Paul E. McKenney wrote: > On Wed, Jan 30, 2008 at 11:45:01AM +0100, Wolfgang Grandegger wrote: >> Paul E. McKenney wrote: >>> On Wed, Jan 30, 2008 at 09:18:49AM +0100, Wolfgang Grandegger wrote: >>>> Paul E. McKenney wrote: >>>>> On Tue, Jan 29, 2008 at 02:38:04PM +0100, Wolfgang Grandegger wrote: >>>>>> Luotao Fu wrote: >>>>>>> Hi, >>>>>>> >>>>>>> Wolfgang Grandegger wrote: >>>>>>> .......... >>>>>>>> Do you still get high latencies with: >>>>>>>> >>>>>>>> CONFIG_PREEMPT_RCU_BOOST=y >>>>>>>> CONFIG_RCU_TRACE=y >>>>>>>> CONFIG_NO_HZ is not set >>>>>>>> >>>>>>>> With this setting I have not yet realized latencies > 150us. Could you >>>>>>>> please give it a try? If I change one of the parameters above, latency >>>>>>>> increases in short time. >>>>>>> I played through some combination of the RCU options and can back your >>>>>>> observation this time: With the rcu Tracer or the priority boost turned >>>>>>> off I also could measure reliably extraordinory high latencies. If they >>>>>>> are both turned on, no high latencies could be measured. Turning on the >>>>>>> dynamic ticker however doesn't seem to cause high latencies during my >>>>>>> test runs. Seemed like an rcu issue here. >>>>>> I'm just making a long test run on my TQM5200 module with my good >>>>>> settings. After more than 4.5 hours under load, cyclictest shows a >>>>>> maximum latency of 177 us. I'm going to re-check the effect of CONFIG_NO_HZ. >>>>>> >>>>>>> Further such results only appear if the target board is booted with >>>>>>> nfsrootfs. (As I already have mentioned several times before), which >>>>>>> leads my suspection to rcu usage in nfs implementation. In this case >>>>>>> this problem might even be platformindependent. I'd have to do some >>>>>>> tests on one of our arm boards later to test this. Since there're no >>>>>>> reports like this for other architecture as powerpc till now, I doubt >>>>>>> quite if this is verifiable. >>>>>> It's also my suspicion that the high latencies are related to the RCU >>>>>> usage in the network layer, where it's heavily used. What is really >>>>>> wired is that switching off CONFIG_RCU_TRACE has a negative impact on >>>>>> the latency. As I see it, it just adds some trace points, but I might >>>>>> have missed something. >>>>> I would expect that CONFIG_RCU_TRACE=n (as in "no" rather than "module") >>>>> would have low latencies rather than high latencies. So I am quite >>>>> surprised by your result. I will dig into this more. >>>> Thanks a lot. To be clear. I need "CONFIG_PREEMPT_RCU_BOOST=y" *and* >>>> "CONFIG_RCU_TRACE=y" to achieve reasonable latencies below 180us. With >>>> CONFIG_PREEMPT_RCU_BOOST or CONFIG_RCU_TRACE not set or >>>> CONFIG_RCU_TRACE=m is rmeasure latencies up to 600us within a minute or so. >>> OK, thank you for the confirmation. >>> >>> The large latencies were from cyclictest, correct? Did other tests >>> also show these latencies? In either case, could you please send the >>> exact command line you used to run the test? >> Yes, I used "$ cyclictest -n -t1 -p80 -i1000" to measure the latency. So >> far, I have not done other tests. Any recommendation? >> As no-rt load I used "while ls; do ls /bin; done" in one telnet window >> and "while ./hackbench 10; do ./calibrator 400 32M cali; sleep 30; done" >> in another. But already "while ls; do ls /bin; done" is enough to >> trigger the high latencies quickly. Note also, that I work on a root >> files-ystem mounted via NFS resulting in a lot of network traffic and >> utilization. > > I have to ask... > > Did you see large latencies when -not- running on NFS? I have not made test without NFS, but Fu has and he said: "Further such results only appear if the target board is booted with nfsrootfs." Wolfgang. - 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
