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 four times: 615 ms before the storage grant was in place (the app was showing the permission screen), 532 ms after it, 560 ms (min 539, max 698) on an earlier full-suite run, and 574 ms (min 554, max 730) on the run that first completed all four benchmarks. The four runs agree within a phone's patience; the 532 ms figure above is the middle one.

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 by tools/benchmark-fixture.sh and removed with --remove. 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.

What the module has measured

The run that first completed all four tests (MIUI's install dialog answered by tools/benchmark-watch-install.sh, 5 iterations each, CompilationMode.None()):

benchmark	result	what it is
coldStart	574.17 ms P50 (min 553.95, max 730.19)	time to initial display — the fourth measurement of the same thing, alongside 615 / 532 / 560 ms
openAFolderOfAThousandFiles	14 frames P50 (min 13, max 17)	frames drawn from the folder tap until file-0.txt is on screen
scrollAFolderOfAThousandFiles	98 frames P50 (min 93, max 100)	frames drawn across four swipes through the folder
searchInsideAFolderOfAThousandFiles	failed	the app bar's search action is not in the accessibility tree within 20 s of the folder opening, although the same taps by hand find it immediately

Those frame counts are not a jank metric, and the difference matters. FrameTimingMetric normally publishes frameDurationCpuMs and frameOverrunMs — how long each frame took and how far past its deadline it ran. This phone reports neither: the same run's JSON contains frameCount and nothing else, so "14 frames to open the folder" is how many frames the interaction took, not how long any of them lasted. It says the interaction is short (14 frames is about a quarter of a second at 60 Hz); it cannot say whether any of those frames was slow. A device that reports frame durations — an emulator or any non-MIUI phone — is what turns this into a jank number.

Two notes on provenance. The results JSON from that run was deleted by the next run's cleanup before it could be copied a second time, so the figures above come from parsing it once, immediately after the run; re-running on a device without an install gate is the way to confirm them. And the run still ends with a failing search test, so its number is missing rather than zero.

Why the reinstall cannot be answered by the app

macrobenchmark installs its target before measuring it, and on this phone MIUI puts that install behind com.miui.securitycenter/.permcenter.install.AdbInstallActivity. The test cannot answer it itself: CompilationMode.installPackageFromPaths builds pm install -t … and runs it through Shell.executeScriptCaptureStdout, so the call blocks until the child exits and no test code is running while the dialog is up.

tools/benchmark-watch-install.sh answers it from the host instead — but it is a workaround with a cost, and it is not part of the benchmark workflow for that reason: MIUI keeps mCurrentFocus pointing at the installer for a moment after the window has been removed, so a tap sent on that signal can land on the home screen underneath (observed: it opened Telegram). The script now reads the button off the screen before every tap and refuses to tap when the installer is not focused, which narrows the race to the handful of milliseconds between that check and the tap — it does not close it. MIUI's own Install via USB setting is what removes the dialog; with it on, installs need no tap at all.

Three further measurements about that dialog, kept because they explain the workaround's shape:

• It is FLAG_SECURE: a screencap taken while mCurrentFocus was the installer shows the home screen behind it, which is why screenshots of "the dialog" show nothing.
• Its lifetime is not stable. One 0.1 s focus trace caught it as 13:51:27.70 AdbInstallActivity → 13:51:28.38 null — too short to read, let alone tap — while a later trace showed the same window focused for 8.5 s, long enough for a dump and a tap. Presumably the difference is whether MIUI is waiting for an answer or merely announcing the install.
• pm install started from an app uid does not even reach the gate: MIUI's XSpaceManagerServiceImpl throws SecurityException: You either need MANAGE_USERS or CREATE_USERS permission to: query users. The rule's install therefore runs through the shell, which is the path MIUI intercepts.

The four switches that make an unattended run possible at all

1. The app under test is the app itself — same package id, release key — because a new package id cannot be installed on this phone (INSTALL_FAILED_USER_RESTRICTED, and an APK installed by tapping ended up visible to pm list packages but invisible to PackageManager). The cost is recorded in :app's build file: this build replaces a release install; reinstalling the release APK puts it back.
2. The test module declares a benchmark build type. AGP 9's com.android.test extension has no testBuildType and no targetVariant — only targetProjectPath — and the target variant is matched by name against the test module's own build types. Without that type every run installed app-debug.apk, so the numbers would have been debug numbers. The type also has to initWith(getByName("debug")): a declared type starts unsigned, and an unsigned test APK fails with INSTALL_PARSE_FAILED_NO_CERTIFICATES.
3. The storage grant is walked through the UI, not set with appops. The rule uninstalls and reinstalls the app, and an uninstall clears app-ops, so a grant applied by the fixture script is gone by the time the app starts. appops set from inside the test does not work either — even under adoptShellPermissionIdentity, the command fails to stderr (which executeShellCommand does not return) and appops get still reads No operations. What does work is tapping what a user taps: the app's "Grant full access" button, then the platform's consent, then leaving the all-files page. grantStorageAccess in BrowserBenchmark does exactly that, in setupBlock, unmeasured.
4. The fixture is created through adb, because UiAutomation.executeShellCommand runs with the test app's uid and cannot write /sdcard.

The workflow

./tools/benchmark-fixture.sh                      # /sdcard/AABenchmark, 1000 empty files
./tools/benchmark-watch-install.sh --seconds 900 & # answers MIUI's install dialog if it stays up
./gradlew :benchmark:connectedBenchmarkAndroidTest

coldStart alone can be run with -Pandroid.testInstrumentationRunnerArguments.class=…BrowserBenchmark#coldStart. The results land in benchmark/build/outputs/androidTest-results/connected/benchmark/, and — worth knowing, because it cost an hour — the per-test JSON that holds the numbers is written to /sdcard/Android/media/<test-pkg>/additional_test_output/ and deleted together with the test app when the run ends, often before Gradle has copied it. Copying it in a loop while the run is in progress is what recovered the numbers above.

Not measured yet, and why

number	status
Folder open with 1 000 files	14 frames P50 — measured, but as a frame count, not a jank figure
Folder open with 5 000 / 20 000 files	not measured: the fixture script counts to 1 000, and every larger run needs another install cycle
Scrolling that folder	98 frames P50 over four swipes — again a frame count
Typing into search	failed in the automated run: the app bar's search action is not in the accessibility tree within 20 s of the folder opening, although the same taps by hand find it immediately
Thumbnail paint while scrolling a photo grid	not measured: no test for it yet, and no frame durations on this device

What the interaction numbers need is a device that reports frame durations; what the repeated runs need is a device that installs unattended. This phone gives neither, and the two are independent: a non-MIUI phone or an emulator with the system language set to English would answer both, and the fixture is a folder of empty files, so nothing device-specific is being measured.

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.