5 ms·

Sorry if this is incredibly ignorant, but I can't believe flight control systems are running Linux? Do these systems not have hard real-time requirements about

Sorry if this is incredibly ignorant, but I can't believe flight control systems are running Linux? Do these systems not have hard real-time requirements about the execution time and periodicity of tasks which can't be guaranteed by the time-sharing scheduling algorithms in Linux?

komaromy 11y ago
There are several real-time Linux variants.
- jacquesm 11y ago
  There are - to my knowledge, feel free to correct me - no real time linux versions (or even any version of linux) that are currently certified for avionics (DO-178B certification is required for that, there are multiple levels and I don't know of any linux distro (or just the kernel) with that certification).
  - komaromy 11y ago
    Avionics certifications are well past the extent of my knowledge and I will gladly accept your point. I was generally addressing the idea that the need for preemptive scheduling precludes the use of a Linux-like kernel.
    - anderspitman 11y ago
      I've never heard of anyone getting Linux running under hard real time constraints. You can get it pretty good (excellent for real time audio, for example), but can never be 100% sure you're going to meet the deadline. The people I've talked to who tried said by the time you strip out enough of the kernel to approach hard real time, you've lost enough of the advantage of using Linux that you may as well switch to an RTOS.
      - jacquesm 11y ago
        Real time audio only if you use a large enough buffer and a hardware component to clock the data out. It's all about the latency guarantees and that means that you're going to have to inspect each and every path through the code for length. With the complexity of the linux kernel that's a pretty tough job and I suspect that anything lower than a few hundred milliseconds (guaranteed!) is out of the question. I built a little controller using linux that required hard real time during a long (multiple minutes) of operation and the way I hacked it was to simply disable all interrupts and recover the various drivers as good as possible once that phase was over. It worked well but was mostly deaf to input during that time except for polling one 'stop' switch which would cause the machinery to coast down to a halt after which interrupts would be enabled. Good enough for tinkering but I certainly would not bet anything in production on that strategy. Real time is hard, soft real time is hard enough (without guarantees but with a best effort and a very large fraction of the deadlines satisfied), hard real time (no misses at all, guaranteed) is hard for a kernel of any complexity.
jacquesm 11y ago
Real time systems will be running a RTOS: VxWorks or QnX or something equivalent to that. They'll definitely build a prototype using Linux but they won't get that certified so it literally 'won't fly', it's just a means to speed up initial development.
- Kliment 11y ago
  Is the order of magnitude of lines of code in QNX different from that of linux? At a first approximation, I don't see why it would be.
  - jacquesm 11y ago
    The QnX kernel is very small compared to the Linux kernel. Small enough that I could-reimplement it in approximately 3500 lines of code + another 850 for the virtual memory management.
    - Kliment 11y ago
      Wow, I had no idea. Since their source is closed and untouchable I had no way to check either. Is there any reason there aren't several certified open RTOSes around?
      - jacquesm 11y ago
        I don't know if there aren't any open certified RTOS's around, but I can explain the 'why' part easily: if you pay for the certification of an open RTOS then everybody that can use one will say 'thank you' for the effort and that's that, since the certification would apply to any and all copies of that particular version. So you're essentially paying for the privilege of cutting your competitors a break. This could only work if the entity paying for the certification had a way of making that money back somehow and I don't see how that could be done.
        walterbell 11y ago

komaromy 11y ago

There are several real-time Linux variants.

jacquesm 11y ago
There are - to my knowledge, feel free to correct me - no real time linux versions (or even any version of linux) that are currently certified for avionics (DO-178B certification is required for that, there are multiple levels and I don't know of any linux distro (or just the kernel) with that certification).
- komaromy 11y ago
  Avionics certifications are well past the extent of my knowledge and I will gladly accept your point. I was generally addressing the idea that the need for preemptive scheduling precludes the use of a Linux-like kernel.
  - anderspitman 11y ago
    I've never heard of anyone getting Linux running under hard real time constraints. You can get it pretty good (excellent for real time audio, for example), but can never be 100% sure you're going to meet the deadline. The people I've talked to who tried said by the time you strip out enough of the kernel to approach hard real time, you've lost enough of the advantage of using Linux that you may as well switch to an RTOS.
    - jacquesm 11y ago
      Real time audio only if you use a large enough buffer and a hardware component to clock the data out. It's all about the latency guarantees and that means that you're going to have to inspect each and every path through the code for length. With the complexity of the linux kernel that's a pretty tough job and I suspect that anything lower than a few hundred milliseconds (guaranteed!) is out of the question. I built a little controller using linux that required hard real time during a long (multiple minutes) of operation and the way I hacked it was to simply disable all interrupts and recover the various drivers as good as possible once that phase was over. It worked well but was mostly deaf to input during that time except for polling one 'stop' switch which would cause the machinery to coast down to a halt after which interrupts would be enabled. Good enough for tinkering but I certainly would not bet anything in production on that strategy. Real time is hard, soft real time is hard enough (without guarantees but with a best effort and a very large fraction of the deadlines satisfied), hard real time (no misses at all, guaranteed) is hard for a kernel of any complexity.

jacquesm 11y ago

Real time systems will be running a RTOS: VxWorks or QnX or something equivalent to that. They'll definitely build a prototype using Linux but they won't get that certified so it literally 'won't fly', it's just a means to speed up initial development.

Kliment 11y ago
Is the order of magnitude of lines of code in QNX different from that of linux? At a first approximation, I don't see why it would be.
- jacquesm 11y ago
  The QnX kernel is very small compared to the Linux kernel. Small enough that I could-reimplement it in approximately 3500 lines of code + another 850 for the virtual memory management.
  - Kliment 11y ago
    Wow, I had no idea. Since their source is closed and untouchable I had no way to check either. Is there any reason there aren't several certified open RTOSes around?
    - jacquesm 11y ago
      I don't know if there aren't any open certified RTOS's around, but I can explain the 'why' part easily: if you pay for the certification of an open RTOS then everybody that can use one will say 'thank you' for the effort and that's that, since the certification would apply to any and all copies of that particular version. So you're essentially paying for the privilege of cutting your competitors a break. This could only work if the entity paying for the certification had a way of making that money back somehow and I don't see how that could be done.
      - walterbell 11y ago