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continous cached:
 7 wallclock secs ( 6.33 usr +  0.14 sys =  6.47 CPU) @ 15455.95/s (n=100000)
filling up:
13 wallclock secs (12.44 usr +  0.11 sys = 12.55 CPU) @ 398.41/s (n=5000)
random get:
 9 wallclock secs ( 8.93 usr +  0.12 sys =  9.05 CPU) @ 2209.94/s (n=20000)
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