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:

# 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.