perf (fetch): use less promises for ReadableStream#4457
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KhafraDev
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Aug 25, 2025
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Yes, undici might have less promises, but there's likely an equal number being created in node core. If we ignore node core, then what exactly are we testing?
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@KhafraDev Before: {
'new ReadableStream (node:internal/webstreams/readablestream:256:30)': 2,
'setupReadableByteStreamController (node:internal/webstreams/readablestream:3290:5)': 2,
'setupReadableByteStreamController (node:internal/webstreams/readablestream:3289:3)': 2,
'Object.start (/home/aras/workspace/undici/lib/core/util.js:612:19)': 1,
'createDeferredPromise (/home/aras/workspace/undici/lib/util/promise.js:18:19)': 1,
'readableStreamReaderGenericInitialize (node:internal/webstreams/readablestream:2163:30)': 1,
'readAllBytes (/home/aras/workspace/undici/lib/web/fetch/util.js:1026:29)': 1,
'new DefaultReadRequest (node:internal/webstreams/readablestream:790:20)': 3,
'readAllBytes (/home/aras/workspace/undici/lib/web/fetch/util.js:1032:51)': 3,
'TestContext.<anonymous> (/home/aras/workspace/undici/test/fetch/readable-stream-from.js:47:18)': 1,
'node:internal/per_context/primordials:605:3': 1,
'new SafePromise (node:internal/per_context/primordials:451:29)': 6,
'node:internal/per_context/primordials:483:35': 2,
'Test.run (node:internal/test_runner/test:1088:13)': 1,
'Test.processPendingSubtests (node:internal/test_runner/test:763:18)': 1,
'Promise.then (<anonymous>)': 4,
'invokePromiseCallback (node:internal/webstreams/util:171:37)': 3,
'pull (/home/aras/workspace/undici/lib/core/util.js:616:29)': 4,
'pull (/home/aras/workspace/undici/lib/core/util.js:617:50)': 8,
'[Symbol.asyncIterator] (/home/aras/workspace/undici/test/fetch/readable-stream-from.js:40:7)': 2,
'readableByteStreamControllerCallPullIfNeeded (node:internal/webstreams/readablestream:3144:3)': 3,
'[Symbol.asyncIterator] (/home/aras/workspace/undici/test/fetch/readable-stream-from.js:41:7)': 2,
'pull (/home/aras/workspace/undici/lib/core/util.js:628:28)': 1,
'[Symbol.asyncIterator] (/home/aras/workspace/undici/test/fetch/readable-stream-from.js:42:7)': 2
}after: {
'new ReadableStream (node:internal/webstreams/readablestream:256:30)': 2,
'setupReadableByteStreamController (node:internal/webstreams/readablestream:3290:5)': 2,
'setupReadableByteStreamController (node:internal/webstreams/readablestream:3289:3)': 2,
'createDeferredPromise (/home/aras/workspace/undici/lib/util/promise.js:18:19)': 1,
'readableStreamReaderGenericInitialize (node:internal/webstreams/readablestream:2163:30)': 1,
'readAllBytes (/home/aras/workspace/undici/lib/web/fetch/util.js:1026:29)': 1,
'new DefaultReadRequest (node:internal/webstreams/readablestream:790:20)': 3,
'readAllBytes (/home/aras/workspace/undici/lib/web/fetch/util.js:1032:51)': 3,
'TestContext.<anonymous> (/home/aras/workspace/undici/test/fetch/readable-stream-from.js:47:18)': 1,
'node:internal/per_context/primordials:605:3': 1,
'new SafePromise (node:internal/per_context/primordials:451:29)': 6,
'node:internal/per_context/primordials:483:35': 2,
'Test.run (node:internal/test_runner/test:1088:13)': 1,
'Test.processPendingSubtests (node:internal/test_runner/test:763:18)': 1,
'invokePromiseCallback (node:internal/webstreams/util:171:37)': 3,
'Object.pull (/home/aras/workspace/undici/lib/core/util.js:616:25)': 4,
'[Symbol.asyncIterator] (/home/aras/workspace/undici/test/fetch/readable-stream-from.js:40:7)': 2,
'Object.pull (/home/aras/workspace/undici/lib/core/util.js:616:32)': 4,
'readableByteStreamControllerCallPullIfNeeded (node:internal/webstreams/readablestream:3144:3)': 3,
'Promise.then (<anonymous>)': 4,
'[Symbol.asyncIterator] (/home/aras/workspace/undici/test/fetch/readable-stream-from.js:41:7)': 2,
'[Symbol.asyncIterator] (/home/aras/workspace/undici/test/fetch/readable-stream-from.js:42:7)': 2
} |
mcollina
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Aug 25, 2025
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lgtm - removing as many as possible would speed us up a tiny bit. Note that this code is functionally equivalent, so there is no downside here.
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What isn't true? The whole point is that if we are counting the number of promises in webstreams, we should include the ones from node core. After I made that comment I had to dig into the streams spec to find how the value was getting wrapped in a promise (in a sane world I would have expected |
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I cant write a test for created Promises in node core. I tried that but it is not working, because the created Promises are different on versions of nodejs. I could write tests based on the nodejs version, if this solves the blocking. |
KhafraDev
approved these changes
Aug 25, 2025
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slagiewka
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Feb 14, 2026
* perf: use less promises for ReadableStream * remove test
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before the test used 19 promises. now it uses only 13.
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