homarr/docs/perf/NEXT-STEPS.md

Where this left off, and what to do next

Written as a handoff. The branch is green and everything below is measured except where explicitly marked.

1. Verify the unmeasured changes

Four commits apply the off-screen-row treatment beyond the one widget where it was measured. All are typechecked and lint-clean, none are A/B'd — they are the only performance claims on this branch not verified before commit.

commit	widget	measured on this board?
7550766f6	mediaReleases	yes — 135 → 55 image requests, 8346 → 5117 layout objects
c526dd2b7	mediaRequests-requestList	no — but overdraw confirmed: 1,512 px of cards in a 405 px tile, 60 images
040f10594	rssFeed, notifications, calendar event list	no — these do not overdraw on this board

The last row needs care when reading a result. On the test backup rssFeed renders 7 nodes and the calendar's 7 rows fit their tile, so a measurement will correctly show no change. That is not a failure of the change — the benefit only exists for a user whose integrations return more rows than the tile can show. To observe it, use a backup with a busier feed, or shrink the tile.

For c526dd2b7, which does overdraw here, a real effect is expected:

# restore two containers on identical data, one per image, then:
STRESS_BASE_URL=http://127.0.0.1:PORT STRESS_BOARD=default \
  node --experimental-strip-types scripts/benchmarks/measure-board-paint.mts

Expect image requests to fall. Do not expect JS heap to move: decoded bitmaps live outside JSHeapUsedSize. Three repeats per side — a single run wrongly suggested a −16% heap win on the first widget. If image requests do not fall, revert it.

1b. What was deliberately not changed

26 widgets use ScrollArea; the treatment was applied to five. content-visibility is not free — it costs containment work, and on content that measures itself (charts, editors, the notebook) skipping layout can be actively wrong. The rule used: apply it where list length is decided by remote data, leave fixed-size and chart widgets alone.

beszelSystemStats, beszelSystemTable and beszelSystemGrid report large row counts in the overdraw tool, but that is the tool misreading SVG groups as rows — their content fits their tiles.

React cache() and memoisation — already in place

Recorded because it looks like a gap and is not.

React cache() is server-only: it dedupes within a single RSC render and does nothing for client widget rendering. It is already applied where it belongs — getBoardByNameAsync, getHomeBoardAsync, getRscServerSettingsAsync and getQueryClient are all wrapped, the last with a comment explaining the dedup.

Client-side data is cached by tRPC/react-query, whose gcTime was already checked and found to be 5 minutes, not the 24 hours the original fix catalogue assumed. React.memo is already used in the clock, calendar and releases widgets.

2. The one lever with real headroom left

The tRPC router imports every integration eagerly. A single 16.10 MiB subtree of 460 files is reached by every API route, and because Next preloads route bundles it is resident before the first request. This pays twice:

• ~41 MiB of retained bundle source (28% of the live server heap) — V8 keeps a script's source alive while any function in it may still be lazily compiled
• ~38 MiB of V8 compiler zone memory, which is proportional to how much JavaScript V8 compiles and is not reachable by any V8 flag (--max-opt is a null result; only --jitless removes it, and that breaks rendering)

Nothing else has this profile. Every allocator and V8-level avenue is closed — see REJECTED.md.

Start from analyze-require-graph.mjs, which found the shared subtree, and [root-of-the-server]__0g06wx7._.js as the chunk reaching 460 files.

3. Known-good but deliberately deferred

item	prize	why it was not done
ASCII-escape server bundles	9.45 MiB	Proven mechanism — 2.00 vs 1.00 bytes/char; one char above U+00FF doubles a whole source string. Needs a build-time rewrite of minified vendor output, so it deserves its own PR and safety argument
Lazy mysql2 / pg	21.4 MiB	Three implementations, all broken; see REJECTED.md. Structurally blocked because createDb must stay synchronous
beszelSystemStats at 1,156 nodes	unknown	Second-heaviest widget, but its cost is chart SVG (<g>×317), so reducing it changes what is drawn
documents reads 18–32	unknown	Never investigated; each document carries overhead

4. For the clean PR

The instruction was to close this experimental PR and open one with only the verified fixes, judged on lines-of-code to performance ratio. On the evidence, that set is:

change	size	measured effect
--max-semi-space-size=4	1 line	−19% idle, −21% peak
preloadEntriesOnStart: false	1 line	−52% anon at idle; 310 fewer files even at full load
Umami: stop fetching 100k raw events per refresh	1 file	−26% peak, −93% transient buffers, largest allocation 46.4 → 2.4 MiB
serverExternalPackages (mysql2/pg + 8 more)	1 block	−21 MiB idle
content-visibility on media-release cards	4 lines	−59% image requests, −41% layout objects
ssh2 WASM heap (docker-modem patch)	1 patch	−15 to −18 MiB external at every stage

Together these produce the headline: −26% peak anon, −52% at idle, no overlapping ranges at any stage, for +18% CPU.

Two things to leave out or flag:

• The gRPC patch removes 112 files and 6 MiB from an isolated require, but is not separately visible in a full run. Bundle hygiene, not a measured memory win.
• Median board load reads +30% but is not established — individual loads were [506, 383, 1233] and [848, 431, 574] before against [780, 808, 515] and [621, 579, 1004] after, heavily overlapping with no cold-then-warm pattern. If the latency trade matters, measure it properly with many more samples; the suspicion is that it is real and concentrated in preloadEntriesOnStart.

5. Read this before measuring anything

METHODOLOGY.md. Every wrong number in this investigation came from an instrument, not the application, and several were published before being caught. The short version:

• a heap snapshot cannot explain a peak — V8 GCs first and reports survivors only
• V8 truncates snapshot string contents to ~1 KiB
• snapshot objects are not live native handles
• measuring without a forced GC measures V8's growth, not a module's cost
• taking a CDP heap snapshot added 134 MiB to the process being measured
• React per-component attribution silently returns nothing on a production build
• a fixed port can measure a leftover container
• when a server-side change moves a client-side metric, the workload changed — this is how --jitless was caught faking a 24% win by under-rendering the page