I think I still owe some performance numbers to show what is wrong with systems using pcc-cpufreq with Linux after commit 554c8aa8ecad.
Following are results for kernbench tests (from MMTests test suite). That's just a kernel compile with different number of compile jobs. As result the time is measured, 5 runs are done for each configuration and average values calculated. I've restricted maximum number of jobs to 30. That means that tests were done for 2, 4, 8, 16, and 30 compile jobs. I had bound all tests to node 0. (I've used something like "numactl -N 0 ./run-mmtests.sh --run-monitor <test_name>" to start those tests.) Tests were done with kernel 4.18.0-rc3 on an HP DL580 Gen8 with Intel Xeon CPU E7-4890 with latest BIOS installed. System had 4 nodes, 15 CPUs per node (30 logical CPUs with HT enabled). pcc-cpufreq was active and ondemand governor in use. I've tested with different number of online CPUs which better illustrates how idle online CPUs interfere with compile load on node 0 (due to the jitter caused by pcc-cpufreq and its locking). Average mean for user/system/elapsed time and standard deviation for each subtest (=number of compile jobs) are as follows: (Nodes) N0 N01 N0 N01 N0123 (CPUs) 15CPUs 30CPUs 30CPUs 60CPUs 120CPUs Amean user-2 640.82 (0.00%) 675.90 (-5.47%) 789.03 (-23.13%) 1448.58 (-126.05%) 3575.79 (-458.01%) Amean user-4 652.18 (0.00%) 689.12 (-5.67%) 868.19 (-33.12%) 1846.66 (-183.15%) 5437.37 (-733.73%) Amean user-8 695.00 (0.00%) 732.22 (-5.35%) 1138.30 (-63.78%) 2598.74 (-273.92%) 7413.43 (-966.67%) Amean user-16 653.94 (0.00%) 772.48 (-18.13%) 1734.80 (-165.29%) 2699.65 (-312.83%) 9224.47 (-1310.61%) Amean user-30 634.91 (0.00%) 701.11 (-10.43%) 1197.37 (-88.59%) 1360.02 (-114.21%) 3732.34 (-487.85%) Amean syst-2 235.45 (0.00%) 235.68 (-0.10%) 321.99 (-36.76%) 574.44 (-143.98%) 869.35 (-269.23%) Amean syst-4 239.34 (0.00%) 243.09 (-1.57%) 345.07 (-44.18%) 621.00 (-159.47%) 1145.13 (-378.46%) Amean syst-8 246.51 (0.00%) 254.83 (-3.37%) 387.49 (-57.19%) 786.63 (-219.10%) 1406.17 (-470.42%) Amean syst-16 110.85 (0.00%) 122.21 (-10.25%) 408.25 (-268.31%) 644.41 (-481.36%) 1513.04 (-1264.99%) Amean syst-30 82.74 (0.00%) 94.07 (-13.69%) 155.38 (-87.80%) 207.03 (-150.22%) 547.73 (-562.01%) Amean elsp-2 625.33 (0.00%) 724.51 (-15.86%) 792.47 (-26.73%) 1537.44 (-145.86%) 3510.22 (-461.34%) Amean elsp-4 482.02 (0.00%) 568.26 (-17.89%) 670.26 (-39.05%) 1257.34 (-160.85%) 3120.89 (-547.46%) Amean elsp-8 267.75 (0.00%) 337.88 (-26.19%) 430.56 (-60.80%) 978.47 (-265.44%) 2321.91 (-767.18%) Amean elsp-16 63.55 (0.00%) 71.79 (-12.97%) 224.83 (-253.79%) 403.94 (-535.65%) 1121.04 (-1664.09%) Amean elsp-30 56.76 (0.00%) 62.82 (-10.69%) 66.50 (-17.16%) 124.20 (-118.84%) 303.47 (-434.70%) Stddev user-2 1.36 (0.00%) 1.94 (-42.57%) 16.17 (-1090.46%) 119.09 (-8669.75%) 382.74 (-28085.60%) Stddev user-4 2.81 (0.00%) 5.08 (-80.78%) 4.88 (-73.66%) 252.56 (-8881.80%) 1133.02 (-40193.16%) Stddev user-8 2.30 (0.00%) 15.58 (-578.28%) 30.60 (-1232.63%) 279.35 (-12064.01%) 1050.00 (-45621.61%) Stddev user-16 6.76 (0.00%) 25.52 (-277.80%) 78.44 (-1060.97%) 118.29 (-1650.94%) 724.11 (-10617.95%) Stddev user-30 0.51 (0.00%) 1.80 (-249.13%) 12.63 (-2354.11%) 25.82 (-4915.43%) 1098.82 (-213365.28%) Stddev syst-2 1.52 (0.00%) 2.76 (-81.04%) 3.98 (-161.58%) 36.35 (-2287.16%) 59.09 (-3781.09%) Stddev syst-4 2.39 (0.00%) 1.55 (35.25%) 3.24 ( -35.92%) 51.51 (-2057.65%) 175.75 (-7262.43%) Stddev syst-8 1.08 (0.00%) 3.70 (-241.40%) 6.83 (-531.33%) 65.80 (-5977.97%) 151.17 (-13864.10%) Stddev syst-16 3.78 (0.00%) 5.58 (-47.53%) 4.63 ( -22.44%) 47.90 (-1167.18%) 99.94 (-2543.88%) Stddev syst-30 0.31 (0.00%) 0.38 (-22.41%) 3.01 (-862.79%) 27.45 (-8688.85%) 137.94 (-44072.77%) Stddev elsp-2 55.14 (0.00%) 55.04 (0.18%) 95.33 ( -72.90%) 103.91 (-88.45%) 302.31 (-448.29%) Stddev elsp-4 60.90 (0.00%) 84.42 (-38.62%) 18.92 ( 68.94%) 197.60 (-224.46%) 323.53 (-431.24%) Stddev elsp-8 16.77 (0.00%) 30.77 (-83.47%) 49.57 (-195.57%) 79.02 (-371.16%) 261.85 (-1461.28%) Stddev elsp-16 1.99 (0.00%) 2.88 (-44.60%) 28.11 (-1311.79%) 101.81 (-5012.88%) 62.29 (-3028.36%) Stddev elsp-30 0.65 (0.00%) 1.04 (-59.06%) 1.64 (-151.81%) 41.84 (-6308.81%) 75.37 (-11445.61%) Overall test time for each mmtests invocation was as follows (this is also given for number-of-cpu configs for which I did not provide details above). N0 N01 N0 N012 N0123 N01 N0123 N0123 N012 N0123 N0123 15CPUs 30CPUs 30CPUs 45CPUs 60CPUs 60CPUs 75CPUs 90CPUs 90CPUs 105CPUs 120CPUs User 17196.67 18714.36 30105.65 19239.27 19505.35 53089.39 22690.33 26731.06 38131.74 47627.61 153424.99 System 4807.98 4970.89 8533.95 5136.97 5184.24 16351.67 6135.29 7152.66 10920.76 12362.39 32129.74 Elapsed 7796.46 9166.55 11518.51 9274.77 9030.39 25465.38 9361.60 10677.63 15633.49 18900.46 60908.28 The results given for 120 online CPUs on nodes 0-3 indicate what I meant with the "system being almost unusable". When trying to gather results with kernel 4.17.5 and 120 CPUs, one iteration of kernbench (1 kernel compile) with 2 jobs even took about 6 hours. Maybe it was an extreme outlier but I dismissed to further use that kernel (w/o modifications) for further tests. Andreas