Multithreaded Instantiation

Unity Engine
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1

I am able to spawn entities in a job, but I’m not sure how to do it in a multithreaded job.

var spawndata = SystemAPI.GetSingleton<SpawnData>();
  EntityCommandBuffer entityCommandBuffer = new EntityCommandBuffer(Allocator.TempJob);
  EntityCommandBuffer.ParallelWriter parallelWriter = entityCommandBuffer.AsParallelWriter();

  new TestSpawnMultithreaded
  {
      ecb = parallelWriter,
      spawnData = spawndata,

  }.ScheduleParallel();

[BurstCompile]
public partial struct TestSpawnMultithreaded : IJobEntity
{
    public EntityCommandBuffer.ParallelWriter ecb;
    public SpawnData spawnData;
    void Execute([ChunkIndexInQuery] int chunkIndex)
    {
        var stone = new NativeArray<Entity>(25, Allocator.Temp);
        var coal = new NativeArray<Entity>(20, Allocator.Temp);

        ecb.Instantiate(chunkIndex, spawnData.Stone, stone);
        ecb.Instantiate(chunkIndex, spawnData.Coal, coal);
    }
}
2

Use IJobFor then use ScheduleParallel(). You’re not querying for anything.

3

The reason the code example IJobEntity is not querying for anything is because it is pseudo code. Assuming this code is using a Query (i.e. a turret that fires at targets), How would you Instantiate a entity using a multithreaded IJobEntity.

4

I barely use IJobEntity but it should be the same:

public struct SpawnJob : IJobChunk
{
    public Entity prefab;
    public EntityCommandBuffer.ParallelWriter commandBuffer;

    public void Execute(in ArchetypeChunk chunk, int unfilteredChunkIndex, bool useEnabledMask, in v128 chunkEnabledMask)
    {
        for(...) {
            Entity instance = commandBuffer.Instantiate(unfilteredChunkIndex, this.prefab);

            // Prepare components of the instance here but use the commandBuffer
        }
    }
}

// Scheduling
SpawnJob spawnJob = new() {
    prefab = yourPrefabEntity,
    commandBuffer = commandBufferSystem.CreateCommandBuffer().AsParallelWriter()
};
this.Dependency = spawnJob.ScheduleParallel(yourQuery, this.Dependency);
5

Just a clarification. You are not spawning the entity in the job. You are registering a command to instantiate an entity later on.
The instantiation itself will happen in a single thread (most likely the main thread)

6

If you arent using one of the builtin ECBSystems, and you arent playing back your commands, all you are doing is recording (as there is no entityCommandBuffer.Playback in your example).

7

Is there a way to set Playback to work on other threads?

8

For some specific scenarios yes
https://docs.unity3d.com/Packages/com.unity.entities@1.0/api/Unity.Entities.ExclusiveEntityTransaction.html

9

IJobEntity and IJobChunk only works with EntityQuery. Using IJobFor, IJob,IJobParallel do somthing in job