parallel arrays and sorting

Herhut, Stephan A stephan.a.herhut at intel.com
Thu Apr 18 21:47:05 PDT 2013


Thanks, this indeed is an interesting use case.

Previously, I thought that using a function to map objects to a sort key is at least as expressive as having two arrays, as a naïve implementation of the mapping function could just be to read the appropriate value from the key array (assuming the mapping function would know the index). However, such a solution does not generate a sorted key array and thus the correlation between the sorted objects and their keys is lost.

After reading the blog post you linked, I am still not sure whether that actually is an issue. It seems for a rendering pipeline only the sorted objects are of importance.

So, potential runtime efficiency issues aside, do you know a use case where one also needs the array of sorted keys?

Stephan

From: Kevin Gadd [mailto:kevin.gadd at gmail.com]
Sent: Thursday, April 11, 2013 11:43 AM
To: Norm Rubin
Cc: Herhut, Stephan A; es-discuss at mozilla.org<mailto:es-discuss at mozilla.org>; dev-tech-js-engine-rivertrail at lists.mozilla.org<mailto:dev-tech-js-engine-rivertrail at lists.mozilla.org>
Subject: Re: parallel arrays and sorting

One simple-ish real world example of a use case that can benefit from a parallel sort:
It's common in games and other realtime rendering scenarios to want to sort your queue of rendering operations by various attributes, in order to minimize the number of hardware state changes and allow you to batch up operations.

http://realtimecollisiondetection.net/blog/?p=86 is an article that describes an approach used to generate 64-bit 'sort keys' for drawing operations that was used in some current-gen console games. At present, I use a similar technique to sort drawing operations in my buffers, and more importantly, I do all the sorting and preparation operations in parallel. For complex scenes I could imagine it being quite common to want to leverage parallel machinery (or even GPU hardware) to sort the thousands of individual drawing operations in a given scene before clustering them up into larger hardware operations.
The proposed compromise of providing a function that maps the objects being sorted to 'sort keys' seems a suitable choice for this kind of scenario - the draw calls can have a sort key implicitly (like described in that blog post), or you can trivially manufacture one (that's what I do). This is definitely a use case where only being able to sort primitives would not be adequate. However, a sort primitive that operates on a pair of arrays would also suffice - Norm calls this 'sort by key', and the .NET BCL exposes this as an overload of Array.Sort. It's quite useful and it seems like it might be feasible to expose this, even if only the key-sorting occurs in parallel (and the value sorting can't because the values are ordinary JS objects).

On Thu, Apr 11, 2013 at 11:35 AM, Norm Rubin <nrubin at nvidia.com<mailto:nrubin at nvidia.com>> wrote:
Good set of questions about sort,  thanks for looking at this carefully.  You guys have  been great about responding to my emails.



One sorting application that I would expect is physics particle simulation, where the code  might sort objects based on distance from the viewer. This could be used so that  far away objects get rendered smaller.

Another might be  computing a histogram  over a visual image – sort the data first so nearby values will go into the same bucket, without needing atomic synchronization.

Sorting with an optional compare seems ok  to me- even if implementations can only optimize for  some  comparisons

One  interesting  question you brought up as  part of sorting by key is pretty tricky.

In general parallel data arrays  would like to be stored in transposed  order from classic cpu styles.
We usually call this array of structures or structures of arrays
Do we sort  an array of objects each with multiple fields stored  one object after another or do we sort multiple arrays,  with all the values of field1 followed by all the values of field2 etc

I’d hope we allow  implementations to reorder the data within a parallelArray as they like without needing to expose the layout to developers.  Thrust, on the other hand, pushes the layout  to the developer and thus has a significant set of transpose routines.

Just for a point of reference there are  8  versions of sort in the thrust library. As  you can see, Thrust never aimed to be a minimal set it just gained operations as applications appeared. AMD has a similar library called bolt which uses the same interface.
Two different parallel  libraries with the same routines does suggest that this set is enough  to do useful work.


Internally thrust notices  that the compare is  <  on primitive  types and  uses radix sort (on the gpu).

Thurst includes:
  Stable and unstable forms
Ascending only or with a comparison function
Single array or a pair of keys and values


1)      Sort(array)  - sorts the array into ascending order, not guaranteed to be stable

2)      Sort (array)– with a comparison function

3)      Sort by key (keys, values) returns both the reordered keys and  the reordered values, sorted by the array keys. This one is kind of confusing so here is an example

// an example of key sorting
#include <thrust/sort.h<http://docs.thrust.googlecode.com/hg/sort_8h.html>>
  const int N = 6;
  int    keys[N] = {  1,   4,   2,   8,   5,   7};
  char values[N] = {'a', 'b', 'c', 'd', 'e', 'f'};
  thrust::sort_by_key<http://docs.thrust.googlecode.com/hg/group__sorting.html#ga2bb765aeef19f6a04ca8b8ba11efff24>(keys, keys + N, values, thrust::greater<int><http://docs.thrust.googlecode.com/hg/structthrust_1_1greater.html>());
  // keys is now   {  8,   7,   5,   4,   2,   1}
  // values is now {'d', 'f', 'e', 'b', 'c', 'a'}




4)      Sort by key with a  comparison function

5-8 the stable forms


Based on the thrust primitives, one  parallel array sort  might be

Sort an array of objects
Besides the array the arguments might be
- a flag indicating if the sort must be  stable
- an optional user compare function that defaulted to < on primitive types


From: Herhut, Stephan A [mailto:stephan.a.herhut at intel.com<mailto:stephan.a.herhut at intel.com>]
Sent: Thursday, April 11, 2013 1:09 PM
To: Norm Rubin; es-discuss at mozilla.org<mailto:es-discuss at mozilla.org>
Subject: RE: parallel arrays and sorting

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> [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<mailto: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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