4 ms·

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.

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.