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pthread_trace

This tool allows capturing pthread events into a perfetto trace, which allows visualizing pthread synchronization events on a timeline for debugging and optimization purposes.

To use it, compile the tool to a shared library:

c++ -shared -fPIC -ldl -O2 -DNDEBUG -std=c++17 pthread_trace.cc -o pthread_trace.so

Then, run your program with pthread_trace.so as an LD_PRELOAD:

PTHREAD_TRACE_PATH=trace.proto LD_PRELOAD=$(pwd)/pthread_trace.so <pthread using program> <program arguments...>

Now, navigate to ui.perfetto.dev, and load the resulting trace.proto file.

Traced events

This tool captures the following information:

  • Time threads are blocked in pthread_mutex_lock
  • Time threads are blocked in pthread_cond_wait, pthread_cond_timedwait
  • Time threads are blocked in pthread_once
  • Time threads are blocked in pthread_join
  • Time yielded via sched_yield, sleep, usleep, nanosleep
  • Time threads hold a mutex (i.e. time between pthread_mutex_lock or a successful pthread_mutex_trylock and pthread_mutex_unlock)
  • pthread_cond_broadcast and pthread_cond_signal calls

Examples

Thread pool end to end

This trace is of a program that starts a simple thread pool, runs a very small group of extremely fine grained tasks, and then shuts down:

Example trace

Dequeuing tasks in a thread pool

This trace is of a more realistic workload, showing threads completing tasks (the empty spaces in traces generated by pthread_trace) and dequeuing the next tasks to work on:

Less contended example

std::lock of 4 mutexes

Here is a look at std::lock's deadlock avoidance algorithm in action:

std::lock of 4 mutexes

How it works

This works by providing wrapper implementations of the various pthreads APIs that add tracing logic. Traces are accumulated in a thread-local buffer before being flushed to the trace file. The trace file is a circular buffer of blocks. When the trace file fills up, the oldest blocks will be overwritten by newer blocks. The size of the trace file circular buffer can be controlled with the PTHREAD_TRACE_BUFFER_SIZE_KB environment variable.

Tracing overhead

While this project makes some effort to reduce tracing overhead, even very lightweight tracing will be significant compared to uncontended thread synchronization primitives. It is possible (likely even) that enabling this tracing tool will alter the behavior of programs using pthreads. It might only slow the program down, but it could dramatically alter the behavior of synchronization primitives.