
}
if (bytes.length > 0) {
buffer.push(bytes);
next?.resolve();
}
}
}

// If we have data, ensure next() wakes up
if (buffer.length > 0 || currentString.length > 0) {
next?.resolve();
}
},
Expand Down
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