parallel arrays and sorting

Herhut, Stephan A stephan.a.herhut at intel.com
Thu Apr 11 10:08:30 PDT 2013


Rick is travelling, so let me chime in.

We have discussed this back and forth but have not come to a conclusion. Generally, we agree that adding a sort primitive makes a lot of sense, in particular as the Array object in JavaScript already has a sort method. Also, as you too mentioned, implementing an efficient sort as a library function without knowing the details of the parallel hardware used is difficult, to say the least. So sort ticks all the boxes to become a primitive.

The other, and arguably more difficult question, is what a sort method should look like. If we take JavaScript's existing Array.sort, the sort method would get an (optional) comparator function. However, using a comparator would preclude the use of radix sort.
An alternative would be to implement sorting of primitive types only. This brings back more choice in sort algorithms but limits use. For such a design, we considered a function as optional argument to sort that, given a value from the ParallelArray to be sorted, returns a key used for comparison, which again needs to be a primitive. This would at least enable sorting of objects by a field and, at some runtime cost, sorting of general data.

The tradeoff between these two approaches, and probably other designs, is hard to judge without knowing what sort is used for. So we decided to wait for some good use cases before deciding on a specific design.

Sorry, no answers only further questions.
  Stephan

From: es-discuss-bounces at mozilla.org [mailto:es-discuss-bounces at mozilla.org] On Behalf Of Norm Rubin
Sent: Monday, April 08, 2013 7:46 AM
To: es-discuss at mozilla.org
Subject: parallel arrays and sorting

In comparing ParallelArrays (rivertrail) to the cuda thrust library,  I noticed  that Sorting using parallelArrays looks like a missing primitive  operation.

Sorting is  special because the good (high performance) algorithm on a gpu is radix sort, while the good (high performance) algorithm on the cpu is parallel merge sort,  The other way around radix sort of cpu, or parallel merge sort of a gpu is very slow and often worse than serial implementations

Sadly it is well past a jit to take the code for one flavor of sort and transform it into the other,  while  it would be pretty simple for a run-time to pick a good sort, if only it knew that a sort was going on.  Run times would not be required to do  so  here since they can always pick a slow sort.

I know this is a slippery road, since once you add another prim, adding more prims becomes ever easier  but sorting seems pretty important.   And array already has a sort, so adding a version that works on ParallelArrays does not seem so bad.

What do you guys think?


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