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? Thanx, Paul - 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
