Lars, Please give us an update from time to time on what you find, the kernels you are using, etc.
Thanks, and look forward to seeing this work... EBo -- On Thu, 16 Sep 2010 07:46:41 +0200, Lars Segerlund <[email protected]> wrote: > Hi again, > > I take 'there are still some quirks in the system' as 'I don't know' > .... as I said I will set up a box and check the latencies, there have > been a lot of work on RT-Preempt, and I would say that the MP problem > is solved and most work now is for device drivers and filesystems, ( > locking in the filesystems are a bugger ), also ioctls are fixed. > With the spinlocks for MP systems we actually have the situation > where MP might be beneficial for a RT workload, and it should be > possible to use cgoroups for 'quality of service' ie. reserving system > capacity for the non realtime parts of EMC on a multitasking system. > > There should not be any worse latencies than the sum of the hardware > latency, and the cost of interrupt handling and a context switch, at > least when all of the ongoing RT-Preempt work is done. > > If we compare this with RT-Linux / RTAI / Xenomai ... the main > difference is the context switch associated with the interrupt, and on > a 200 MHz Pentium the context switch time for a task with its memory > locked in RAM ( and stack ) is less than 10 us, from modern hardware I > do expect it to be slightly smaller. > So in time I 'think' we should se jitter comparable to RTAI/RT-Linux > and a worst case time which gets really close. > > For now I think the best procedure is to measure the jitter and worst > case. > > Anyhow, I am not really interested in relative merits since RTAI will > always get lower jitter/latency hanging off an interrupt, I want to > increase the availability of EMC to a large number of people ( me and > the voices in my head ). > > So I think RT-Preempt is good enough for some cases, and will get to > the point where it's god for all cases later, right now there is > indeed some things to look out for but IMHO it should at least > theoreticly be able to get a lot better. > > I looked some more, and it seems if there were a rtapi-preempt , it > could be used for 'NON realtime' systems instead of the stub that is > now used, thus it could be run on a vanilla kernel. ( for demo/testing > purposes ) > > SO thanks for all the help and warnings, I think I should go and have > a look at the code and see if I can get it running on RT-Preempt, > wether it's good enough remains to be seen. > > / regards, Lars Segerlund. > > 2010/9/16 Jon Elson <[email protected]>: >> Michael Büsch wrote: >>> I got it below 20 microseconds when I last tested it with the patches I >>> posted. >>> However, there were still some quirks as in certain actions in the >>> system >>> induced huge latency spikes. However, if one doesn't do those >>> activities, >>> >> 20 us is not bad, but would likely not be acceptable for many stepgen >> users. >> It might be OK for servo cards, however. But, can you TRUST that those >> big latency spikes will NEVER happen when using the system for CNC >> control? >> That is a real worry. >> >> Jon >> >> ------------------------------------------------------------------------------ >> Start uncovering the many advantages of virtual appliances >> and start using them to simplify application deployment and >> accelerate your shift to cloud computing. >> http://p.sf.net/sfu/novell-sfdev2dev >> _______________________________________________ >> Emc-developers mailing list >> [email protected] >> https://lists.sourceforge.net/lists/listinfo/emc-developers >> > > ------------------------------------------------------------------------------ > Start uncovering the many advantages of virtual appliances > and start using them to simplify application deployment and > accelerate your shift to cloud computing. > http://p.sf.net/sfu/novell-sfdev2dev > _______________________________________________ > Emc-developers mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-developers ------------------------------------------------------------------------------ Start uncovering the many advantages of virtual appliances and start using them to simplify application deployment and accelerate your shift to cloud computing. http://p.sf.net/sfu/novell-sfdev2dev _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
