From: "robert.berger" <[email protected]> Reply-To: <[email protected]> Date: Sunday, February 23, 2014 at 3:26 AM To: <[email protected]> Cc: <[email protected]> Subject: [beagleboard] Re: BBB + PREEMPT_RT
> Hi, > > On Saturday, February 22, 2014 2:17:24 PM UTC+2, [email protected] wrote: >> Just a few thoughts ... >> >> It is not possible to have a fully deterministic real-time operating system >> on a processor that uses instruction/data caches. ie you have to turn off the >> cacheing to achieve determinism and eliminate performance jitter (which then >> degrades the average performance). > > Yep, but, that's the easy part. How about pipelines and instructions > reordering done by the compiler and the processor? How about interrupts? How > about multi-cores? How about the drift of the crystal you use as the clock > source of your CPU? You might be shocked now, but as you can see it's > impossible to have a hard real-time system with state of the art (multi-core) > processors. Is it? I think that you need to come up with a realistic test > suite to see if preempt-rt (with or without CPU isolation) is good enough, or > if you need Xenomai (still you will see issues if Xenomai and Linux use the > same caches), or some dedicated hardware like PRU. There is also some > interesting work by Jan Kiszka - not yet on ARM.[1] I think there is some confusion about what real-time really means. It doesn¹t mean fast or even consistent, it just means that it will respond to some event in a required time. If your requirement is that something respond in 1 second, then Linux kernel is good for real-time. If you want a response of less than 1ms, then the Linux interrupt latency may not meet this requirement. Remember, latency tests are conducted when the processor is under load. Xenomai running on the BBB can achieve 50uS interrupt latency whereas preempt-rt is more like 200uS. Regards, John > > > Regards, > > Robert > > [1] http://lwn.net/Articles/574273/ > > Shameless self promotion: > > [2] http://www.reliableembeddedsystems.com/pdfs/2010_03_04_rt_linux.pdf > [3] > http://www.embedded.com/design/operating-systems/4204740/Getting-real--time--a > bout-embedded-GNU-Linux > > -- > For more options, visit http://beagleboard.org/discuss > --- > You received this message because you are subscribed to the Google Groups > "BeagleBoard" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/groups/opt_out. -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/groups/opt_out.
