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PX4 Lockstep Mode#

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 that they use the same clock time. This makes PX4 behave normally even during unusually long delays in simulator response due to performance lags.

Recommendation

When you are running a lockstep enabled version of PX4 in software-in-the-loop (SITL) mode, set AutonomySim to use a SteppableClock and set UseTcp and LockStep to true.

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

This causes AutonomySim to not use a real-time clock, but instead to advance the clock in-step with each sensor update sent to PX4. This way, PX4 perceives time is progressing smoothly no matter how long it takes AutonomySim to process the update loop.

This has the following advantages:

  • AutonomySim can be run on slow machines that cannot process updates quickly.
  • You can debug AutonomySim, hit a breakpoint, and when you resume, PX4 will behave normally.
  • You can enable very slow sensors such as LiDAR with a large number of simulated points and PX4 will still behave normally.

However, the lockstep feature does have side effects. These include slower update loops caused by running AutonomySim on an underpowered machine or by expensive sensors (e.g., LiDAR), which can create visible jerkiness in the simulated flight, if you view the on-screen updates in real-time.

Disabling Lockstep Mode#

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 AutonomySim 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 AutonomySim.