Cost of External Merge Sort
•Number of passes:
•Think differently
–Given B = 4KB, M = 64MB, R = 0.1KB
–Pass 1: runs of length M/R = 640000
•Have now sorted runs of 640000
records
–Pass 2: runs increase by a factor of M/B – 1 =
16000
•Have now sorted runs of
10,240,000,000 = 1010 records
–Pass 3: runs increase by a factor of M/B – 1 =
16000
•Have now sorted runs of 1014 records
•Nobody has so much data !
•Can sort everything in 2 or 3
passes !
