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aminer10
Beginner
57 Views

About parallel merging ...

Hello,

I have just read the following paper on Parallel Merging:

http://www.economyinformatics.ase.ro/content/EN4/alecu.pdf


And i have implemented this algorithm just to see what is the performance.

And i have noticed that the serial algorithm is 8 times slower
than the merge function that you find in the serial mergesort algorithm.
So 8 times slower, it's too slow.

It's better to use the following algorithm;

http://www.drdobbs.com/parallel/parallel-merge/229204454?queryText=parallel%2Bsort


The idea is simple:

Let's assume we want to merge sorted arrays X and Y. Select X median
element in X. Elements in X[ .. m-1] are less than or equal to X.
Using binary search find index k of the first element in Y greater than
X.
Thus Y[ .. k-1] are less than or equal to X as well. Elements in X[m+1 .. ]
are greater than or equal to X and Y[k .. ] are greater. So merge(X, Y)
can be defined as concat(merge(X[ .. m-1], Y[ .. k-1]), X, merge(X[m+1 .. ], Y[k .. ]))
now we can recursively in parallel do merge(X[ .. m-1], Y[ .. k-1]) and
merge(X[m+1 .. ], Y[k .. ]) and then concat results.

And then you can continue to apply this method recursivily.



Thank you,
Amine Moulay Ramdane.

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2 Replies
Andrey_Tikhonov
Beginner
57 Views

Very interesting, thank you.
SergeyKostrov
Valued Contributor II
57 Views

Thanks for these web-links. >>...And i have noticed that the serial algorithm is 8 times slower >>than the merge function that you find in the serial mergesort algorithm. >>So 8 times slower, it's too slow. I'm not surprised with your results because MergeSort is easily parallelizable by design: it splitts a source ( input ) data set in half on every recursion call until it reaches a threshold value two or four. Note: A threshold value depends if a data-mining step ( pair-switching ) was performed before processing. >>...Using binary search find index k of the first element in Y greater than X... It is clear that application of a binary search inside of MergeSort will create additional overhead and possibly affect a performance.
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