-----Oprindelig meddelelse----- Fra: linux-rt-users-owner@xxxxxxxxxxxxxxx [mailto:linux-rt-users-owner@xxxxxxxxxxxxxxx] På vegne af Wolfgang Wallner Sendt: 26. august 2013 14:29 Til: linux-rt-users@xxxxxxxxxxxxxxx Emne: Configuration of real-time tasks in PREEMPT_RT kernels > 2.6.33 > >Dear RT community, > >I have a question regarding the configuration of real-time tasks in PREEMPT _RT kernels > 2.6.33. >Up to now, I only have experience with 2.6.33-rt. > >I use the openPOWERLINK[1] real-time ethernet stack, and have it running on an x86-box with Ubuntu 13.04. >I try to use it with two different kernel versions: >* 2.6.33.7.2-rt30 >* 3.2.48-rt69 > >When I use kernel version 2.6.33, I get very good results (jitter of ethernet frames on the network < +/-10us). >But when I use kernel version 3.2, the results are much worse. The jitter on the network increases up to +/-50us. > >In my configuration, the openPOWERLINK stack consists of a kernel module (data link layer) and a user space application. > >I know that in 2.6.33 there were splitted softirqs, and that it was possible to configure each thread with its own priority. >What are the steps that need to be done in 3.x to achieve similar results as in 2.6.33? > >kind regards, >Wolfgang Wallner > >[1] http://sourceforge.net/projects/openpowerlink/ > >PS: Running cyclictest (e.g. sudo cyclictest -p 99 -n -i 500) on both kernels shows very similar results (avg latency: 15us, max latency 35us). Hi Wolfgang, >From the openPOWERLINK documentation (linux-x86.txt): Kernel 3.x The behaviour of a 3.X real-time kernel changed. Splitted softirq threads are no longer available. To get a good real-time behaviour the following steps have to be made: 1. A multicore processor is required for these optimizations! 2. Ensure that the following configuration options are set for your real-time kernel: CONFIG_RT_GROUP_SCHED is not set CONFIG_RCU_BOOST=y CONFIG_RCU_BOOST_PRIO=99 3. Isolate the second core of the multi-core processor to be used exclusively for openPOWERLINK by specifying the kernel-commandline parameter isolcpus=1 4. Set the default interrupt affinity to all other cores, e.g. for a dual-core system to core 0 by setting the kernel-commandline parameter irqaffinity=0 5. Disable IRQ balancing by disable irqbalance. This depends on your Linux distribution. For example in Ubuntu edit /etc/default/irqbalance So to get similar performance on 3.x, it seems that you'll need to assign a core solely for the openPOWERLINK stack. I've wondered a bit about whether there might be a different solution myself, but hadn't had the time to investigate much further. Best regards, Simon Falsig -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
