On Wed, 21 Sep 2011, Clark Williams wrote: > While kernel.org is sorting out the security stuff, the rt-tests code > may be pulled from: > > git://github.com/clrkwllms/rt-tests.git > > Note that we're now at version 0.82. Presently I only have the git > archive available (no tarballs). > > While investigating latency spikes in the 3.0.x-rt kernels, Thomas > spotted a case where an Intel quad-core Xeon was going into deep > sleep states and were all fighting to come out of sleep at the same > time (and consequently causing a big latency spike in cyclictest). > > While trying to figure out how to prevent deep cstates I remembered a > conversation I had with Arjan at the last Plumbers conference in > Boston. He mentioned the /dev/cpu_dma_latency interface to the power > managment code and that if you opened it and wrote a zero to it, you > effectively put the system into "idle=poll" mode until you closed the > file descriptor (see: Documentation/power/pm_qos_interface.txt). > > I've added a set_latency_target() function to cyclictest that by > default opens /dev/cpu_dma_latency and writes a zero to it, then holds > the file descriptor open for the duration of the cyclictest run. This > made a *huge* difference on some Intel Xeon's. Without this option, when > I was running cyclictest with the -b option, I saw latencies over > 300us. When I added it, while tracing I never saw a latency over 30us. > Turning of -b, I never saw it go over 10us. I am doing further testing > now with other x86_64 systems. > > Of course this is very architecture specific, so YMMV, but I think it's > a valid mechanism to be used when measuring latency and I believe a > technique that many latency-sensitive applications might use to good > effect. This is not a big surprise as the kernel got more agressive going into deep C-states since 2.6.33-rt especially with "intel_idle" on modern (Nehalem+) cpus. Thanks, tglx -- 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
