# External Sorting Adapt fastest internalsort methods Quick sort

- Slides: 25

External Sorting • Adapt fastest internal-sort methods. ü Quick sort …best average run time. • Merge sort … best worst-case run time.

Internal Merge Sort Review • Phase 1 § Create initial sorted segments • Natural segments • Insertion sort • Phase 2 § Merge pairs of sorted segments, in merge passes, until only 1 segment remains.

External Merge Sort • • Sort 10, 000 records. Enough memory for 500 records. Block size is 100 records. t. IO = time to input/output 1 block (includes seek, latency, and transmission times) • t. IS = time to internally sort 1 memory load • t. IM = time to internally merge 1 block load

External Merge Sort • Two phases. § Run generation. ØA run is a sorted sequence of records. § Run merging.

Run Generation 10, 000 records 100 blocks MEMORY 500 records 5 blocks • • Input 5 blocks. Sort. Output as a run. Do 20 times. DISK • • 5 t. IO t. IS 5 t. IO 200 t. IO + 20 t. IS

Run Merging • Merge Pass. § Pairwise merge the 20 runs into 10. § In a merge pass all runs (except possibly one) are pairwise merged. • Perform 4 more merge passes, reducing the number of runs to 1.

Merge 20 Runs R 1 R 2 R 3 R 4 R 5 R 6 R 7 R 8 R 9 R 10 R 11 R 12 R 13 R 14 R 15 R 16 R 17 R 18 R 19 R 20 S 1 S 2 T 1 S 3 S 4 S 5 T 2 S 6 S 7 T 4 T 3 S 10 S 9 T 5 U 3 U 2 U 1 S 8 V 2 V 1 W 1

Merge R 1 and R 2 Output Input 0 • • Input 1 DISK Fill I 0 (Input 0) from R 1 and I 1 from R 2. Merge from I 0 and I 1 to output buffer. Write whenever output buffer full. Read whenever input buffer empty.

Time To Merge R 1 and R 2 • • • Each is 5 blocks long. Input time = 10 t. IO. Write/output time = 10 t. IO. Merge time = 10 t. IM. Total time = 20 t. IO + 10 t. IM.

Time For Pass 1 (R • Time to merge one pair of runs = 20 t. IO + 10 t. IM. • Time to merge all 10 pairs of runs = 200 t. IO + 100 t. IM. S)

Time To Merge S 1 and S 2 • • • Each is 10 blocks long. Input time = 20 t. IO. Write/output time = 20 t. IO. Merge time = 20 t. IM. Total time = 40 t. IO + 20 t. IM.

Time For Pass 2 (S • Time to merge one pair of runs = 40 t. IO + 20 t. IM. • Time to merge all 5 pairs of runs = 200 t. IO + 100 t. IM. T)

Time For One Merge Pass • • Time to input all blocks = 100 t. IO. Time to output all blocks = 100 t. IO. Time to merge all blocks = 100 t. IM. Total time for a merge pass = 200 t. IO + 100 t. IM.

Total Run-Merging Time • (time for one merge pass) * (number of passes) = (time for one merge pass) * ceil(log 2(number of initial runs)) = (200 t. IO + 100 t. IM) * ceil(log 2(20)) = (200 t. IO + 100 t. IM) * 5

Factors In Overall Run Time • Run generation. 200 t. IO + 20 t. IS § Internal sort time. § Input and output time. • Run merging. (200 t. IO + 100 t. IM) * ceil(log 2(20)) § § Internal merge time. Input and output time. Number of initial runs. Merge order (number of merge passes is determined by number of runs and merge order)

Improve Run Generation • Overlap input, output, and internal sorting. DISK MEMORY DISK

Improve Run Generation • Generate runs whose length (on average) exceeds memory size. • Equivalent to reducing number of runs generated.

Improve Run Merging • Overlap input, output, and internal merging. DISK MEMORY DISK

Improve Run Merging • Reduce number of merge passes. § Use higher-order merge. § Number of passes = ceil(logk(number of initial runs)) where k is the merge order.

Merge 20 Runs Using 5 -Way Merging R 1 R 2 R 3 R 4 R 5 R 6 R 7 R 8 R 9 R 10 R 11 R 12 R 13 R 14 R 15 R 16 R 17 R 18 R 19 R 20 S 1 S 2 S 3 T 1 Number of passes = 2 S 4

I/O Time Per Merge Pass • Number of input buffers needed is linear in merge order k. • Since memory size is fixed, block size decreases as k increases (after a certain k). • So, number of blocks increases. • So, number of seek and latency delays per pass increases.

I/O Time Per Merge Pass I/O time per pass merge order k

Total I/O Time To Merge Runs • (I/O time for one merge pass) * ceil(logk(number of initial runs)) Total I/O time to merge runs merge order k

Internal Merge Time O R 1 R 2 R 3 R 4 R 5 R 6 • Naïve way => k – 1 compares to determine next record to move to the output buffer. • Time to merge n records is c(k – 1)n, where c is a constant. • Merge time per pass is c(k – 1)n. • Total merge time is c(k – 1)nlogkr ~ cn(k/log 2 k) log 2 r.

Merge Time Using A Tournament Tree O R 1 R 2 R 3 R 4 R 5 R 6 • Time to merge n records is dnlog 2 k, where d is a constant. • Merge time per pass is dnlog 2 k. • Total merge time is (dnlog 2 k) logkr = dnlog 2 r.

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