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04.02.2008 at 07:58PM PDT, ID: 23291593
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Parallel External Mergesort

I posted this question in C/C++ because I usually program in those languages, but this is really a general question relating to an algorithm.

Recently I've been experimenting with different implementations of algorithms designed to sort very large files using external merge sorting.  This entails a two-phase approach - the first phase reads in the file in chunks and sorts each chunk, then writes the sorted chunk to disk.  The second phase then merges all chunks together into a single sorted file.

Parallelization is of course the key here to doing this efficiently.  It's quite easy to parallelize the first phase of the algorithm.  Each chunk of the file can be passed to a separate worker thread, so the sorting can be done in parallel.  But is it possible to parallelize the merging phase?

Does any expert here know of a general algorithm for parallelizing the merging phase of an external merge sort?  
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Question Stats
Zone: Programming
Question Asked By: chsalvia
Solution Provided By: sdstuber
Participating Experts: 2
Solution Grade: A
Views: 0
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04.03.2008 at 07:39AM PDT, ID: 21273146

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04.03.2008 at 11:16AM PDT, ID: 21275308

Rank: Guru

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04.03.2008 at 07:39AM PDT, ID: 21273146
The merging must be singly threaded by the nature of the algorithm.

The chunk sorting can be parellized because they are independent operations.
You might be able to parallelize "sub-merges", but I'm somewhat sceptical of the benefits of doing such.

For example  8 chunks -8 threads.
4 threads merge 2 chunks each
2 threads merge 2 double-chunks each
1 thread merges 2 quad-chunks.



Accepted Solution
 
04.03.2008 at 11:16AM PDT, ID: 21275308

Rank: Guru

Hi chsalvia,

I haven't seen you on here for a while. Good to see you back. :)

As has been pointed out, the merge process would be difficult to parallelise as its main bottleneck is disk I/O. However, if there is something you can assume about the sort key there may be something you can do.

Perhaps you could bucket the chunks. If you are sorting ascii strings, you could split the data 26 ways on your first read+chunk+sort pass. You could then merge each of the resultant 26 sets of sorted chunks in a separate thread. This may drastically impact on your disk I/O though.

Generally, when you are using an external mergesort you are usually doing it because you have a LOT of data. Optimisation of cpu usage is unlikely to have much of an impact on throughput.

Paul

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