While testing our maximum SSerial update rates, (linuxCNC 2.5.1 Ubuntu 10.4) I noticed something somewhat interesting/disturbing about latency test on the servo thread on my Atom 330 based motherboard (Jetway). After disabling hyperthreading in the BIOS and adding the isolcpus=1 line to grub.cfg, the latency test gets consistant ~6 usec max latencies. This sounds really good but I was unable to run faster than 2 KHz servo threads without occasional real time errors.
To check what was going on I toggled an I/O bit at the servo thread rate and looked at it on the oscilloscope, and the latency on the servo thread is much worse than 6 usec, occasionally getting above 100 usec! . It gets noticable worse when the screen is re-drawn. I do not know why the latency test does not "see" this. This problem seems specific to the Atom MBs and explains why others have found it impossible to run faster servo threads than about 1500 Hz reliably on the Atom MBs (the newer D2500/2700/2800 may be different) After this test I moved the hard drive to a Intel D41 core duo min ITX motherboard (which has a latency number of about 16 usec for the servo thread) and did the same toggling bit test and found only about 20 usec of jitter with the 'scope. This motherboard will run with 6 KHz servo threads with lots of Sserial I/O with no complaints so it seems this may be a Atom MB specific issue. Peter Wallace Mesa Electronics ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-developers mailing list Emc-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-developers
