Question Confusion about what kinds of data I can use in IJobEntity when using ScheduleParallel

My understanding originally was that when scheduling multithreaded jobs you could only use blittable data types, and data structures couldn't just be shared between entities without making them readonly or as a parallel writer. however I recently realized I was accidently using some static variables from my ISystem class, and it was just working. what exactly am I not understanding here?

here is an example of working code to demonstrate what I am talking about

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using Unity.Burst;
4. using Unity.Entities;
5. using Unity.Jobs;
6. using Unity.Transforms;
7.  
8. public partial struct DotsOdditiyDemonstration : ISystem
9. {
10.     static string[] entitiesLocations;
11.     static int nextIndex;
12.     static EntityQueryDesc entitiesWithTransformDesc;
13.     public void OnCreate(ref SystemState state)
14.     {
15.     }
16.  
17.     public void OnDestroy(ref SystemState state)
18.     {
19.        
20.     }
21.  
22.     public void OnUpdate(ref SystemState state)
23.     {
24.         if (UnityEngine.Input.GetKeyDown("p"))
25.         {
26.             entitiesWithTransformDesc = new EntityQueryDesc()
27.             {
28.                 All = new ComponentType[] { ComponentType.ReadOnly<LocalTransform>() }
29.             };
30.  
31.             EntityQuery entitiesWithTransformQuery = state.EntityManager.CreateEntityQuery(entitiesWithTransformDesc);
32.  
33.             entitiesLocations = new string[entitiesWithTransformQuery.CalculateEntityCount()];
34.             nextIndex = 0;
35.  
36.             JobHandle EntityLocationOutputHandle = new EntityLocationOutputJob()
37.             {
38.             }.ScheduleParallel(entitiesWithTransformQuery, state.Dependency);
39.  
40.             EntityLocationOutputHandle.Complete();
41.  
42.             foreach (string s in entitiesLocations)
43.             {
44.                 UnityEngine.Debug.Log(s);
45.             }
46.         }
47.     }
48.  
49.     public partial struct EntityLocationOutputJob : IJobEntity
50.     {
51.         public void Execute(Entity entity, in LocalTransform localTransform)
52.         {
53.             entitiesLocations[nextIndex] = $"Entity {entity} is at {localTransform.Position}";
54.             nextIndex++;
55.         }
56.     }
57. }

 

You aren't burst compiling your job. Statics that aren't readonly are disallowed in burst jobs but work fine in normal ones.

What you're doing is not thread safe though.

 

I was under the impression that the compiler wouldn't let me do anything that isn't thread safe. So to be clear, I have to manually make sure that I make it thread safe?

 

If you use native containers then unity will provide safety. There is no safety in managed land.

 

Ok, so what I first noticed was that I was reading from a static persistent native array. If I understand you right, that would be fine until I want to burst? Basically, can I use this for debugging and remove it later?

Thank you very much for helping clear things up.

 

If you were only reading from a shared array that would be fine. However nextIndex++ does also write to a static int. So multiple threads could read/write the same field at the same time.

 

My advice is you should reason things in ECS without involving statics. That means: remove statics from your ECS code. That would give back to you something better.