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PartialPrefetching Causes Four Renders Per On-Demand Page, Timer-Retained Render State Leads To Large Memory Spikes

With partialPrefetching enabled, dynamic pages are rendered four times per request (request, runtime prefetch, shell upgrade x2). Timers scheduled during each render capture AsyncLocalStorage context and retain large React Flight request and cache state, causing memory retention of ~3.8 MiB per request for a 10-minute timer, up to 45 GiB at 20 rps.

highConfidence 92%Next.js Affected V16.3.5 Affected V16.3.6 Affected V16.4.0-Canary.39

Origin Analysis

Next.js spawns two additional background render phases when partialPrefetching is active: spawnRuntimePrefetchWithFilledCaches and the fallback shell upgrade via responseCache.revalidate. Each phase executes user code and schedules timers; those timers inherit the render's AsyncLocalStorage store, which holds workUnitStore, React Flight Request, cacheController and abort-reason Error frames. The references are not cleared after background renders settle, so timers pin the entire render state until they fire.

1. Clone https://github.com/giovannetti-eric/next-partial-prefetching-memory-repro and npm install. 2. Build/start with PARTIAL_PREFETCHING=true and NODE_OPTIONS=--expose-gc. 3. Run count-timers.mjs to observe 4 renders per request. 4. Run load.mjs to measure retained heap after forced GC; compare with PARTIAL_PREFETCHING=false. 5. Observe 3.8 MiB retained per request for 600s timers vs 0.55 MiB without flag.

Fixing Code Block

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0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

Disabling partialPrefetching loses the prefetch performance benefit and may not be acceptable for apps that rely on partial prefetching. Setting gcTime=Infinity on server may change cache eviction behavior for client-side hydration? Actually it only affects server, so safe. The config change is easily reversible; if you revert, the memory issue returns. Upgrading Next.js may not fix until a patch is released. Test thoroughly for regressions in caching and PPR behavior.