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turbostat uses /dev/cpu/*/msr interface to read MSRs. For modern systems, it reads 10 MSR/CPU. This can be observed as 10 "Function Call Interrupts" per CPU per sample added to /proc/interrupts. This overhead is measurable on large idle systems, and as Yoquan Song pointed out, it can even trick cpuidle into thinking the system is busy. Here turbostat re-schedules itself in-turn to each CPU so that its MSR reads will always be local. This replaces the 10 "Function Call Interrupts" with a single "Rescheduling interrupt" per sample per CPU. On an idle 32-CPU system, this shifts some residency from the shallow c1 state to the deeper c7 state: # ./turbostat.old -s %c0 GHz TSC %c1 %c3 %c6 %c7 %pc2 %pc3 %pc6 %pc7 0.27 1.29 2.29 0.95 0.02 0.00 98.77 20.23 0.00 77.41 0.00 0.25 1.24 2.29 0.98 0.02 0.00 98.75 20.34 0.03 77.74 0.00 0.27 1.22 2.29 0.54 0.00 0.00 99.18 20.64 0.00 77.70 0.00 0.26 1.22 2.29 1.22 0.00 0.00 98.52 20.22 0.00 77.74 0.00 0.26 1.38 2.29 0.78 0.02 0.00 98.95 20.51 0.05 77.56 0.00 ^C i# ./turbostat.new -s %c0 GHz TSC %c1 %c3 %c6 %c7 %pc2 %pc3 %pc6 %pc7 0.27 1.20 2.29 0.24 0.01 0.00 99.49 20.58 0.00 78.20 0.00 0.27 1.22 2.29 0.25 0.00 0.00 99.48 20.79 0.00 77.85 0.00 0.27 1.20 2.29 0.25 0.02 0.00 99.46 20.71 0.03 77.89 0.00 0.28 1.26 2.29 0.25 0.01 0.00 99.46 20.89 0.02 77.67 0.00 0.27 1.20 2.29 0.24 0.01 0.00 99.48 20.65 0.00 78.04 0.00 cc: Youquan Song <youquan.song@intel.com> Signed-off-by: Len Brown <len.brown@intel.com> |
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