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PREEMPT-RT Locking Infrastructure Possibly Ready For Linux 5.15

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  • #11
    Originally posted by Linuxxx View Post
    In these cases a soft-realtime Linux kernel like Ubuntu's "lowlatency" flavor is more than adequate; plus it doesn't introduce security problems as described here:
    Soft-realtime still can benefit from hard realtime features restricted to kernel space. Lowlatency mode does not mean hard real-time features are not being used at all. That ubuntu write up focuses on that the real-time patch allows you to have true hard real-time user processes this skips over that a real-time patch applied kernel does have better lowlatency behavour and the fact ubuntu cherry picks a few parts out of the real-time patch set into their default kernel.

    This locking infrastructure improvement from the PREEMPT-RT set is one of the parts that help lowlatency/soft realtime and hard realtime that had not got mainline yet.

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    • #12
      Originally posted by Linuxxx View Post
      In these cases a soft-realtime Linux kernel like Ubuntu's "lowlatency" flavor is more than adequate; plus it doesn't introduce security problems as described here:
      These Ubuntu statements are very old and you can square them away as advertisement for 'how they do things'.

      Soft real time/low latency can benefit gamers, consumer high-res audio, etc.
      Hard real time is a different beast. It is for situations where your life depends on it (like reading and processing the video camera information of an autonomous car) or also where your work depends on it (like in a sound studio when you record a high $ musician).

      Also, regarding the Ubuntu statements, it it is very difficult to completely lock a system with multicore cpu. Anyhow if you overrun the system capabilities, this tends to be the expected/preferred outcome in many hard-real-time systems as opposed to missing 'interrupts'.

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      • #13
        Originally posted by Linuxxx View Post

        In these cases a soft-realtime Linux kernel like Ubuntu's "lowlatency" flavor is more than adequate; plus it doesn't introduce security problems as described here:

        Yeah, this is non-sense. There is no reason a RT kernel must allocate all CPU to 1 process. All it must do is complete a task before a dead line. In fact, all CPU time after completing the task until the dead line is in principle available to other processes.

        Also, the mentioned security problem happens on all linux kernels when you allocate so much memory the kernel starts swapping 100% of the time. A good way to try this is gitk on the linux git repo, while you have 4GB RAM (and 8GB swap). While gitk causes git to allocate more then physical RAM, swapping time is not considered by the kernel scheduler. This will lockup even your mouse and keyboard preventing you from terminating gitk (you might be able to ssh into your machine and SIGSTOP git, this will cause git to be swapped out and other processes to receive CPU time again).

        Another fabulous example is building nodejs with > 4 threads on a machine with 16GB RAM (during linking there are 5 threads each requiring 4GB).

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        • #14
          Originally posted by Linuxxx View Post

          Just curious:
          Do you have some safety-critical hardware running in your basement or why do you exactly want a hard-realtime Linux kernel for your needs?
          Becuase I don't like random stutter on my low-latency audio.

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          • #15
            Originally posted by tildearrow View Post

            Becuase I don't like random stutter on my low-latency audio.
            Did you try pipewire with preempt-rt?

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            • #16
              Originally posted by mppix View Post
              Did you try pipewire with preempt-rt?
              This is a interesting one. A Linux kernel with the preempt-rt patch set applied but still built like a normal kernel still today has lower jitter than the mainline 5.14 kernel . It will be interesting to see if the merge of the preempt-rt locking in 5.15 changes this.

              https://gitlab.freedesktop.org/pipew...ormance-tuning
              Yes pipewire is designed to take advantage of hard realtime. It also is designed to use rtkit for setting up hard realtime that is a watchdog against the worst things a hard realtime process can do. As in you attempt to have a hard realtime application so something system destroying the rtkit watchdog will kill the application.

              The ubuntu case against hard realtime also does not allow for items like rtkit that make hard realtime fairly safe.

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              • #17
                Originally posted by wertigon View Post
                ...we want to cut our costs by using Linux, since this OS is massively popular, often known by new recruits...
                Wow, are we still talking about Linux here? In my country's enterprise/industrial sector we're still largely dependent on either Microsoft's products or some obscure ancient proprietary software from before I was born; and most new recruits only know "PC" and "Mac".

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                • #18
                  Originally posted by mppix View Post

                  Did you try pipewire with preempt-rt?
                  No, because PipeWire still is kind of incomplete.
                  It's the Wayland of audio. Better, modern and secure; but several things are missing compared to its predecessor.

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                  • #19
                    Originally posted by Linuxxx View Post

                    In these cases a soft-realtime Linux kernel like Ubuntu's "lowlatency" flavor is more than adequate; plus it doesn't introduce security problems as described here:

                    Hello, our dear Ubuntu troll...


                    How many times will you paste that Ubuntu propaganda when someone talks about RT? Please stop your boring astroturfing, you're becoming boring and annoying.

                    Please use your own words, if you can. I think you're unable to understand the topics, so you just paste text and repeat terms like a parrot

                    You're not helping. Please stop NOW.
                    Last edited by timofonic; 19 August 2021, 08:34 AM.

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                    • #20
                      Originally posted by Nocifer View Post

                      Wow, are we still talking about Linux here? In my country's enterprise/industrial sector we're still largely dependent on either Microsoft's products or some obscure ancient proprietary software from before I was born; and most new recruits only know "PC" and "Mac".
                      Yeah, that's the legacy product stack; lots of different things there, a few dead RTOSes, a couple of homegrown ones and even I think a Windows Embedded product.

                      This latest iteration we kinda got a little nudge since our supplier either wanted $$$$$ to make the new SoC platform drivers work with the old driver system, or use Linux RT. Lesser of two evils here, and now management are super happy they went this route.

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