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px4_lockstep.md

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LockStep

The latest version of PX4 supports a new lockstep feature when communicating with the simulator over TCP. Lockstep is an important feature because it synchronizes PX4 and the simulator so they essentially use the same clock time. This makes PX4 behave normally even during unusually long delays in Simulator performance.

It is recommended that when you are running a lockstep enabled version of PX4 in SITL mode that you tell AirSim to use a SteppableClock, and set UseTcp to true and LockStep to true.

    {
        "SettingsVersion": 1.2,
        "SimMode": "Multirotor",
        "ClockType": "SteppableClock",
        "Vehicles": {
            "PX4": {
                "VehicleType": "PX4Multirotor",
                "UseTcp": true,
                "LockStep": true,
                ...

This causes AirSim to not use a "realtime" clock, but instead it advances the clock in step which each sensor update sent to PX4. This way PX4 thinks time is progressing smoothly no matter how long it takes AirSim to really process that update loop.

This has the following advantages:

  • AirSim can be used on slow machines that cannot process updates quickly.
  • You can debug AirSim and hit a breakpoint, and when you resume PX4 will behave normally.
  • You can enable very slow sensors like the Lidar with large number of simulated points, and PX4 will still behave normally.

There will be some side effects to lockstep, namely, slower update loops caused by running AirSim on an underpowered machine or from expensive sensors (like Lidar) will create some visible jerkiness in the simulated flight if you look at the updates on screen in realtime.

Disabling LockStep

If you are running PX4 in cygwin, there is an open issue with lockstep. PX4 is configured to use lockstep by default. To disable this feature, first disable it in PX4:

  1. Navigate to boards/px4/sitl/ in your local PX4 repository
  2. Edit default.cmake and find the following line: 
    set(ENABLE_LOCKSTEP_SCHEDULER yes)
  3. Change this line to: 
    set(ENABLE_LOCKSTEP_SCHEDULER no)
  4. Disable it in AirSim by setting LockStep to false and either removing any "ClockType": "SteppableClock" setting or resetting ClockType back to default: 
        {
        ...
        "ClockType": "",
        "Vehicles": {
            "PX4": {
                "VehicleType": "PX4Multirotor",
                "LockStep": false,
                ...
  5. Now you can run PX4 SITL as you normally would (make px4_sitl_default none_iris) and it will use the host system time without waiting on AirSim.