[mlpack-git] [mlpack/mlpack] LSHSearch Parallelization (#700)

[Yannis Mentekidis]

> @@ -811,9 +841,18 @@ void LSHSearch<SortPolicy>::Search(const arma::mat& querySet,
>  
>    Timer::Start("computing_neighbors");
>  
> -  // Go through every query point sequentially.
> -  for (size_t i = 0; i < querySet.n_cols; i++)
> +  // Parallelization to process more than one query at a time.
> +  // use as many threads possible but not more than allowed number
> +  size_t numThreadsUsed = maxThreads;
> +  #pragma omp parallel for \
> +    num_threads ( numThreadsUsed )\
> +    shared(avgIndicesReturned, resultingNeighbors, distances) \
> +    schedule(dynamic)

About static vs dynamic scheduling, I ran some tests:

Sift100k
```
dynamic: 0.086 +/- 0.008 s
 static: 0.093 +/- 0.014 s
```

phy
```
dynamic: 3.35 +/- 0.124 s
 static: 3.46 +/- 0.203 s
```

Corel
```
dynamic: 0.228 +/- 0.03 s
 static: 0.234 +/- 0.03 s
```

Miniboone
```
dynamic: 0.711 +/- 0.07 s
 static: 0.701 +/- 0.09 s
```

In the first 3, I'd say dynamic is slightly faster. It's hard to tell for Miniboone because the standard deviation is much larger than the difference. I'll run covertype and pokerhand in a while when my PC is not used.

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