file-manager/docs/PERFORMANCE.md

# Performance

What has been measured, on what, and — explicitly — what has **not** been measured at all. The rule
this file exists to enforce comes from the competitive survey: the market leader's top complaint is
thumbnails that load late, its own support reply to a speed complaint was *"your SD card is slow"*,
and the lesson recorded there was to measure what was measured and never close a performance
complaint by blaming the hardware.

Until this file existed, the app had **no performance number of any kind**.

## Cold start

| build | device | runs (ms) | median |
|---|---|---|---|
| debug, `1.0.0-debug` | Xiaomi phone, Android 15 | 1 758 · 1 773 · 1 814 · 1 926 · 2 043 | **1 814 ms** |
| `benchmark` (R8, not debuggable, profileable) | same | — | **532 ms** |

The first is `./tools/measure-startup.sh 5` (`am start -W`, one warm-up start discarded because the first
launch after an install pays for dexopt). The second is `BrowserBenchmark#coldStart` in the `:benchmark`
module: `StartupTimingMetric`, five iterations, `CompilationMode.None()`, measured as time to initial
display — min 499 ms, median 532 ms, max 805 ms. Measured twice: 615 ms before the storage grant was in
place (the app was showing the permission screen) and 532 ms after, which is the figure above.

Two caveats belong to that number rather than to a footnote. The tool itself warns *"Unable to clear
Runtime Image, subsequent launches/iterations may exhibit faster startup than production due to
accelerated class loading"*, so it is optimistic by an unmeasured amount on this Android version. And
`CompilationMode.None()` means no compilation profile is applied — what a fresh install does — so a
baseline profile would lower it further and is the obvious next measurement.

The debug build is deliberately the *slow* case — StrictMode on, LeakCanary attached, no R8 — which is
why the same screen takes it three times as long. That is the honest illustration of why a debug number
must never be published as the app's performance.

### What StrictMode found on the first cold start

The same debug build now logs StrictMode violations, and the first cold start produced five, against a
1 814 ms start:

| duration | where | ours? |
|---|---|---|
| 492 ms | `BrowserViewModel.Factory` → `app.cacheDir` | **yes** — `Context.getCacheDir()` checks and creates the private cache directory while the view model is being created, on the main thread |
| 397 ms | `BrowserViewModel.rootsOf` → `StorageProvider.space` → `VolumeRepository.stat` | **yes** — free space for every volume is read with `StatFs` inside `refresh()`, which the screen calls from a `LaunchedEffect` |
| 477 ms | `FileManagerTheme` → `typography = FileManagerTypography` | no — the platform loading its own typefaces on first composition |
| 54 ms × 2 | `miui.util.font.FontSettings` via `MainActivity.onCreate` | no — the OEM's font hook doing `File.exists` while the activity is created |

That is **~1.5 s of main-thread disk I/O inside a 1.8 s start**, of which **~0.9 s is ours**. Both of
ours are mechanical to move and neither is fixed yet: the cache directory is only needed when a preview
copies something, and the free-space line on the roots screen can arrive a frame later. They are the
first items of the speed work.

## Measured on demand: the `:benchmark` module

`benchmark/` holds four macrobenchmark tests that measure the interaction numbers above on a benchmark
build of the app: release rules (R8 on, not debuggable), debug-signed, and profileable — under its own
package id, `…filemanager.perf`, so it can never collide with the test APK or take over a release
install on the device.

| benchmark | metric | what it answers |
|---|---|---|
| `coldStart` | `StartupTimingMetric` | tap → first frame |
| `openAFolderOfAThousandFiles` | `FrameTimingMetric` | jank while a 1 000-file folder fills in |
| `scrollAFolderOfAThousandFiles` | `FrameTimingMetric` | jank while scrolling that folder |
| `searchInsideAFolderOfAThousandFiles` | `FrameTimingMetric` | jank while typing into search |

They share a fixture — `/sdcard/AABenchmark`, a thousand empty files, created once per run and deleted
in `@AfterClass` — and the storage grant is applied with `appops`, so an unattended run is possible.
`CompilationMode.None()` is deliberate: no profile is applied, which is the honest cold-install
baseline. `CompilationMode.Partial()` plus a baseline profile is the follow-up, and needs the same
module.

**Two of these have produced a number** (`coldStart`, above); the three interaction ones have not, and
the reason is the device rather than the code. Getting them to run at all took four fixes, each of which
is written next to the code it belongs to: the app under test needs its own package id; it needs to be
installed by hand once, because MIUI refuses `adb install` of a new package id without a human
(`INSTALL_FAILED_USER_RESTRICTED`); the test APK needs package-visibility for the app it measures, or
macrobenchmark reports `NameNotFoundException` for an app that `pm list packages` shows installed; and
the fixture has to be created through adb, because `UiAutomation.executeShellCommand` runs with the
*test app's* uid and cannot write to `/sdcard`.

The workflow that avoids all of it after the first install:

```bash
# once, by hand on the phone (Files → Download → tap), because MIUI gates adb installs:
#   /sdcard/Download/PlainFiles-perf.apk        (app under test, package …filemanager.perf)
#   /sdcard/Download/PlainFiles-benchmark-test.apk

./tools/benchmark-fixture.sh                  # create the fixture and grant storage access
adb shell am instrument -w \
    -e class de.softwareapp_hb.filemanager.benchmark.BrowserBenchmark \
    de.softwareapp_hb.filemanager.benchmark/androidx.test.runner.AndroidJUnitRunner
```

`am instrument` is preferred over `./gradlew -PbenchmarkBuild :benchmark:connectedDebugAndroidTest` on a
device with an install gate: the Gradle task installs *and uninstalls* the APKs around the run, so every
run needs a human again — and an uninstall between runs is what made the app under test disappear once.

## Not measured yet, and why

| number | how it would be taken | why it is not here |
|---|---|---|
| Folder open, 1 000 / 5 000 / 20 000 files | macrobenchmark `StartupTimingMetric` over a fixture volume, or `TraceSectionMetric` around the listing | needs a `:benchmark` module and a profileable build installed under the **release package id**; on this MIUI phone the first install of a new package id waits for a confirmation dialog, so it cannot run unattended |
| Thumbnail paint while scrolling a photo grid | `FrameTimingMetric` | same |
| Keystroke → search result | `TraceSectionMetric` around the query and the index read | same |

The module does not exist yet, deliberately: half-configured build wiring that nobody can run is worse
than an honest gap. It is the next measurement step, and it belongs with the baseline-profile work,
which needs the same module.

## What is measured instead, meanwhile

The gate measures things that are not latency but do not rot: the test count, lint, detekt against its
baseline, and whether `docs/CODE_MAP.md` still matches the tree. Those are the numbers that can be
kept true by automation; the three above need a device and a decision about which device.

## Two rules

- **No number that was not taken from a real build on a real device.** Not from a debug build
  pretending to be release, and not from reading the code.
- **No performance change judged by reading the code.** The bigger-thumbnail change (28 → 48 dp in a
  row, 40 → 96 dp in a grid) is the standing example: it may be exactly right, and nobody measured
  either side of it. `docs/COMPETITIVE_ANALYSIS.md` had already written "a thumbnail that is bigger and
  slower is strictly worse than one that is smaller and instant" — which is a hypothesis until it is a
  number.