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104 lines
3.9 KiB
104 lines
3.9 KiB
## Inferno
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### Description
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Inferno is a flamegraph generator for native (C/C++) Android apps. It was
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originally written to profile and improve surfaceflinger performance
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(Android compositor) but it can be used for any native Android application
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. You can see a sample report generated with Inferno
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[here](./report.html). Report are self-contained in HTML so they can be
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exchanged easily.
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Notice there is no concept of time in a flame graph since all callstack are
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merged together. As a result, the width of a flamegraph represents 100% of
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the number of samples and the height is related to the number of functions on
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the stack when sampling occurred.
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In the flamegraph featured above you can see the main thread of SurfaceFlinger.
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It is immediatly apparent that most of the CPU time is spent processing messages
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`android::SurfaceFlinger::onMessageReceived`. The most expensive task is to ask
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the screen to be refreshed as `android::DisplayDevice::prepare` shows in orange
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. This graphic division helps to see what part of the program is costly and
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where a developer's effort to improve performances should go.
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### Example of bottleneck
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A flamegraph give you instant vision on the CPU cycles cost centers but
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it can also be used to find specific offenders. To find them, look for
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plateaus. It is easier to see an example:
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In the previous flamegraph, two
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plateaus (due to `android::BufferQueueCore::validateConsistencyLocked`)
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are immediately apparent.
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### How it works
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Inferno relies on simpleperf to record the callstack of a native application
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thousands of times per second. Simpleperf takes care of unwinding the stack
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either using frame pointer (recommended) or dwarf. At the end of the recording
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`simpleperf` also symbolize all IPs automatically. The record are aggregated and
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dumps dumped to a file `perf.data`. This file is pulled from the Android device
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and processed on the host by Inferno. The callstacks are merged together to
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visualize in which part of an app the CPU cycles are spent.
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### How to use it
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Open a terminal and from `simpleperf/scripts` directory type:
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```
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./inferno.sh (on Linux/Mac)
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inferno.bat (on Windows)
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```
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Inferno will collect data, process them and automatically open your web browser
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to display the HTML report.
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### Parameters
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You can select how long to sample for, the color of the node and many other
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things. Use `-h` to get a list of all supported parameters.
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```
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./inferno.sh -h
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```
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### Troubleshooting
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#### Messy flame graph
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A healthy flame graph features a single call site at its base (see [here](./report.html)).
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If you don't see a unique call site like `_start` or `_start_thread` at the base
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from which all flames originate, something went wrong. : Stack unwinding may
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fail to reach the root callsite. These incomplete
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callstack are impossible to merge properly. By default Inferno asks
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`simpleperf` to unwind the stack via the kernel and frame pointers. Try to
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perform unwinding with dwarf `-du`, you can further tune this setting.
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#### No flames
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If you see no flames at all or a mess of 1 level flame without a common base,
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this may be because you compiled without frame pointers. Make sure there is no
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` -fomit-frame-pointer` in your build config. Alternatively, ask simpleperf to
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collect data with dward unwinding `-du`.
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#### High percentage of lost samples
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If simpleperf reports a lot of lost sample it is probably because you are
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unwinding with `dwarf`. Dwarf unwinding involves copying the stack before it is
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processed. Try to use frame pointer unwinding which can be done by the kernel
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and it much faster.
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The cost of frame pointer is negligible on arm64 parameter but considerable
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on arm 32-bit arch (due to register pressure). Use a 64-bit build for better
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profiling.
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#### run-as: package not debuggable
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If you cannot run as root, make sure the app is debuggable otherwise simpleperf
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will not be able to profile it.
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