4 ms·

Garbage collection is the least of your problems in that case. You can't use a standard OS for that kind of scenario either, Linux has far bigger jitter than th

Garbage collection is the least of your problems in that case. You can't use a standard OS for that kind of scenario either, Linux has far bigger jitter than that. And if you have a hard real-time system, you could have a hard real-time garbage collector too. Garbage collection gets a bad rep from garbage collectors that are tuned for throughput rather than latency, but there is no lower limit to latency for garbage collection and there are GCs out there that prove it (like Azul). If you can afford dynamic memory allocation, you can afford garbage collection. And if you can't afford it, you can just preallocate and turn off the garbage collector.

lmilcin 5y ago
> You can't use a standard OS for that kind of scenario either, Linux has far bigger jitter than that. Bullshit. We were using Linux and it was just fine. But I agree, it requires more knowledge than your run of the mill software dev will ever have.
- saagarjha 5y ago
  Be kind; don’t be snarky. You’re here to share your knowledge, not sneer down at people who bring up fairly reasonable questions.
  - FpUser 5y ago
    >"not sneer down at people who bring up fairly reasonable questions" Downvoting reasonable posts that do not fit somebody's point of view without providing any constructive explanations is of course way better.
    - lmilcin 5y ago
      I don't care about downvotes. If somebody is here for votes then they should reassess their life's choices.
      - FpUser 5y ago
        I do not care either. Real choices are made in real life. We are just talking here.
  - lmilcin 5y ago
    The person put a blank statement that the Linux is unusable for the task (without giving any more information) and I responded that it is usable because we are using it for it. As to "run of the mill software dev" this is not a snarky comment, this is just reality. Most devs have absolutely no idea how to configure Linux at all, other than to make their application work. Not to mention configure for real time operation. So I stand by my comment that comes from over two decades of experience in the field.
    - saagarjha 5y ago
      "Linux" as most people know it is unusable for this kind of task. They are correct that the standard scheduler will not really let you run anything that needs to run in real time. Your response to that is literally just "no, we did it, and you're just to dumb to understand". Imagine a third grader and and they told you that you can't take the square root of a negative number. Of course you know that imaginary numbers exist, but you'd be a jerk if you just replied to them with "you can actually but I guess some people just don't know how to do it". Hacker News is a site that values intellectual curiosity, not snark: you should have replied with an explanation of what you are actually doing to get that kind of thing out of the Linux kernel. What value is your experience in the field to this community if you refuse to share it with others? In this case my analogy is even stronger because you could argue that explaining imaginary numbers to a third grader may be difficult, but "I set CONFIG_whatever to this value and set the kernel to never schedule my application off this core" is exactly the kind of response that would be expected in this context.
- stonemetal12 5y ago
  What distro or patch set did you use? According to wikipedia, the best RTLinux could do was within 15 microseconds on interrupt handlers, and within 35 microseconds of its scheduled time for processes. It is my understanding that the other real time linux attempts ended up in similar territory, good but not sub 5 microsecond good.
rigtorp 5y ago
You're incorrect, garbage collection would be the biggest problem for that use case. You have to be really careful even with your C/C++ code, warming up the branch predictor between packets etc, see my article for some pitfalls: https://rigtorp.se/virtual-memory/ https://rigtorp.se/virtual-memory/
erichocean 5y ago
> You can't use a standard OS for that kind of scenario either, Linux has far bigger jitter than that. This is 100% false. It's easy to configure Linux itself to only occupy 1 or 2 cores (one real + the hyper-thread for that real core), and then pin your own application threads to the other cores where no other code will run. This setup has identical application performance (other than not having those two Linux cores) to having no OS at all. 100% predictable, ZERO jitter application code (including user-space networking). Except…you can still SSH in to your "no OS" Linux box, you have a file system, can run cron jobs, git, gdb, WireGuard, etc. So it's much, much better in practice and what actual high-performance network developers actually do. Essentially no one has run a literal "no OS" box for at least a decade; everyone who is running on normal hardware does what I just described with Linux because the cost is dirt cheap and it's way too convenient.

lmilcin 5y ago

> You can't use a standard OS for that kind of scenario either, Linux has far bigger jitter than that. Bullshit. We were using Linux and it was just fine. But I agree, it requires more knowledge than your run of the mill software dev will ever have.

saagarjha 5y ago
Be kind; don’t be snarky. You’re here to share your knowledge, not sneer down at people who bring up fairly reasonable questions.
- FpUser 5y ago
  >"not sneer down at people who bring up fairly reasonable questions" Downvoting reasonable posts that do not fit somebody's point of view without providing any constructive explanations is of course way better.
  - lmilcin 5y ago
    I don't care about downvotes. If somebody is here for votes then they should reassess their life's choices.
    - FpUser 5y ago
      I do not care either. Real choices are made in real life. We are just talking here.
- lmilcin 5y ago
  The person put a blank statement that the Linux is unusable for the task (without giving any more information) and I responded that it is usable because we are using it for it. As to "run of the mill software dev" this is not a snarky comment, this is just reality. Most devs have absolutely no idea how to configure Linux at all, other than to make their application work. Not to mention configure for real time operation. So I stand by my comment that comes from over two decades of experience in the field.
  - saagarjha 5y ago
    "Linux" as most people know it is unusable for this kind of task. They are correct that the standard scheduler will not really let you run anything that needs to run in real time. Your response to that is literally just "no, we did it, and you're just to dumb to understand". Imagine a third grader and and they told you that you can't take the square root of a negative number. Of course you know that imaginary numbers exist, but you'd be a jerk if you just replied to them with "you can actually but I guess some people just don't know how to do it". Hacker News is a site that values intellectual curiosity, not snark: you should have replied with an explanation of what you are actually doing to get that kind of thing out of the Linux kernel. What value is your experience in the field to this community if you refuse to share it with others? In this case my analogy is even stronger because you could argue that explaining imaginary numbers to a third grader may be difficult, but "I set CONFIG_whatever to this value and set the kernel to never schedule my application off this core" is exactly the kind of response that would be expected in this context.
stonemetal12 5y ago
What distro or patch set did you use? According to wikipedia, the best RTLinux could do was within 15 microseconds on interrupt handlers, and within 35 microseconds of its scheduled time for processes. It is my understanding that the other real time linux attempts ended up in similar territory, good but not sub 5 microsecond good.

rigtorp 5y ago

You're incorrect, garbage collection would be the biggest problem for that use case. You have to be really careful even with your C/C++ code, warming up the branch predictor between packets etc, see my article for some pitfalls: https://rigtorp.se/virtual-memory/ https://rigtorp.se/virtual-memory/

erichocean 5y ago

> You can't use a standard OS for that kind of scenario either, Linux has far bigger jitter than that. This is 100% false. It's easy to configure Linux itself to only occupy 1 or 2 cores (one real + the hyper-thread for that real core), and then pin your own application threads to the other cores where no other code will run. This setup has identical application performance (other than not having those two Linux cores) to having no OS at all. 100% predictable, ZERO jitter application code (including user-space networking). Except…you can still SSH in to your "no OS" Linux box, you have a file system, can run cron jobs, git, gdb, WireGuard, etc. So it's much, much better in practice and what actual high-performance network developers actually do. Essentially no one has run a literal "no OS" box for at least a decade; everyone who is running on normal hardware does what I just described with Linux because the cost is dirt cheap and it's way too convenient.