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. Clark
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