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 six times, on five different runs: 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), 574 ms (min 554, max 730), 519.5 ms (min 503.6, max 544.7) and 531.6 ms (min 475.6, max 552.0) — the last two on the runs that used CompilationMode.Ignore() and measured all four benchmarks. They 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 was ours. Both of ours were mechanical to move — the cache directory is only needed when a preview copies something, and the free-space line can arrive a frame later — and both are fixed now; what they bought, measured, is below.

After the fix: what moved, and what did not

Both are now fixed — scratchRoot is a Lazy<File> resolved on first use, and the free-space probes run in Dispatchers.IO and update the state when they return (fillRootFreeSpace, fillFreeSpace). Measured afterwards:

what	before	after
StrictMode at start, ours	492 ms cacheDir, 397 ms StatFs	gone
StrictMode at start, not ours	477 ms typefaces, 54 ms × 2 MIUI font hook	50–78 ms in pairs, all from miui.util.font.FontSettings.loadFontSetting / TypefaceUtils.loadFontSettings / contentcatcher.InterceptorProxy.create via MainActivity.onCreate
debug cold start, 5 runs	median 1 814 ms	median 1 803 ms (1 697 · 1 752 · 1 803 · 1 823 · 2 507)
coldStart on the benchmark build	519.5 / 531.6 ms	485.6 / 568.9 / 600.0 ms

The numbers did not improve, and that is worth stating rather than dressing up. The two violations were first-call costs: creating the cache directory happens once per install, and StatFs is slow when its page cache is cold. Neither is paid on a warm device, which is what all five timed runs are — and the run-to-run spread on this phone (±115 ms between the three benchmark runs above) is larger than anything the change could have bought. What the fix does buy is that the main thread no longer does this work at all, which is exactly the case the benchmark's first iteration shows: its max is 648 ms after a fresh install, against 550 ms on a warm one.

The honest conclusion is that the remaining 1.8 s of a debug start is not made of these two calls. It is R8-less code, Compose's first composition, the typefaces, and MIUI's font hook — and the release-like build already pays 0.5 s, not 1.8 s. Making a number out of any further work needs either a quieter device or many more runs; making the app faster needs a baseline profile (next section).

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 two runs that completed all four tests — same APK, CompilationMode.Ignore(), 5 iterations each, no install during either run:

benchmark	run 1	run 2	what it is
coldStart	519.5 ms (503.6–544.7)	531.6 ms (475.6–552.0)	time to initial display
openAFolderOfAThousandFiles	14 frames (11–17)	12 frames (12–17)	frames from the folder tap until file-0.txt is on screen
scrollAFolderOfAThousandFiles	94 frames (48–96)	97 frames (92–100)	frames across four swipes through the folder
searchInsideAFolderOfAThousandFiles	38 frames (33–44)	39 frames (36–40)	frames from opening search until file-99.txt is the result

The two runs agree closely, which is the point of running them twice: the method is stable enough that a change in the app would show up as a change in these numbers rather than as noise.

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.

One bug had to be fixed before search could be measured, and it is worth knowing about because it looked like a UI problem and was a state problem: the search iteration ends with search mode open, and with Ignore() nothing reinstalls the app between iterations, so the next iteration found a search field where it expected an app bar — the assertion fired every time. The hierarchy dump taken at the point of failure showed a TextView whose text was "Search this folder" and a desc='Clear search' beside it: the app was already searching. openFixture now calls killProcess() before every iteration, so each one starts on the roots screen. The dump itself stays: on a missing action it writes search-action-missing.xml into the test app's external files directory, which is what made this diagnosable instead of a guess.

The install moved out of the test — that is the fix

The problem was never the dialog; it was who installs. CompilationMode.None() reports shouldReset = true, and that is what makes resetAndCompile uninstall the app and reinstall it with pm install -t --skip-verification before every single test. On this phone that install is intercepted by com.miui.securitycenter/.permcenter.install.AdbInstallActivity, which the test cannot answer: the install call blocks until pm exits, so no test code is running while the dialog is up.

CompilationMode.Ignore() reports shouldReset = **false** — the library installs nothing and measures what is there. So the APK is installed once, from the host, and the run contains no install at all:

./tools/benchmark-fixture.sh                                    # /sdcard/AABenchmark, 1000 empty files
adb install -r app/build/outputs/apk/benchmark/app-benchmark.apk
adb install -r benchmark/build/outputs/apk/benchmark/benchmark-benchmark.apk
adb shell am instrument -w -e class de.softwareapp_hb.filemanager.benchmark.BrowserBenchmark \
    de.softwareapp_hb.filemanager.benchmark/androidx.test.runner.AndroidJUnitRunner

Measured: with that command the four tests run with zero installs — tools/benchmark-watch-install.sh logs heartbeats and nothing else — and three of them pass. Ignore() applies no compilation profile either, so the numbers still describe code that has never run; a freshly installed APK has no profile to begin with. Running through Gradle still works, but Gradle's own install steps happen before the tests and are gated too, which is why am instrument is the documented path.

tools/benchmark-watch-install.sh is kept for the one thing it is good at: the two host-side adb install commands above. It reads the Install button off the screen and taps only while the installer is focused. It is not a solution for a run — MIUI keeps mCurrentFocus on the installer for a moment after the window is gone, so a tap sent on that signal can land on the home screen behind it (observed: it opened Telegram).

Why the dialog could not simply be answered from the test, kept because it explains the shape of all this:

• The dialog 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 another showed the same window focused for 8.5 s.
• 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, the path MIUI intercepts.
• MIUI's SecurityCenter cannot be disabled to get out of the way: pm disable-user answers SecurityException: Cannot disable miui core packages.

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 survives because nothing uninstalls any more. An uninstall clears app-ops, so while the rule was reinstalling the app, a grant applied by the fixture script was gone by the time the app started — and appops set from inside the test does not work either (even under adoptShellPermissionIdentity it fails to stderr, which executeShellCommand does not return, and appops get still reads No operations). With Ignore() the grant lands once and stays. grantStorageAccess in BrowserBenchmark still walks the app's own "Grant full access" flow when the app comes up without access, which costs nothing when it is not needed.
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
adb install -r app/build/outputs/apk/benchmark/app-benchmark.apk  # the build under measure
adb install -r benchmark/build/outputs/apk/benchmark/benchmark-benchmark.apk
adb shell am instrument -w -e class de.softwareapp_hb.filemanager.benchmark.BrowserBenchmark \
    de.softwareapp_hb.filemanager.benchmark/androidx.test.runner.AndroidJUnitRunner

One test can be run by naming it: -e class …BrowserBenchmark#coldStart. The numbers are printed in the instrumentation output itself, and the per-test JSON and Perfetto traces are left in /sdcard/Android/media/de.softwareapp_hb.filemanager.benchmark/ — they used to be deleted with the test app when a run ended, which is why the earlier numbers had to be copied mid-run.

Still not measured

number	status
Folder open with 5 000 / 20 000 files	not measured yet: ./tools/benchmark-fixture.sh --count 20000 builds that fixture, and the open/scroll numbers follow from re-running the same command
Thumbnail paint while scrolling a photo grid	not measured: no test for it yet
Any jank figure at all	not measurable here: this phone's FrameTimingMetric output is frameCount only, with no frameDurationCpuMs or frameOverrunMs

What the interaction numbers still need is a device that reports frame durations — an emulator or any non-MIUI phone would do, and the fixture is a folder of empty files, so nothing device-specific is being measured. The install side is solved: CompilationMode.Ignore() plus installing from the host means a run touches the package manager zero times.

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.