The number of failures given above is for 1000 runs of the test program. Notice the jump between N = 2 and N = 4 ? And the plateau for higher values of N ? To explain those numbers, I need to give more information on the test Skylake machine. It has 4 physical cores and 8 logical cores, since HT is enabled. Two of the cores were busy with two long-running tests (not mine) in the background, but ot