Real-Time Factor

The ratio of simulated time to real elapsed time, measuring whether a simulation runs faster or slower than reality.

Real-Time Factor = time advanced inside the simulation ÷ time elapsed in the real world. RTF = 1 means the simulation runs at the same speed as reality; above 1 is faster than real time, so RTF = 10 means 10 seconds of simulation happen in 1 real second; below 1 is slower, common when a scene is complex, has many sensors, or has heavy rendering. Gazebo's interface shows RTF live at the bottom of the screen; in Gazebo Classic, the step size max_step_size multiplied by the update rate real_time_update_rate gives RTF's upper bound, and setting the update rate to 0 runs the simulation as fast as possible. RTF matters greatly for reinforcement learning, which needs huge amounts of interaction — the higher the RTF, the more experience can be collected per wall-clock hour; GPU-parallel simulators instead usually report throughput as steps per second summed across all environments. Conversely, hardware- and software-in-the-loop testing must stay synchronized with a real controller, so RTF is locked to 1. Note that RTF in speech recognition is defined the opposite way (processing time ÷ audio duration, smaller is faster).

ExampleIn ETH's 2019 ANYmal locomotion work, the simulator advanced nearly 500,000 timesteps per second, roughly a thousand times faster than real time (RTF ≈ 1000), letting the whole training finish on one computer in under eleven hours.

Also called

RTF, Simulation Speed Multiplier

Related

Simulator · Simulation Timestep · Simulation Throughput (Steps / Frames Per Second) · GPU-Accelerated Parallel Simulation · Hardware-in-the-Loop / Software-in-the-Loop Simulation · Gazebo

Sources

Gazebo Classic 教程：Physics Parameters（real_time_factor） (Chinese)
Gazebo 文档：Understanding the GUI（RTF 显示） (Chinese)
Learning agile and dynamic motor skills for legged robots (arXiv 1901.08652)