Date: Thu, 10 Jul 2003 15:55:13 +0200 (CEST) Subject: IDE-HDD accesses disturbs realtime timing (RTNET) Hi everybody, my setup is a P4 PC (2.4 GHz) with an IDE HDD, I disabled DMA for the IDE HDD. On the HDD I use the reiserfs filesystem, no swap partition is in use. I have an Ethernet (Intel eepro100) adapter plugged in the PC, this is used together with rtnet (realtime ethernet add on to RTAI). Kernel: 2.4.20, rtai-24.1.11, rthal-5g, rtnet-0.2.9. I use the ethernet to transfer data in a 10 millisecond cycle to a remote device. My remote device is configured to change a voltage on an output pin, the value of this voltage is transfered within the ethernet message. When sending two alternating values, the result is a rectangle waveform that can be measured with an oscilloscope. It works fine (stable rectangle, nearly no jitter) until an HDD access occurs. This leads to a terrible jitter (up to 40 milliseconds) on the rectangle signal. The HDD is used for logging, I cannot disable it for the time the system is running. It looks like there is a conflict on the PCI bus. Is this possible? Can usage of DMA for the IDE improve the behaviour? The realtime driver of the eepro100 from rtnet itself uses DMA - will there be a conflict? The reiserfs does regular accesses to the HDD (about every 5 seconds) can this be stopped somehow? Any ideas on this problem are highly welcome! Thanks for any help Mathias
> For any who may be interested, I found the culprit. > I started to disable all IO peripherals in BIOS that I was > not using and found that the On Chip USB and USB Legacy Support were the > cause. Once these were disabled, I get 15usec max latency. I have seen this problem with other RTOS systems on other SBCs. It is usually the USB Legacy Support that causes the problem. Periodic NMIs of 300 microseconds! Eeek!.