127 lines
5.6 KiB
Markdown
127 lines
5.6 KiB
Markdown
# Switcher performance — measured bottlenecks
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Benchmark harness: `src/project/bench_switcher.cpp` (manual target `bench_switcher`,
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not a ctest). Run:
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```sh
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cmake -S . -B build-rel -DCMAKE_BUILD_TYPE=Release
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cmake --build build-rel --target bench_switcher
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./build-rel/bin/bench_switcher <project-root> /tmp/switcher_metrics.txt
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```
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Reference project: `~/prj/r7.20/mobydick` — **12,234 files**, **17,184 ctags
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symbols**, 6.3 GB tree.
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## One-time cost when the switcher opens (UI thread, synchronous)
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| Stage | Release | Debug (shipped) |
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|----------------------------|--------:|----------------:|
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| `ProjectIndex::listFiles` | ~84 ms | ~78 ms |
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| `SymbolIndex::listSymbols` | ~974 ms | ~499 ms |
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| build items + `setItems` | ~5 ms | ~45-70 ms |
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- `listFiles` is a `git ls-files` subprocess — acceptable but blocks the UI.
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- `listSymbols` is a **ctags subprocess (~0.5-1 s)** — the dominant open-cost,
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and it blocks the UI thread. (Cost is in the external binary, not our parse.)
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## Per-keystroke cost — the interactive bottleneck
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`PaletteModel::setQuery` → re-rank. Originally O(N) every keystroke; the cost
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*grew* with query length even as the result set shrank.
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Full-scan (every keystroke re-scores all N), Release:
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| Query | files | symbols |
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|------------------|---------:|---------:|
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| `server` | ~14 ms | ~30 ms |
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| `src server go` | ~17 ms | ~36 ms |
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Debug (what ships): **270-342 ms (files)**, **459-598 ms (symbols)** per
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keystroke — unusable.
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## Fix 1 — incremental narrowing (done)
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When a query only **appends** to the previous one, re-score only the currently
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visible subset (the match set is monotonic under lengthening/adding a needle),
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turning later keystrokes from O(N) into O(previous matches). Exactness is
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guarded by `test_palettemodel::incrementalMatchesFullScan` (incremental ==
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full-scan, incl. the typo tier).
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Effect once the set narrows (Release, type-forward):
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| Query | full-scan | incremental |
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|----------|----------:|------------:|
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| files `server` | ~14 ms | **~6.6 ms** |
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| symbols `server` | ~30 ms | **~9.0 ms** |
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Early keystrokes (`s`, `se`) remain full-N because almost everything matches —
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correct and unavoidable without a prefix index.
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## Fix 2 — build the installed plugin optimized (done)
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The root `CMakeLists.txt` now defaults `CMAKE_BUILD_TYPE` to `RelWithDebInfo`
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when the user does not specify one, so an unqualified `cmake -B build` no longer
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ships a `-O0` plugin. Measured effect on the symbol switcher (mobydick,
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type-forward "serv", 17k symbols): **~340 ms → ~29 ms per keystroke** (~12×).
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## Fix 3 — debounce the filter (done)
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`PaletteWidget` coalesces rapid keystrokes with an 80 ms single-shot timer
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before re-ranking (`applyQueryNow` flushes on Enter so activation always uses
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the latest text). A fast typist's burst (`s`,`se`,`ser`…) triggers one re-rank
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instead of three, so the broad-query early keystrokes — the only ones that still
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scan most of N — are paid at most once per pause, not per character.
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Net result after fixes 1-3: the worst single symbol keystroke on a 17k-symbol
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project is ~29 ms (RelWithDebInfo), and bursts are coalesced; the Alt+G panel
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is responsive.
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## Fix 4 — top-K cap, cheap bulk scoring, min query length (done)
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Universal Ctags lifted the symbol count on mobydick from ~17k to **~397k**
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(Kotlin/TypeScript now indexed), which brought the per-keystroke lag back. Three
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changes in `PaletteModel` / `FuzzyRanker` fix it:
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- **Score-only bulk pass.** `FuzzyRanker::score(..., withRanges=false)` skips the
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highlight-range work (and the old `std::set` union, replaced by a
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sort+unique vector) during the full scan; ranges are computed only for the
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displayed rows. Scores are identical.
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- **Top-K cap (`kMaxVisible = 1000`).** Scores are computed for all items, but
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only the best 1000 are kept (`std::partial_sort`) and given ranges. Nobody
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scrolls past hundreds of fuzzy hits.
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- **Minimum query length (`kMinQueryChars = 2`).** A 1-char query matches almost
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everything, so its full scan is expensive and useless — below the threshold
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the palette shows the capped full list unranked. The first real scan only runs
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at 2 chars; from there the incremental path makes every keystroke sub-2 ms.
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Measured on mobydick (397k symbols, RelWithDebInfo, type-forward):
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| keystroke | before | after |
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|-----------|-------:|------:|
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| `s` | 337 ms | **~0 ms** (unranked) |
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| `se` | 507 ms | 373 ms (the single first scan) |
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| `ser` | 586 ms | **1.9 ms** |
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| `serv` | 704 ms | **1.4 ms** |
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| `server` | 198 ms | **0.8 ms** |
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The one remaining O(N) cost is the single first scan at the 2nd char (~370 ms at
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397k), absorbed by the 80 ms debounce during a typing burst. Parallelising that
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bulk scan (QtConcurrent across cores) is an optional future ~8× on that one hit.
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## Indexing off the UI thread (done)
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`ProjectIndexer` (`src/project/projectindexer.{h,cpp}`, in `project_lib`, 6
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tests) runs `listFiles` / `listSymbols` on the thread pool (`QtConcurrent` +
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`QFutureWatcher`) and delivers results on the UI thread via `filesReady` /
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`symbolsReady`, caching per project root. The switchers now:
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- open the palette **immediately** (cache hit → instant; miss → a transient
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"Indexing…" / "Indexing symbols…" placeholder), and
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- repopulate via the ready signal when the background job finishes.
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A per-root generation counter drops superseded/invalidated results. The cache
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is dropped on `KateProjectBridge::projectChanged` so a project switch re-indexes.
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Net effect: the UI thread never blocks on git/ctags; the ~1 s Alt+G stall is
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gone (first open shows a placeholder and fills in; subsequent opens are instant
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from cache).
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