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fs: improve promises.readFile performance #37127

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addaleax wants to merge 3 commits into from
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fs: improve promises.readFile performance #37127

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69c87cc
fs: only use Buffer.concat in promises.readFile when necessary
addaleax Jan 29, 2021
5ce769e
fs: read full size if known in promises.readFile
addaleax Jan 29, 2021
759440f
fixup! fs: read full size if known in promises.readFile
addaleax Jan 29, 2021
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fs: read full size if known in promises.readFile 
If we have an estimate of the file size available from the previous
stat call, use that for the size of the first chunk to be read.
This increases performance by reading more data (and, most likely,
all data) at once without incurring memory overhead in most situations.
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@addaleax
addaleax committed Jan 29, 2021
commit 5ce769e2381631f23d56b4c01c8065f661695515
11 changes: 6 additions & 5 deletions lib/internal/fs/promises.js
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Original file line number Diff line number Diff line change
Expand Up @@ -315,20 +315,21 @@ async function readFileHandle(filehandle, options) {
throw new ERR_FS_FILE_TOO_LARGE(size);

const chunks = [];
const chunkSize = size === 0 ?
kReadFileMaxChunkSize :
MathMin(size, kReadFileMaxChunkSize);
let isFirstChunk = true;
const firstChunkSize = size === 0 ? kReadFileMaxChunkSize : size;
const chunkSize = Math.min(firstChunkSize, kReadFileMaxChunkSize);
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let endOfFile = false;
do {
if (signal?.aborted) {
throw lazyDOMException('The operation was aborted', 'AbortError');
}
const buf = Buffer.alloc(chunkSize);
const buf = Buffer.alloc(isFirstChunk ? firstChunkSize : chunkSize);
const { bytesRead, buffer } =
await read(filehandle, buf, 0, chunkSize, -1);
await read(filehandle, buf, 0, buf.length, -1);
endOfFile = bytesRead === 0;
if (bytesRead > 0)
ArrayPrototypePush(chunks, buffer.slice(0, bytesRead));
isFirstChunk = false;
} while (!endOfFile);

const result = chunks.length === 1 ? chunks[0] : Buffer.concat(chunks);
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