Converting and visualizing a trace

The Fuchsia trace system supports various file formats for recording traces. Each data format requires a specific tool to visualize trace results.


Before you attempt to convert or analyze a trace file, make sure you've recorded a trace.

Fuchsia trace file formats

The following types of file formats can store Fuchsia trace data:

  • FXT (or Fuchsia trace format) is a binary format that is a direct encoding of the original trace data that is produced by the various programs. For more information, see FXT trace.
  • JSON is used for viewing trace data on Chrome. For more information, see JSON trace.
  • HTML is a standalone file that includes both the viewer and trace data. For more information, see HTML trace.

Convert Fuchsia trace files

To convert one or more files from FXT to JSON, run the following command:

fx trace2json < <FXT_FILE> > trace.json

Replace FXT_FILE with a trace output file in FXT format, for example:

fx trace2json < trace.fxt > trace.json

Visualize a trace

There are different ways to visualize a trace based on the format of the trace:

FXT trace

To visualize an FXT trace, use the Perfetto viewer{:.external}, which also allows you to use SQL to query your trace data{:.external}.

JSON trace

To visualize a JSON trace, use Chromium‘s Trace-Viewer{:.external}. The viewer is built into Chrome and can be loaded with chrome://tracing. For more information on Chrome’s trace viewer, see The Trace Event Profiling Tool{:.external}.

HTML trace

To generate an HTML trace (which is a viewer bundled with trace data), you need to run trace2html from the Chromium Catapult Repository{:.external}.

From the Catapult repository, run the following command:

./tracing/bin/trace2html <JSON_TRACE_FILE>

Analyze a trace file

Note: The instructions in this section detail how to visualize an JSON trace with a Chrome browser.

To analyze a JSON trace file:

  1. Open a new tab in Chrome.
  2. Navigate to chrome://tracing.
  3. Click the Load button.
  4. Open your JSON trace file.

Navigation controls

As there is a lot of information in a trace file, there are some useful keyboard shortcuts that you can use:

Note: Near the top right of the page, there is a small ? icon that you can click to see help information.

  • w and s: Zoom in and zoom out, respectively. The zoom function is based on the current position of your mouse.
  • W and S: Zoom in and zoom out at a larger scale, respectively. The zoom function is based on the current position of your mouse.
  • a and d: Pan left and right, respectively.
  • A and D: Pan left and right at a larger scale, respectively.
  • You can deselect specific process rows to remove processes that aren't important for your current trace. To deselect a specific process row, click the x in the right corner of the process row.

Interpret the trace data

The example in this section shows a trace of what the system is doing while running the du command, which shows disk usage.

Before you can record trace data, you must start a Fuchsia instance. From your host, if you don't have a Fuchsia target device, you can start a Fuchsia emulator with networking:

Note: For more information on getting started with Fuchsia, see Fuchsia.

ffx emu --net tap

This command configures and starts the Fuchsia emulator.

To record a trace of du, do the following:

  1. In a new terminal, run ffx trace start:

    Note: For more information on recording a trace in Fuchsia, see Recording a Fuchsia trace.

    ffx trace start --buffer-size 64 --categories all

    This command sets a recording buffer size of 64 megabytes and records all tracing categories.

  2. In the Fuchsia emulator's terminal, run the following command:

    /boot/bin/sh -c "'\
       sleep 2 ;\
       i=0 ;\
       while [ \$i -lt 10 ] ;\
       do /bin/du /boot ;\
           i=\$(( \$i + 1 )) ;\

    This command runs du in a loop,

    Note: For more information on creating a process in Fuchsia, see Process creation.

  3. To finish the tracing, press Enter key in the terminal on your host machine.

    When the tracing is finished, it generated an FXT file,

  4. Convert this FXT file to JSON:

    fx trace2json < trace.fxt > trace.json
  5. Generate an HTML trace (see HTML Trace above):

    ./tracing/bin/trace2html trace.json

Screenshot of trace interface

A trace file has a lot of information including a time scale near the top of the trace. In this example, the whole trace lasted about 2.5 seconds.

CPU usage

The region marked by the yellow circle shows the CPU usage area where you can see the overall CPU usage on all CPU cores.

Program execution

The region marked by the green circle shows the program execution.

In this example, you can see 10 invocations of the du program, which is expected since the trace was recorded during a loop of du. Therefore, you can see 10 different du process IDs, one after the other.

blobfs CPU usage

The region marked by the blue circle shows the CPU usage to write to the blobstore filesystem (blobFS).

In this example, you can see little bursts of CPU time that are each related to an invocation of du.

At this high level, it can be difficult to determine the exact correlation between the CPU usage and the filesystem:

  • Is the CPU usage caused by the loading of du from the filesystem?
  • Is the CPU usage caused by the execution du as it runs through the target filesystem to see how much space is in use?

You can zoom in on specific areas of this region to determine the correlation between the CPU usage and the filesystem.

Screenshot of zooming in on cpu and filesystem information

In this example, you can see just two du executions (the first is marked with a green circle). The first blobfs CPU burst actually consists of three main clusters and some smaller spikes. Subsequent blobfs CPU bursts have two clusters.

From analyzing this example, you can see that the blobfs bursts happen before the du program is executed. This information shows that the blobfs bursts are not due to the du program reading the filesystem. Instead, it shows that the bursts are due to loading the du program.

You are now ready to dive further into what is causing the blobs bursts.

Image of blobs trace timings

In this example, notice the time scale that spans a time period from 2,023,500 microseconds to just past 2,024,500 which indicated a time scale of about 1 millisecond.

During that millisecond, blobfs executed code, starting with a process identified as FileReadAt, which then called Blob::Read, which then called Blob::ReadInternal.

To correlate this information with the code, you can click on parts of the report for more detailed information about a specific object.

If you click on FileReadAt, you can see the following information:

Image of FileReadAt information

This information tells you the following:

  • The trace category for FileReadAt is vfs.
  • The length of time of the function execution.

Note: For information on how tracing is performed for FileReatAt, see //src/lib/storage/vfs/cpp/

If you click on Blob::Read, you can see the following information:

Image of Blob::Read information

Note: For information on how tracing is performed for Blob::Read, see //src/storage/blobfs/

The code for Blob::Read is:

zx_status_t Blob::Read(void* data,
                       size_t len,
                       size_t off,
                       size_t* out_actual) {
    TRACE_DURATION("blobfs", "Blob::Read", "len", len, "off", off);
    LatencyEvent event(&blobfs_->GetMutableVnodeMetrics()->read,

    return ReadInternal(data, len, off, out_actual);

This code calls calls the TRACE_DURATION() macro with the category of blobfs, a name of Blob::Read, and a length and offset key and value pairs. All of this information is recorded in the trace file.

After analyzing these examples, you can still see additional objects that are being traced.