Profiling

Flame Graph Formats

In profiling, collapsed and flamegraph are the two most common formats, corresponding to the “raw data” and “visual view” layers respectively.

Collapsed Format

Standard Syntax and Format

The collapsed format (also called folded stacks) was defined by Brendan Gregg and serves as the raw text input format for flame graphs. Each line represents a unique call stack and its sample count.

Basic rule:

frame1;frame2;frame3;...;frameN COUNT

Format details:

• One unique call stack per line; samples with the same stack path have their counts merged
• Frame order: left to right is root → leaf (call chain direction)
• Blank lines and lines starting with # are treated as comments and ignored during parsing
• The semantics of COUNT depend on the analysis mode: for CPU sampling it is the number of samples, for memory allocation it is the number of bytes allocated, for lock analysis it is the contention time in milliseconds

Extended specification:

Some profiling tools (e.g. async-profiler) add frame type annotations on top of the standard format to identify the runtime category of a frame:

frameName_{type} COUNT

Additionally, some tools support a weighted collapsed format for differential flame graphs:

frame1;frame2;frameN WEIGHT

Where WEIGHT is a floating-point number representing the weight of the stack rather than a simple count.

Sample Examples

CPU profiling example (data from the async-profiler official documentation):

FileConverter.main;FileConverter.convertFile;FileConverter.saveResult 21
FileConverter.main;FileConverter.convertFile;FileConverter.saveResult;java/io/DataOutputStream.writeInt 1
FileConverter.main;FileConverter.convertFile;FileConverter.saveResult;java/io/DataOutputStream.writeInt;java/io/ByteArrayOutputStream.write 5
FileConverter.main;FileConverter.convertFile;FileConverter.saveResult;java/io/DataOutputStream.writeUTF;java/io/DataOutputStream.writeUTF 12
FileConverter.main;FileConverter.convertFile;FileConverter.saveResult;java/io/DataOutputStream.writeUTF;java/io/DataOutputStream.writeUTF;java/lang/String.length 3
FileConverter.main;FileConverter.convertFile;FileConverter.saveResult;java/io/DataOutputStream.writeUTF;java/io/DataOutputStream.writeUTF;java/io/DataOutputStream.write 6
start_thread;thread_native_entry;Thread::call_run;VMThread::run;VMThread::inner_execute;VMThread::evaluate_operation;VM_Operation::evaluate;VM_GenCollectForAllocation::doit;GenCollectedHeap::satisfy_failed_allocation;GenCollectedHeap::do_collection;GenCollectedHeap::collect_generation;DefNewGeneration::collect;DefNewGeneration::FastEvacuateFollowersClosure::do_void 12

Example with frame type annotations (async-profiler extension):

Main.run_[j];Service.process_[j];DAO.query_[j];mysql_real_query_[n] 45
Main.run_[j];Service.process_[j];DAO.query_[j];recv_[k] 18

Core Use Cases

Generation command example:

# Using async-profiler as an example
asprof -d 30 -f profile.collapsed -o collapsed <PID>

Flamegraph Format

Standard Syntax and Format

The flamegraph format is a self-contained HTML file with embedded SVG visualization and JavaScript interaction logic, which can be opened directly in a browser.

Structural composition:

flamegraph.html
├── HTML skeleton + CSS styles
├── SVG flame graph body
│   ├── <g> rectangle block for each frame
│   │   ├── <title> frame name + sample count/percentage
│   │   └── <rect> position, width, height, color
│   └── ...
├── JavaScript interaction logic
│   ├── Click to zoom (zoom into subtree)
│   ├── Search & highlight
│   ├── Tooltip on hover
│   └── Reset zoom
└── Metadata (title, total samples, etc.)

Visual encoding rules:

Frame color specification (based on async-profiler):

Note: Flame graph color schemes are not a cross-tool standard. The original flamegraph.pl by Brendan Gregg uses random warm tones with no semantic meaning; perf/bpftrace typically colors by DSO or uses random colors; async-profiler colors by frame type semantics. The following is the async-profiler color specification:

Extended features (based on async-profiler):

• Icicle Graph: Displays the call chain top-down (root at the top), which better suits top-down reading habits. Toggle via the --reverse option or the Reverse button in the browser
• Multi-thread view: Call stacks from different threads are displayed side by side at the root level
• Search highlighting: Matching frames are highlighted in purple; non-matching frames are dimmed
• Sample info tooltip: Hover to display frame name, sample count, and percentage of total samples
• Cutoff frames: Frames marked as [...] indicate stack truncation (e.g. due to stack depth limits)

Sample Examples

Generation command example:

# Using async-profiler as an example
asprof -d 30 -f flamegraph.html <PID>

Interactive operations:

• Click a frame: Zoom to make the frame full-width, showing only its subtree
• Search box: Enter a keyword; matching frames are highlighted
• Hover: Display frame name, sample count, and percentage
• Reset Zoom: Restore the global view

Core Use Cases

Format Comparison

Typical workflow:

Collect ──► collapsed ──► flamegraph.html (manual analysis)
                   │
                   ├──► Differential flame graph (regression detection)
                   ├──► Custom aggregation scripts
                   └──► Archive storage