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Hawkeye - CPU Trace

CPUTrace is used to track the CPU's continuous high-load usage, and will recording which methods are mainly called during the high-load CPU usage.

0x00 Usage

After add ANR Trace to MTHawkeyeClient, by default it'll start after MTHawkeyeClient run, you can change it by following steps:

  1. Tap MTHawkeye floating window, enter the main panel.
  2. Tap navigation title view, show the MTHawkeye panel switching view.
  3. Tap Setting in the upper right corner of the switching view, enter the Setting view home.
  4. Find Energy and go to CPU Trace, turn off CPU Trace

The data recorded by CPU Trace contains following parts:

  • CPU high-load start time
  • CPU high-load duration
  • Average CPU usage during the high-load.
  • stack frame samples during the high-load.

0x01 Storage

While the records generate, the data will store under Records file.

Each CPU high-load record split into two lines, one for the underlying data, and the collection name is cpu-highload

cpu-highload,575204521.78,{{basic-cpu-highload record}}

key is the start time of the CPU high-load record, {{basic-cpu-highload record}} is a JSON string with three fields:

{
    "start": "575204521.78", // the start time of the record
    "lasting": "132.60",     // the duration of the record
    "average": "106"         // the average CPU usage during the high-load, 106%
}

Another line records the sample stack frames during the high-load, the collection name is cpu-highload-stackframe.

cpu-highload-stackframe,575204521.78,{{cpu-highload stackframes samples}}

key is the start time of the CPU high-load record, {{cpu-highload stackframes samples}} is a JSON string with fields bellow:

  • frame: the frame from the sample call stack.
  • proportion: the ratio of this sample to the total sample
  • count: the count the frame was sampled.
  • children: sub-call frames

example

[
    {
        "frame": "0x10234235",
        "proportion": 0.6,
        "count": 60,
        "children": [
            {
                "frame": "0x10234444",
                "proportion": 0.4,
                "count": 40
            },
            {
                "frame": "0x10235555",
                "proportion": 0.2,
                "count": 20
            }
        ]
    },
    {
        "frame": "0x10234111",
        "proportion": 0.3,
        "count": 30,
        "children": [
            {
                "frame": "0x10234112",
                "proportion": 0.3,
                "count": 30
            }
        ]
    },
    {
        "frame": "0x10234000",
        "proportion": 0.1,
        "count": 10
    }
]

You need to read both cpu-highload and cpu-highload-stackframe at the same time, After read out from the Records file, merge the records with the same key together.

0x03 Symbolics

CPU Trace records raw stack frame need to symbolized for reading.

If the running App do have DWARF within, you can directly use CPU Trace panel to view the symbolized result generated internal.

In other cases, if you have your own remote symbolization service and did set, switch on the Remote Symbolics, and then you can view the symbolized stack frame directly in the App.

If you wanna get the raw data from the sandbox, manual symbolize yourself as follows:

  1. Get the dyld-images file in the MTHawkeye storage directory.
  2. Get the raw stack frame data from the records.
  3. Get the dSYM files
  4. for each frame
    • find the match dyld-image by frame value, then match a dSYM file.
    • Use atos command to symbolize the frame.

0x04 Performance Impact

The main time-consuming point is to get the CPU usage detail of all threads and sampling the stacks while CPU high-load.

Under iPhone 6s, iOS 10.3.2, while there are about 30 threads running, the original CPU usage is 90~110%, and the number of threads that need to be sampling stacks is less than 5.

  • For RELEASE, cost 350us to get all the CPU usage detail each time in average. Under DEBUG is not much different.
  • For RELEASE, cost 160us to sampling the call stacks in average. And about 290us under DEBUG.

In summary, the extra time spent in each operation is within 1 millisecond, while the default trigger interval to get the usage is once per second, and when during high-load it's every 0.3 seconds.