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. 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
