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) and profileable, under the app's own package id and signed with the release key, so installing it updates an installed release build and keeps its data. A separate package id (.perf) was the first design and is the tidier one, but on this MIUI phone a new package id cannot be installed without a human: adb install is refused with INSTALL_FAILED_USER_RESTRICTED, and installing by tapping the APK left the package visible to pm list packages yet invisible to PackageManager, which is what macrobenchmark reports as "Unable to find target package". The trade-off is written in :app's build file: this build replaces a release install, and reinstalling the release APK puts it back.

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 a device policy rather than the code. Getting this far took five fixes, each written next to what it constrains: the app under test needs the app's own package id and release key (a new id cannot be installed here); the test APK needs package visibility for it; the fixture has to be created through adb, because UiAutomation.executeShellCommand runs with the test app's uid and cannot write /sdcard; testBuildType has to sit behind -PbenchmarkBuild, or :app:testDebugUnitTest stops existing; and hasProperty is needed to read that flag, because the Provider form read as absent and the runner then installed the debug APK and failed on it.

The workflow:

./tools/benchmark-fixture.sh       # the fixture, and the storage grant the benchmark build lacks
./gradlew -PbenchmarkBuild :benchmark:connectedDebugAndroidTest

The Gradle runner is required, not preferred. Macrobenchmark's rule uninstalls the app at the start of a run and reinstalls it before measuring. That alone is survivable; what is not is the side effect: an uninstall clears the app's app-ops, so the MANAGE_EXTERNAL_STORAGE grant from tools/benchmark-fixture.sh is gone by the time the test navigates. The app then correctly shows its "choose a folder" fallback, the volume row is not on the screen, and the assertion says so. Checked directly — appops get … MANAGE_EXTERNAL_STORAGE reads No operations after a run that passed through the reinstall.

There is no way around that from inside the test: granting an app-op needs the shell's uid, and UiAutomation.executeShellCommand runs with the test app's uid. So on a device where this app needs that grant — which is every device, for the browser screen — the three interaction benchmarks cannot run as macrobenchmark tests here, and the cold-start number above stands alone.

Two paths would give the numbers, and neither is a small change:

• A plain instrumented test instead of a macrobenchmark one (no rule, so no reinstall): install the benchmark build once, grant, then navigate with UiAutomator and read frame durations from FrameMetrics. It measures the release-like build, but it is our own measurement code rather than a maintained library, and FrameTimingMetric exists precisely so nobody has to write that.
• An app that does not need the grant for the flow being measured — for instance a benchmark build that browses its own external files directory, which needs no permission. That measures a different screen than the one users open, so it answers a narrower question than "how fast does opening a 1 000-file folder feel".

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.