Comment # 9 on bug 1131437 from Jan Kara

OK, 4.20 with scsi_mod.use_blk_mq=0:

Amean      1       38.46 (   0.00%)       26.94 (  29.95%)       25.55 ( 
33.58%)       26.92 (  30.00%)       26.01 (  32.38%)
Amean      2       31.93 (   0.00%)       31.75 (   0.55%)       31.68 (  
0.76%)       35.29 ( -10.53%)       31.71 (   0.68%)
Amean      4       41.66 (   0.00%)       35.14 (  15.65%)       38.41 (  
7.81%)       38.30 (   8.08%)       36.31 (  12.84%)
Stddev     1        5.87 (   0.00%)        7.16 ( -21.96%)        5.83 (  
0.74%)        6.77 ( -15.34%)        6.20 (  -5.72%)
Stddev     2        9.03 (   0.00%)        8.73 (   3.30%)        8.85 (  
1.98%)        8.06 (  10.75%)        8.66 (   4.09%)
Stddev     4        6.11 (   0.00%)        7.96 ( -30.25%)        7.69 (
-25.90%)        8.18 ( -33.85%)        9.60 ( -57.09%)

So the variability is still there. Will try even older kernels next week.