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perf(runtime): deduplicate concurrent client manifest loads - #528
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Brooooooklyn merged 2 commits intoOct 21, 2026
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## Problem `createManifestLoader()` caches the parsed `/.vite/manifest.json` only after the fetch and parse finish. Requests that arrive together on a cold process or isolate all miss the cache, so each one fetches the manifest from `ASSETS` and parses it again. Nothing sets `globalThis.__VITE_MANIFEST__`, so production always takes this path. The manifest grows with the app. In the 5,000-page benchmark app it is 2,056,949 B. With 8 concurrent cold calls, the loader made 8 fetches and 8 `response.json()` parses totalling ~28 ms, and returned 8 distinct manifest objects. The parses run on the same thread, so they also delay the other requests. Later calls hit the cache. ## Fix Concurrent cold calls now share one in-flight load. The shared promise is cleared when it settles, so failures behave as before: - A non-ok response returns `null` and is not cached. The next call fetches again. - A rejected fetch rejects every caller that shared the attempt. The next call fetches again. ## Impact Measured with a probe that calls the built loader 8 times at once on a cold instance. A stub `ASSETS.fetch` serves the 5,000-page benchmark manifest verbatim after 5 ms. Node 24.14, 7 fresh processes per variant, median (range): | | Before | After | |---|---:|---:| | `ASSETS.fetch` calls | 8 | 1 | | Total `response.json()` time | 27.9 ms (25.9–30.3) | 3.6 ms (3.5–3.7) | | Wall time for the 8 calls | 44.2 ms (42.9–47.4) | 11.5 ms (11.2–11.7) | | Distinct manifest objects | 8 | 1 | Only the first burst of requests on each cold process or isolate changes. Warm requests already hit the cache. ## Verification - `vp test run packages/void/test/unit/routing-runtime.test.ts`: 40 passed. The new concurrency test fails on the old loader (8 fetches). - `vp run check`, after `vp run build:all`: 33/33 passed. - `vp run test --base origin/main`: 508 files selected, 507 passed and 1 skipped; 10,744 tests passed and 31 skipped. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Brooooooklyn
self-requested a review
October 21, 2026 12:15
Brooooooklyn
approved these changes
Oct 21, 2026
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Issue
createManifestLoader()caches the client manifest after fetching and parsing it. Concurrent HTML renders on a fresh instance can all miss that cache and load separate copies.This duplicates work during overlapping cold loads. Warm requests already reuse the cached manifest.
Fix
Share one pending promise across concurrent manifest loads. Cache successful results and clear the pending promise when it settles, allowing missing manifests and failed loads to be retried.
Verification
Added unit coverage for concurrent loads sharing one fetch and manifest object, completed-cache reuse, and retries after a missing response or rejected fetch.
vp test run packages/void/test/unit/routing-runtime.test.ts: 40 tests passed on Node 24.21.0.Benchmarked eight simultaneous cold SSR requests using real generated manifests, with no artificial asset delay. Median burst completion times across nine shuffled runs on Node 24.21.0:
Single-request controls performed one load on both revisions; warm bursts performed zero additional loads. HTML output matched, and browser hydration and navigation checks passed.
These results describe concurrent cold bursts. Production frequency and full SSG build impact have not been measured.