How to Use DTrace for Application Tracing on FreeBSD

DTrace is a powerful dynamic tracing framework that allows developers and system administrators to observe, debug, and troubleshoot applications and the operating system in real-time with minimal overhead. Originally developed by Sun Microsystems for Solaris, DTrace has been successfully ported to FreeBSD since version 7.1, providing comprehensive observability capabilities that help diagnose performance issues and understand complex system behavior.

Introduction to DTrace on FreeBSD

DTrace provides unprecedented visibility into both the kernel and user applications running on FreeBSD, offering a unified tracing approach that works across the entire software stack. As a dynamic tracing framework, DTrace enables you to:

• Monitor system calls, kernel functions, and user-level functions
• Track file system, network, process, and CPU activity
• Identify performance bottlenecks and resource contention
• Debug application behavior without modifying source code
• Generate statistics on system operations with minimal performance impact

FreeBSD’s implementation of DTrace retains the same architecture and principles found in the original Solaris version, featuring a powerful array of providers, probes, actions, and a dedicated scripting language called D.

Prerequisites

Before you can begin using DTrace on your FreeBSD system, you need to ensure:

1. Your FreeBSD installation includes DTrace support (included by default since FreeBSD 9.2)
2. The DTrace kernel modules are loaded
3. You have appropriate permissions to use DTrace (typically root privileges)

Verifying DTrace Support

To verify that your FreeBSD kernel has DTrace support enabled:

# sysctl kern.features.dtrace

If DTrace is supported, this command will return:

kern.features.dtrace: 1

Loading DTrace Kernel Modules

To load all required DTrace kernel modules:

# kldload dtraceall

This command loads all the essential DTrace modules, including:

• dtrace - Core DTrace module
• dtmalloc - Memory allocation tracing provider
• dtnfscl - NFS client tracing provider
• dtnfssrv - NFS server tracing provider
• fbt - Function Boundary Tracing provider
• sdt - Statically Defined Tracing provider
• profile - Profiling provider
• syscall - System call tracing provider

To verify the modules have been loaded:

# kldstat | grep dtrace

Understanding the D Language

DTrace uses its own scripting language called D, which is syntactically similar to C but with special constructs for tracing operations. D scripts, commonly called “D programs,” define the conditions under which data should be collected and what actions to take when those conditions are met.

A typical D program consists of:

1. Probe descriptions: Define what events to monitor
2. Predicates: Optional filters that determine when a probe action should be executed
3. Actions: Code that executes when a probe fires and passes any predicate tests

Probe Descriptions

Probe descriptions in DTrace follow this format:

provider:module:function:name

Where:

• provider: The DTrace provider (e.g., syscall, fbt, pid)
• module: The kernel module or library
• function: The function name to trace
• name: The specific probe name (often “entry” or “return”)

For example, syscall::open:entry refers to the entry point of the open() system call.

Predicates

Predicates are boolean expressions enclosed in slashes that determine whether the associated action block should execute when a probe fires:

probe-description /predicate/ { actions }

For example:

syscall::open:entry /pid == 1234/ { printf("Process %d opened %s\n", pid, copyinstr(arg0)); }

This only traces open() calls from the process with PID 1234.

Actions

The most common actions in D scripts include:

• printf(): Print formatted output
• trace(): Print simple values
• quantize(), lquantize(): Generate distribution plots
• count(): Increment a counter
• sum(): Add to a sum
• exit(): Exit the DTrace session

Basic DTrace Commands on FreeBSD

Let’s explore some basic DTrace commands that demonstrate its capabilities:

1. Listing Available Probes

To list all available probes on your system:

# dtrace -l | head

To count the total number of probes:

# dtrace -l | wc -l

To find specific probes, use grep. For example, to find all open system call probes:

# dtrace -l | grep open

2. Tracing System Calls

To trace all system calls made by a specific process:

# dtrace -n 'syscall:::entry /pid == $target/ { @[probefunc] = count(); }' -p PID

Replace PID with the process ID you want to trace.

3. Monitoring File I/O

To monitor files being opened across the system:

# dtrace -n 'syscall::open*:entry { printf("%s %s", execname, copyinstr(arg0)); }'

4. Analyzing Process Creation

To track new processes being created:

# dtrace -n 'proc:::exec-success { trace(execname); }'

Application Tracing Techniques

While system-wide tracing is useful, DTrace truly shines when used for application-specific tracing. Here are techniques for tracing applications on FreeBSD:

1. Process Tracing with the PID Provider

The pid provider allows you to trace function calls within a specific process:

# dtrace -n 'pid$target:::entry { @[probefunc] = count(); }' -p PID

This counts all function calls within the specified process.

2. USDT Probes (User Statically-Defined Tracing)

Some applications are instrumented with USDT (User Statically-Defined Tracing) probes. For example, to trace PostgreSQL if it has USDT probes enabled:

# dtrace -n 'postgresql*:::query-start { printf("%s", copyinstr(arg0)); }'

3. Function Boundary Tracing for Applications

To trace function entry and return within a specific library in an application:

# dtrace -n 'pid$target:libc:malloc:entry { @[execname] = count(); }' -p PID

This counts memory allocation calls from the process.

4. Monitoring Application Latency

To measure how long a particular function takes to execute:

# dtrace -n '
pid$target::function_name:entry
{
    self->ts = timestamp;
}

pid$target::function_name:return
/self->ts/
{
    @["function_name latency (ns)"] = quantize(timestamp - self->ts);
    self->ts = 0;
}' -p PID

Replace function_name with the actual function name you want to monitor.

Creating DTrace Scripts for Complex Tracing

For more complex tracing scenarios, it’s better to create D script files rather than using command-line invocations.

Example: HTTP Request Tracing Script

Here’s an example D script (http_trace.d) for tracing HTTP requests in a web server:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
    printf("Tracing HTTP requests... Hit Ctrl-C to end.\n");
}

pid$target::process_request:entry
{
    self->start = timestamp;
    self->path = copyinstr(arg0);
    printf("Request started: %s\n", self->path);
}

pid$target::process_request:return
/self->start/
{
    @time[self->path] = quantize(timestamp - self->start);
    printf("Request completed: %s (%d ns)\n", 
           self->path, timestamp - self->start);
    self->path = 0;
    self->start = 0;
}

dtrace:::END
{
    printf("\nRequest latency distributions (nanoseconds):\n");
    printa("  %s %@d\n", @time);
}

Execute this script with:

# chmod +x http_trace.d
# ./http_trace.d -p PID

Performance Considerations

While DTrace is designed to have minimal overhead, it’s important to keep these considerations in mind:

1. Probe effect: Heavy tracing can impact system performance

2. Buffer size: For high-frequency events, you may need to increase the buffer size:

   # dtrace -b 16m -n '...'
   

3. Aggregation: Use DTrace’s aggregation functions (@) rather than printing for every probe firing

4. Predicates: Use predicates to filter probes early and reduce processing overhead

Advanced DTrace Features on FreeBSD

1. Speculative Tracing

Speculative tracing allows you to record data temporarily and then commit or discard it based on conditions:

speculate(buf);  // Record to speculative buffer
/* Collect data... */
commit(buf);     // Commit if conditions are met
discard(buf);    // Or discard if not needed

2. Destructive Actions

With appropriate privileges and caution, DTrace can perform destructive actions like stopping processes:

# dtrace -w -n 'syscall::reboot:entry { stop(); }'

The -w flag enables destructive actions.

3. Custom Providers

FreeBSD allows creating custom DTrace providers through its kernel module system, extending DTrace capabilities for specific applications.

Troubleshooting DTrace on FreeBSD

Common issues with DTrace on FreeBSD include:

1. Permission errors: Ensure you’re running DTrace as root
2. Missing symbols: If function names aren’t resolved, check if the application was compiled with debug symbols
3. Module loading failures: Verify kernel compatibility and module dependencies
4. No probes matching: Double-check probe syntax and that target processes are running

Conclusion

DTrace on FreeBSD provides an extraordinarily powerful framework for application and system tracing. By leveraging its capabilities, FreeBSD administrators and developers can gain unprecedented insight into system behavior, diagnose complex performance issues, and understand application interactions without modifying code or rebuilding the system.

The dynamic and low-overhead nature of DTrace makes it suitable for production environments, allowing real-time problem diagnosis without bringing systems down or causing significant performance degradation. With the techniques described in this article, you can use DTrace to solve complex problems, from identifying application bottlenecks to understanding system-wide behavior patterns.

As you become more proficient with DTrace, you’ll discover that the combination of its powerful probing mechanisms, the flexibility of the D language, and FreeBSD’s robust implementation creates a truly indispensable tool for system observation and analysis.