On 04/10/2017 09:45 AM, Valerio Bellizzomi wrote: > On Mon, 2017-04-10 at 16:30 +0100, andy pugh wrote: >> On 10 April 2017 at 15:52, Valerio Bellizzomi <vale...@selnet.org> wrote: >>> Running the servo jitter test program shows a value of around 6000-7000, >>> but the default is 1000000 ! >> >> That's because a 1mS servo thread is about right for most machines. >> Just about the only time that you would need to go faster in the base >> thread is if you are doing brushless motor commutation in software. >> > > Yes the latency-test, it shows a figure of around 6000-7000 but when I > use that in the config, linuxcnc doesn't start, and the dmesg shows a > consistent number of messages related to a cpu being stuck
The jitter that latency-test determines is *not* the number you should use for your servo-thread period. Jitter is the error in scheduling time: the difference between when the thread asked to be woken up, and when it actually was woken up. The servo-thread period is *how often* the servo thread wants to be woken up, to perform the next increment of controlling the machine. Jitter is usually on the order of a few microseconds to a few tens of microseconds, and is determined by your hardware. The servo-thread period is usually one millisecond, but sometimes in special circumstances a few times less than that, and is chosen by the machine integrator (the person building and configuring the machine). So if pncconf suggested a 1 ms servo-thread period, it did the right thing. If you try to run with a servo-thread period of 7,000 nanoseconds you'll have a bad time, because the servo thread generally takes longer than that to run. So the servo-thread will be unhappy because it doesn't get to run as often as you told it to, and the rest of the system will be unhappy because there won't be any time for anything *other* than the servo-thread. It's that "no time for anything else" part that caused the hang when you start LinuxCNC. You can see how long the servo-thread takes to do its job by starting LinuxCNC, then running (in a terminal window): halcmd show thread The "Max-Time" is the longest the servo-thread has taken to run (so far, it might be more in the future, depending on what it's doing). If you have a servo-thread period that's shorter than that max-time things aren't going to work well, and if the period is *close* to the thread's max-time there won't be much CPU left over for doing other things (like running your GUI and the rest of your Linux system). -- Sebastian Kuzminsky ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
