C++ Behavior Tree benchmarks

A Behavior Tree is an hierarchical abstraction utilized in robotics and AI (See the Wikipedia page for more information).

This repository aims to benchmark three behavior tree implementations:

• Inheritance-based (OOP-style), using std::unique_ptr to derived classes
• std::function-based
• std::tuple-based

The first two use heap allocations, the second one can be entirely allocated on the stack.

The design

All tree implementations followed similar design:

• The control flow is synchronous and the tree traversal is recursive
• Each tree implementation contains the Conditional, Action, Sequence, Fallback and Parallel nodes and provides support for custom decorators
• The operator () was utilized for the tick and the operator returns bt::Status
• The heap allocated trees utilize std::vector for storage
  • Although this means the trees can be modified at runtime, support for modification was not implemented

The benchmark

The benchmark utilized a simple tree:

- Fallback
  - Conditional: returns a random boolean
  - Sequence
    - Action: always returns Success
    - Action: increments a counter and returns Success

A "default implementation" was also provided, where the behavior tree is reduced to if(!random_boolean()) count++;, in order to compare the overhead of each implementation.

The bench.cpp file contains the benchmark code and can be compiled with Google Benchmark, using the provided CMake file.

Results

GCC 8, Ubuntu 18.04 (WSL1)

-----------------------------------------------------------------
Benchmark                       Time             CPU   Iterations
-----------------------------------------------------------------
oop_style                    9.05 ns         8.79 ns     74666667
functions                    13.4 ns         13.5 ns     49777778
tuples                       1.44 ns         1.41 ns    497777778
default_implementation       1.41 ns         1.41 ns    497777778

Clang 8, Ubuntu 18.04 (WSL1)

-----------------------------------------------------------------
Benchmark                       Time             CPU   Iterations
-----------------------------------------------------------------
oop_style                    13.1 ns         12.8 ns     56000000
functions                    11.6 ns         11.5 ns     64000000
tuples                       1.64 ns         1.61 ns    407272727
default_implementation       1.60 ns         1.60 ns    448000000

Clang 8, Windows 10, x64

-----------------------------------------------------------------
Benchmark                       Time             CPU   Iterations
-----------------------------------------------------------------
oop_style                    12.7 ns         12.2 ns     49777778
functions                    13.1 ns         13.1 ns     56000000
tuples                       1.71 ns         1.69 ns    407272727
default_implementation       1.70 ns         1.69 ns    407272727

MSVC 2017, Windows 10, x64

-----------------------------------------------------------------
Benchmark                       Time             CPU   Iterations
-----------------------------------------------------------------
oop_style                    15.3 ns         15.0 ns     44800000
functions                    14.2 ns         14.2 ns     56000000
tuples                       3.38 ns         3.30 ns    213333333
default_implementation       3.47 ns         3.38 ns    203636364

Observations

• The timing differences were too small between certain implementations to guarantee perfect results, e.g. the difference between tuple and default implementations varied between runs, both negatively and positively.
  • A better test tree must be created for the next iteration of the benchmark.
• GCC (Linux) had the best results for the inheritance-based tests and differed from Clang on the same platform.
• Clang's results on Linux for the std::function were really interesting. Faster than libstdc++'s implementation, even though the vtable performance from the inheritance test was worse than GCC's.
• MSVC's results were the slowest all-around, although I haven't tested using the most recent version of the compiler and the standard library to be certain.
• WSL is awesome! If you develop cross-platform applications on Windows, it's definitely worth a try.
• Using zero-overhead abstractions in C++ still reigns, regardless of compiler or Standard Library implementation. If you don't need something, e.g. reconfigurability in this application, don't pay for it!

(un) License

The code in this repository is public domain, released under The Unlicense.

Feel free to use any standalone Behavior Tree implementation in your applications or as a base for your own implementations!