nativeThreadIndex vs JobsUtility.MaxJobThreadCount

Hey everyone,

I've been using a custom "RandomSystem" in order to get "proper" random numbers in a

JobComponentSystem.Entities.Foreach

. Everything was fine until the number of my entities went above

128

. I was making the (false) assumption that the

nativeThreadIndex

of the

ForEach

would somehow be limited to

JobsUtility.MaxJobThreadCount

.

The docs say this:

So my idea was to just change:

Code (CSharp):

1. var random = randomArray[nativeThreadIndex];

to

Code (CSharp):

1. var random = randomArray[nativeThreadIndex % JobsUtility.MaxJobThreadCount];

and call it a day, but I'm wondering if there is a better solution for this kind of problem, because I'm sure it's going to pop up in some shape or form again soon.

Thanks

Code (CSharp):

1. public class RandomSystem : ComponentSystem {
2.     public NativeArray<Random> randomArray;
3.  
4.     protected override void OnCreate() {
5.         var randomArray = new Random[JobsUtility.MaxJobThreadCount];
6.         var seed = new System.Random();
7.  
8.         for (var i = 0; i < JobsUtility.MaxJobThreadCount; ++i) {
9.             randomArray[i] = new Random((uint) seed.Next());
10.         }
11.  
12.         this.randomArray = new NativeArray<Random>(randomArray, Allocator.Persistent);
13.     }
14.  
15.     protected override void OnDestroy() {
16.         randomArray.Dispose();
17.     }
18.  
19.     protected override void OnUpdate() { /* Nothing */ }
20. }

Code (CSharp):

1. public class UnitAttackTargetSystem : JobComponentSystem {
2.     protected override JobHandle OnUpdate(JobHandle inputDependencies) {
3.         var randomArray = World.GetExistingSystem<RandomSystem>().randomArray;
4.  
5.         return Entities.WithAll<Foo>.WithNativeDisableParallelForRestriction(randomArray).ForEach((int nativeThreadIndex, Entity entity, int entityInQueryIndex) => {
6.             var random = randomArray[nativeThreadIndex];
7.  
8.             // ...
9.  
10.             /* Write back the random struct so that next time it won't be the same: */
11.             randomArray[nativeThreadIndex] = random;
12.         }).Schedule(inputDependencies);
13.     }
14. }

 

I've just run into this myself, and am now very confused. In the tests I've done, nativeThreadIndex is always equal to entityInQueryIndex, given the wording of nativeThreadIndex i believe its mean't to give a value unique to the thread, that wont go above MaxJobThreadCount, but right now, its just giving entityInQueryIndex...

Is it bugged? Is unity passing entityInQueryIndex twice by mistake? have i missed something?

Code (CSharp):

1.                     if(nativeThreadIndex != entityInQueryIndex)
2.                         UnityEngine.Debug.Log(string.Format("{0} - {1}", nativeThreadIndex, entityInQueryIndex));

never prints anything

Code (CSharp):

1.                     if(nativeThreadIndex == entityInQueryIndex)
2.                         UnityEngine.Debug.Log(string.Format("{0} - {1}", nativeThreadIndex, entityInQueryIndex));

always prints (the same value for both variables)

Using entities package 0.8.0 preview8

EDIT: The problem seems to be because both entityInQueryIndex & nativeThreadIndex are present, if only one is present the value is as expected.

EDIT2: I'm having some success avoiding the issue by defining "int entityInQueryIndex" at the begining of the lambda, and "in int nativeThreadIndex" at the end, important point is the "in" decoration. If both are put "int", or both are "in int", both values will be the same.

 

Nice catch, this indeed happens if you use them both and out of order in the lambda declaration.

nativeThreadIndex

should always come after

entityInQueryIndex

and their values will be correct.

Code (CSharp):

1.  .ForEach((int entityInQueryIndex, int nativeThreadIndex, Entity entity, ref Translation translation, ref Rotation rotation) => {})
2.  

 

It seems this is still the case in Entities 0.16

 

Faced the same issue in Entities 0.16

It seems if I put entityInQueryIndex first:

Code (CSharp):

1. .ForEach((int entityInQueryIndex, int nativeThreadIndex,  ref Translation trans)

then both parameters will be equal to nativeThreadIndex.

If nativeThreadIndex first:

Code (CSharp):

1. .ForEach((int nativeThreadIndex,  int entityInQueryIndex, ref Translation trans)

then both parameters will be equal to entityInQueryIndex.

But workaround proposed by Icewave works for me:

Code (CSharp):

1. .ForEach((int entityInQueryIndex,  ref Translation trans, in int nativeThreadIndex)

 

Hate to go off-topic but @nicolasgramlich that looks almost exactly like my code... I noticed the copyright notice was missing, lol. Anyway....

For those curious you can actually install the code as a package, as little as there is, which is part of my monorepo.