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The flight software team did a reddit AMA a while back [0]. They use C++ for writing the flight software (what flies falcon, dragon, etc) running on a customize

by elteto 8y ago

The flight software team did a reddit AMA a while back [0]. They use C++ for writing the flight software (what flies falcon, dragon, etc) running on a customized RT Linux. [0] https://www.reddit.com/r/IAmA/comments/1853ap/we_are_spacex_software_engineers_we_launch/ https://www.reddit.com/r/IAmA/comments/1853ap/we_are_spacex_...

monocasa 8y ago
And FreeRTOS. RTLinux is only barely soft realtime.
- avmich 8y ago
  > RTLinux is only barely soft realtime. Linux used to run as a non-privileged process in hard real-time kernel, which taken together was guaranteeing responses in nanoseconds range. If this is still the case (along with what Wikipedia states) , the whole system is by definition hard real-time.
  - solarkraft 8y ago
    But only the underlying kernel part is actually real-time, the Linux process wouldn't be, right? That leads me to the question: Shouldn't all things in a rocket be real time? Are they "post-poning" stuff like telemetry and other communications?
    - avmich 8y ago
      All critical processes can run outside of Linux, in hard real-time fashion. Linux is used for non-critical things, like networking... The whole RTLinux includes Linux (non-RT part) and a hard RT part, for which you can write your code.
  - monocasa 8y ago
    Wow, that's really silly that someone went out, made a hard realtime system using Linux as the lowest priority level, and then called it RTLinux. RTLinux for years had meant Linux with the PREEMPT_RT patches. In any case, SpaceX doesn't use Linux for hard realtime systems. > They do not have hard realtime requirements https://lwn.net/Articles/540368/ https://lwn.net/Articles/540368/ And I probably wouldn't use the RTLinux for a real hard realtime system if I were making the choice. Putting all of the realtime code in the same memory domain as the rest of the Linux kernel is a huge surface area for corruption. I'd probably use sel4 with a virtualized Linux to get the same semantics.
raptorraver 8y ago
Thanks for this! ”We’re looking to start crew trials as early as 2015.” :)

by elteto 8y ago

monocasa 8y ago

And FreeRTOS. RTLinux is only barely soft realtime.

avmich 8y ago
> RTLinux is only barely soft realtime. Linux used to run as a non-privileged process in hard real-time kernel, which taken together was guaranteeing responses in nanoseconds range. If this is still the case (along with what Wikipedia states) , the whole system is by definition hard real-time.
- solarkraft 8y ago
  But only the underlying kernel part is actually real-time, the Linux process wouldn't be, right? That leads me to the question: Shouldn't all things in a rocket be real time? Are they "post-poning" stuff like telemetry and other communications?
  - avmich 8y ago
    All critical processes can run outside of Linux, in hard real-time fashion. Linux is used for non-critical things, like networking... The whole RTLinux includes Linux (non-RT part) and a hard RT part, for which you can write your code.
- monocasa 8y ago
  Wow, that's really silly that someone went out, made a hard realtime system using Linux as the lowest priority level, and then called it RTLinux. RTLinux for years had meant Linux with the PREEMPT_RT patches. In any case, SpaceX doesn't use Linux for hard realtime systems. > They do not have hard realtime requirements https://lwn.net/Articles/540368/ https://lwn.net/Articles/540368/ And I probably wouldn't use the RTLinux for a real hard realtime system if I were making the choice. Putting all of the realtime code in the same memory domain as the rest of the Linux kernel is a huge surface area for corruption. I'd probably use sel4 with a virtualized Linux to get the same semantics.

raptorraver 8y ago

Thanks for this! ”We’re looking to start crew trials as early as 2015.” :)