Markers are packets of arbitrary data that are added to a profile by the Firefox code, usually to indicate something important happening at a point in time, or during an interval of time.

Each marker has a name, a category, some common optional information (timing, backtrace, etc.), and an optional payload of a specific type (containing arbitrary data relevant to that type).


This guide explains C++ markers in depth. To learn more about how to add a marker in JavaScript or Rust, please take a look at their documentation in Instrumenting JavaScript or Instrumenting Rust respectively.


Short example, details below.

Note: Most marker-related identifiers are in the mozilla namespace, to be added where necessary.

// Record a simple marker with the category of DOM.

// Create a marker with some additional text information. (Be wary of printf!)
PROFILER_MARKER_TEXT("Marker Name", JS, MarkerOptions{}, "Additional text information.");

// Record a custom marker of type `ExampleNumberMarker` (see definition below).
PROFILER_MARKER("Number", OTHER, MarkerOptions{}, ExampleNumberMarker, 42);
// Marker type definition.
struct ExampleNumberMarker {
  // Unique marker type name.
  static constexpr Span<const char> MarkerTypeName() { return MakeStringSpan("number"); }
  // Data specific to this marker type, serialized to JSON for
  static void StreamJSONMarkerData(SpliceableJSONWriter& aWriter, int a number) {
    aWriter.IntProperty("number", a number);
  // Where and how to display the marker and its data.
  static MarkerSchema MarkerTypeDisplay() {
    using MS = MarkerSchema;
    MS schema(MS::Location::MarkerChart, MS::Location::MarkerTable);
    schema.SetChartLabel("Number: {}");
    schema.AddKeyLabelFormat("number", "Number", MS::Format::Number);
    return schema;

How to Record Markers

Header to Include

If the compilation unit only defines and records untyped, text, and/or its own markers, include the main profiler markers header:

#include "mozilla/ProfilerMarkers.h"

If it also records one of the other common markers defined in ProfilerMarkerTypes.h, include that one instead:

#include "mozilla/ProfilerMarkerTypes.h"

And if it uses any other profiler functions (e.g., labels), use the main Gecko Profiler header instead:

#include "GeckoProfiler.h"

The above works from source files that end up in libxul, which is true for the majority of Firefox source code. But some files live outside of libxul, such as mfbt, in which case the advice is the same but the equivalent headers are from the Base Profiler instead:

#include "mozilla/BaseProfilerMarkers.h" // Only own/untyped/text markers
#include "mozilla/BaseProfilerMarkerTypes.h" // Only common markers
#include "BaseProfiler.h" // Markers and other profiler functions

Untyped Markers

Untyped markers don’t carry any information apart from common marker data: Name, category, options.

    // Name, and category pair.
    "Marker Name", OTHER,
    // Marker options, may be omitted if all defaults are acceptable.
    MarkerOptions(MarkerStack::Capture(), ...));

PROFILER_MARKER_UNTYPED is a macro that simplifies the use of the main profiler_add_marker function, by adding the appropriate namespaces, and a surrounding #ifdef MOZ_GECKO_PROFILER guard.

  1. Marker name

    The first argument is the name of this marker. This will be displayed in most places the marker is shown. It can be a literal C string, or any dynamic string object.

  2. Category pair name

    Choose a category + subcategory from the the list of categories. This is the second parameter of each SUBCATEGORY line, for instance LAYOUT_Reflow. (Internally, this is really a MarkerCategory object, in case you need to construct it elsewhere.)

  3. MarkerOptions

    See the options below. It can be omitted if there are no other arguments, {}, or MarkerOptions() (no specified options); only one of the following option types alone; or MarkerOptions(...) with one or more of the following options types:

    • MarkerThreadId

      Rarely used, as it defaults to the current thread. Otherwise it specifies the target “thread id” (aka “track”) where the marker should appear; This may be useful when referring to something that happened on another thread (use profiler_current_thread_id() from the original thread to get its id); or for some important markers, they may be sent to the “main thread”, which can be specified with MarkerThreadId::MainThread().

    • MarkerTiming

      This specifies an instant or interval of time. It defaults to the current instant if left unspecified. Otherwise use MarkerTiming::InstantAt(timestamp) or MarkerTiming::Interval(ts1, ts2); timestamps are usually captured with TimeStamp::Now(). It is also possible to record only the start or the end of an interval, pairs of start/end markers will be matched by their name. Note: The upcoming “marker sets” feature will make this pairing more reliable, and also allow more than two markers to be connected.

    • MarkerStack

      By default, markers do not record a “stack” (or “backtrace”). To record a stack at this point, in the most efficient manner, specify MarkerStack::Capture(). To record a previously captured stack, first store a stack into a UniquePtr<ProfileChunkedBuffer> with profiler_capture_backtrace(), then pass it to the marker with MarkerStack::TakeBacktrace(std::move(stack)).

    • MarkerInnerWindowId

      If you have access to an “inner window id”, consider specifying it as an option, to help to classify them by tab.

Text Markers

Text markers are very common, they carry an extra text as a fourth argument, in addition to the marker name. Use the following macro:

    // Name, category pair, options.
    "Marker Name", OTHER, {},
    // Text string.
    "Here are some more details."

As useful as it is, using an expensive printf operation to generate a complex text comes with a variety of issues string. It can leak potentially sensitive information such as URLs can be leaked during the profile sharing step. cannot access the information programmatically. It won’t get the formatting benefits of the built-in marker schema. Please consider using a custom marker type to separate and better present the data.

Other Typed Markers

From C++ code, a marker of some type YourMarker (details about type definition follow) can be recorded like this:

    "YourMarker name", OTHER,
    YourMarker, "some string", 12345, "", someTimeStamp);

After the first three common arguments (like in PROFILER_MARKER_UNTYPED), there are:

  1. The marker type, which is the name of the C++ struct that defines that type.

  2. A variadic list of type-specific argument. They must match the number of, and must be convertible to, StreamJSONMarkerData parameters as specified in the marker type definition.

“Auto” Scoped Interval Markers

To capture time intervals around some important operations, it is common to store a timestamp, do the work, and then record a marker, e.g.:

void DoTimedWork() {
  TimeStamp start = TimeStamp::Now();
  PROFILER_MARKER_TEXT("Timed work", OTHER, MarkerTiming::IntervalUntilNowFrom(start), "Details");

RAII objects automate this, by recording the time when the object is constructed, and later recording the marker when the object is destroyed at the end of its C++ scope. This is especially useful if there are multiple scope exit points.

AUTO_PROFILER_MARKER_TEXT is `the only one implemented <`_ at this time.

void MaybeDoTimedWork(bool aDoIt) {
  AUTO_PROFILER_MARKER_TEXT("Timed work", OTHER, "Details");
  if (!aDoIt) { /* Marker recorded here... */ return; }
  /* ... or here. */

Note that these RAII objects only record one marker. In some situation, a very long operation could be missed if it hasn’t completed by the end of the profiling session. In this case, consider recording two distinct markers, using MarkerTiming::IntervalStart() and MarkerTiming::IntervalEnd().

Where to Define New Marker Types

The first step is to determine the location of the marker type definition:

  • If this type is only used in one function, or a component, it can be defined in a local common place relative to its use.

  • For a more common type that could be used from multiple locations:

How to Define New Marker Types

Each marker type must be defined once and only once. The definition is a C++ struct, its identifier is used when recording markers of that type in C++. By convention, the suffix “Marker” is recommended to better distinguish them from non-profiler entities in the source.

struct YourMarker {

Marker Type Name

A marker type must have a unique name, it is used to keep track of the type of markers in the profiler storage, and to identify them uniquely on (It does not need to be the same as the struct’s name.)

This name is defined in a special static member function MarkerTypeName:

// …
  static constexpr Span<const char> MarkerTypeName() {
    return MakeStringSpan("YourMarker");

Marker Type Data

All markers of any type have some common data: A name, a category, options like timing, etc. as previously explained.

In addition, a certain marker type may carry zero of more arbitrary pieces of information, and they are always the same for all markers of that type.

These are defined in a special static member function StreamJSONMarkerData.

The first function parameters is always SpliceableJSONWriter& aWriter, it will be used to stream the data as JSON, to later be read by

// …
  static void StreamJSONMarkerData(SpliceableJSONWriter& aWriter,

The following function parameters is how the data is received as C++ objects from the call sites.

  • Most C/C++ POD (Plain Old Data) and trivially-copyable types should work as-is, including TimeStamp.

  • Character strings should be passed using const ProfilerString8View& (this handles literal strings, and various std::string and nsCString types, and spans with or without null terminator). Use const ProfilerString16View& for 16-bit strings such as nsString.

  • Other types can be used if they define specializations for ProfileBufferEntryWriter::Serializer and ProfileBufferEntryReader::Deserializer. You should rarely need to define new ones, but if needed see how existing specializations are written, or contact the perf-tools team for help.

Passing by value or by reference-to-const is recommended, because arguments are serialized in binary form (i.e., there are no optimizable move operations).

For example, here’s how to handle a string, a 64-bit number, another string, and a timestamp:

// …
                                   const ProfilerString8View& aString,
                                   const int64_t aBytes,
                                   const ProfilerString8View& aURL,
                                   const TimeStamp& aTime) {

Then the body of the function turns these parameters into a JSON stream.

When this function is called, the writer has just started a JSON object, so everything that is written should be a named object property. Use SpliceableJSONWriter functions, in most cases ...Property functions from its parent class JSONWriter: NullProperty, BoolProperty, IntProperty, DoubleProperty, StringProperty. (Other nested JSON types like arrays or objects are not supported by the profiler.)

As a special case, TimeStamps must be streamed using aWriter.TimeProperty(timestamp).

The property names will be used to identify where each piece of data is stored and how it should be displayed on (see next section).

Here’s how the above functions parameters could be streamed:

// …
    aWriter.StringProperty("myString", aString);
    aWriter.IntProperty("myBytes", aBytes);
    aWriter.StringProperty("myURL", aURL);
    aWriter.TimeProperty("myTime", aTime);

Marker Type Display Schema

Now that we have defined how to stream type-specific data (from Firefox to, we need to describe where and how this data will be displayed on

The static member function MarkerTypeDisplay returns an opaque MarkerSchema object, which will be forwarded to

// …
  static MarkerSchema MarkerTypeDisplay() {

The MarkerSchema type will be used repeatedly, so for convenience we can define a local type alias:

// …
    using MS = MarkerSchema;

First, we construct the MarkerSchema object to be returned at the end.

One or more constructor arguments determine where this marker will be displayed in the UI. See the MarkerSchema::Location enumeration for the full list.

Here is the most common set of locations, showing markers of that type in both the Marker Chart and the Marker Table panels:

// …
    MS schema(MS::Location::MarkerChart, MS::Location::MarkerTable);

Some labels can optionally be specified, to display certain information in different locations: SetChartLabel, SetTooltipLabel, and SetTableLabel; or SetAllLabels to define all of them the same way.

The arguments is a string that may refer to marker data within braces:

  • {}: Marker name.

  • {}: Type-specific data, as streamed with property name “X” from StreamJSONMarkerData (e.g., aWriter.IntProperty("X", a number);

For example, here’s how to set the Marker Chart label to show the marker name and the myBytes number of bytes:

// …
    schema.SetChartLabel("{} – {}"); will apply the label with the data in a consistent manner. For example, with this label definition, it could display marker information like the following in the Firefox Profiler’s Marker Chart:

  • “Marker Name – 10B”

  • “Marker Name – 25.204KB”

  • “Marker Name – 512.54MB”

For implementation details on this processing, see src/profiler-logic/marker-schema.js in the profiler’s front-end.

Next, define the main display of marker data, which will appear in the Marker Chart tooltips and the Marker Table sidebar.

Each row may either be:

  • A dynamic key-value pair, using one of the MarkerSchema::AddKey... functions. Each function is given:

    • Key: Element property name as streamed in StreamJSONMarkerData.

    • Label: Optional prefix. Defaults to the key name.

    • Format: How to format the data element value, see MarkerSchema::Format for details.

    • Searchable: Optional boolean, indicates if the value is used in searches, defaults to false.

  • Or a fixed label and value strings, using MarkerSchema::AddStaticLabelValue.

// …
        "myString", "My String", MS::Format::String, true);
        "myBytes", "My Bytes", MS::Format::Bytes);
        "myUrl", "My URL", MS::Format::Url);
        "myTime", "Event time", MS::Format::Time);

Finally the schema object is returned from the function:

// …
    return schema;

Any other struct member function is ignored. There could be utility functions used by the above compulsory functions, to make the code clearer.

And that is the end of the marker definition struct.

// …

Performance Considerations

During profiling, it is best to reduce the amount of work spent doing profiler operations, as they can influence the performance of the code that you want to profile.

Whenever possible, consider passing simple types to marker functions, such that StreamJSONMarkerData will do the minimum amount of work necessary to serialize the marker type-specific arguments to its internal buffer representation. POD types (numbers) and strings are the easiest and cheapest to serialize. Look at the corresponding ProfileBufferEntryWriter::Serializer specializations if you want to better understand the work done.

Avoid doing expensive operations when recording markers. E.g.: printf of different things into a string, or complex computations; instead pass the printf/computation arguments straight through to the marker function, so that StreamJSONMarkerData can do the expensive work at the end of the profiling session.

Marker Architecture Description

The above sections should give all the information needed for adding your own marker types. However, if you are wanting to work on the marker architecture itself, this section will describe how the system works.

  • Briefly describe the buffer and serialization.

  • Describe the template strategy for generating marker types

  • Describe the serialization and link to profiler front-end docs on marker processing (if they exist)