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JL
Uploaded byJothi Lakshmi
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external sorting

External sorting algorithms are needed to sort data that is too large to fit into main memory. Traditional sorting algorithms require the entire input to reside in memory. External sorting techniques divide the large input into chunks that can fit in memory, sort each chunk, and then merge the sorted chunks. The document discusses how external accesses to data on disk are much slower than accessing data in memory, necessitating new algorithms to minimize disk I/O for large datasets that cannot fit in memory.

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External Sorting By V.Jothi lakshmi
External Sorting • All the algorithms that we have learnt still requires the input to be fit into main memory. • There are some applications where the input is too large to fit into main memory. • To solve this problem external sorting algorithms which are designed to handle very large inputs.
Need of new algorithms • Users usually want data sorted. • Sorting useful for eliminating duplicates. • Internal sorting algorithms take advantage of the fact that memory is directly addressable. • Shell sort – A[i] and A[i-hk] in one time unit • Heap sort -- A[i] and A[i*2+1] in one time unit • Quick sort -- 3 median of 3 partitioning  A[left] A[center] A[right]
Need of new algorithms • If input on tape – all these operations lose their efficiency. • Elements on tape can be accessed -sequent • If it is in disk- lose of efficiency –delay required to spin the disk and move the disk head.
Need of new algorithms • To see how external accesses really are  create a random file that is large. But not too big to fit in main memory. Read the file and sort using efficient algorithm. Time to sort input compared to time to read input. Even though sorting is an O(N log N) operation and reading O(N)
Need of new algorithms • To see how external accesses really are  create a random file that is large. But not too big to fit in main memory. Read the file and sort using efficient algorithm. Time to sort input compared to time to read input. Even though sorting is an O(N log N) operation and reading O(N)