Comment # 6 on bug 1131437 from Mel Gorman

(In reply to Jan Kara from comment #5)
> OK, when the machine is booted with intel_idle.max_cstate=1, the numbers are
> much more stable (did run just for 1, 2, and 4 clients):
> 
> Amean      1       36.13 (   0.00%)       35.76 (   1.03%)       36.18 ( 
> -0.14%
> )       36.44 (  -0.86%)       36.43 (  -0.82%)
> Amean      2       34.63 (   0.00%)       31.68 (   8.50%)       31.83 (  
> 8.08%
> )       31.59 (   8.77%)       31.70 (   8.45%)
> Amean      4       40.73 (   0.00%)       38.24 (   6.13%)       39.73 (  
> 2.46%
> )       40.53 (   0.51%)       40.29 (   1.09%)
> Stddev     1        4.65 (   0.00%)        4.17 (  10.35%)        4.44 (  
> 4.43%
> )        3.59 (  22.76%)        4.39 (   5.45%)
> Stddev     2        6.42 (   0.00%)        6.18 (   3.81%)        6.16 (  
> 4.09%)        6.38 (   0.58%)        6.10 (   4.97%)
> Stddev     4        5.94 (   0.00%)        5.57 (   6.11%)        5.89 (  
> 0.80%)         5.76 (   2.97%)        5.64 (   4.97%)
> 
> I'm going to verify now how stock 4.20 dbench numbers are / aren't stable to
> see whether this instability is a recent thing.

If 4.20 looks good, I would suggest taking a close look / revert of
8e3b40395450 ("cpufreq: intel_pstate: Fix up iowait_boost computation") and the
base commit it relies on b8bd1581aa61 ("cpufreq: intel_pstate: Rework iowait
boosting to be less aggressive") because they have "potential to regress
workloads that pause on IO for short periods" written all over them.