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continous cached:
 8 wallclock secs ( 7.07 usr +  0.10 sys =  7.17 CPU) @ 13947.00/s (n=100000)
filling up:
 1 wallclock secs ( 0.92 usr +  0.00 sys =  0.92 CPU) @ 5434.78/s (n=5000)
random get:
 2 wallclock secs ( 2.08 usr +  0.09 sys =  2.17 CPU) @ 9216.59/s (n=20000)
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