RFR 8003981: Support Parallel Array Sorting - JEP 103

[Remi Forax]

On 12/20/2012 06:31 PM, Chris Hegarty wrote:
> This is a review request for the addition of utility methods to 
> java.util.Arrays that provide sorting of arrays in parallel, JEP 103 [1].
>
> Webrev:
>   http://cr.openjdk.java.net/~chegar/8003981/ver.00/
>
> Current sorting implementations provided by the Java Collections 
> Framework (Collections.sort and Arrays.sort) all perform the sorting 
> operation sequentially in the calling thread. This enhancement will 
> offer the same set of sorting operations currently provided by the 
> Arrays class, but with a parallel implementation that utilizes the 
> Fork/Join framework. These new API’s are still synchronous with regard 
> to the calling thread as it will not proceed past the sorting 
> operation until the parallel sort is complete.
>
> The actual sorting API this proposal adds will leverage the 
> ForkJoinPool.commonPool (default Fork/Join pool defined as part of JEP 
> 155). The sorting algorithm is that used in Doug Lea’s ParallelArray 
> implementation.
>
> This work was originally done over in the lambda/lambda repo by David 
> Holmes, and is now being cleaned up and brought into jdk8.
>
> Open issues/differences from version in lambda:
> 1) Is is necessary for MIN_ARRAY_SORT_GRAN to be in the public API?
>    It is an implementation detail (easy to remove).
> 2) The use of FJP.commonPool is an implementation detail, not
>    part of the spec. Should not be a problem, just worth pointing
>    out, as it differs from what is in lambda/lambda.
>
> -Chris.
>
> [1] http://openjdk.java.net/jeps/103

Hi Chris,
in Arrays.parallelSort(TYPE[] a, int fromIndex, int toIndex),
there is no need to have the two supplementary local variable origin and 
fence,
ws is not a great better name  (workspaceArray?), I had to read the code 
of ArraySortHelpers to understand.

ArraySortHelpers should be renamed to ArrayParallelSortHelpers.

In ArraySortHelpers, all Sorter, Merger, etc. should declare only one 
field by line, currently, the formatting of the fields is weird.

In all Sorter, instead of calling Arrays.sort(), you should call 
DualPivotQuicksort.sort to avoid unnecessary range checks.

In compute() of Sorter and Merger, you should copy a and w in local 
variables (a = this.a) to help the VM to figure out that they never changed,
it will also reduce the bytecode size.

In Sorter.compute, n should be loaded in a local field,
   int l = origin;
   int g = gran;
   int n = this.n;

In Merger.compute, the sequence of inits before the second while should be:
   int l = lo;
   int lFence = l + nleft;  // instead of lo + nleft
   int r = ro;
   int rFence = r + nright;  // instead of ro + nright
   int k = wo;
it should not changed the performance but it's more inline with the 
coding style of the rest of the code IMO.

cheers,
Rémi