Search Unity

Efficient random values in job

Discussion in 'Data Oriented Technology Stack' started by rkas1222, Oct 6, 2018.

  1. rkas1222

    rkas1222

    Joined:
    Aug 9, 2017
    Posts:
    6
    Best approach for efficiently generating random values in a parallel job?

    I've used ThreadLocal System.Random objects in jobs, without ECS, and that seems to work. But my understanding is that doing anything with managed memory inside of a job is potentially very slow -- presumably from under-the-covers locking.

    Alternatively, if there are guarantees on allocations within System.Random (i.e. there aren't many/any), perhaps that's good enough, given that they are thread local. But if there's a better or more idiomatic way to do this with jobs, I'd like to know.
     
  2. dartriminis

    dartriminis

    Joined:
    Feb 3, 2017
    Posts:
    156
    There is a Unity.Mathematics.Random struct available for use. There's also Unity.Mathematics.noise if that floats your boat. ;)
     
  3. rkas1222

    rkas1222

    Joined:
    Aug 9, 2017
    Posts:
    6
    Looks perfect. Thanks!
     
  4. Ryetoast

    Ryetoast

    Joined:
    Mar 8, 2015
    Posts:
    40
    Are there any good examples of using Unity.Mathematics.Random out there? I'm unclear on what good practices are for seeding, or dealing with random values inside of a parallel job, like IJobProcessComponentData.

    From testing, similar seeds produce similar results, which means;
    • Using something like the index as a seed offset isn't good enough
    • Using tick count would be bad if you are creating these structs on the fly
    The solutions I can think of off the top of my head are:
    • Trying to use a single Unity.Mathematics.Random instance for most things that just lives for the duration of the program, which doesn't really work with parallel jobs since getting a random is a modifying action.
    • Having an instance of Unity.Mathematics.Random on each entity that is going to be doing random things. Unfortunately this still has the issue that I basically need to manually randomize the seed in order to not have entities have very similar starting patterns.
    Is there some simple solution I'm missing?
     
    Mikael-H likes this.
  5. Ryetoast

    Ryetoast

    Joined:
    Mar 8, 2015
    Posts:
    40
    After spending some time messing around with it, the best I could come up with is:
    1. Use an [Inject] and IJobParallelFor instead of IJobProcessComponentData so I can know how many seeds I need
    2. Generating a NativeArray<uint> of seeds using (uint)UnityEngine.Random.Range(int.MinValue, int.MaxValue)
    3. Make new Unity.Mathematics.Random(Seed[index]) inside the job
    I couldn't just directly make a NativeArray<Unity.Mathematics.Random> and then call InitState because the constructor errors on state not being initialized yet... (checks the state member variable isn't 0). Still doesn't seem ideal to me even if I could do that, but I guess it works.
     
    Mikael-H likes this.
  6. Mikael-H

    Mikael-H

    Joined:
    Apr 26, 2013
    Posts:
    281
    EDIT: I've found that the method below does not produce entirely random numbers for some reason. If anyone can spot the error I'd be very grateful. So for now either produce random numbers beforehand or set an array of random generators with their seeds set.


    Thanks for the inpyt @Ryetoast! I ended up setting just a base seed for the job and then adding the index value to this seed, that way I don't need to assign an extra native array.

    Code (CSharp):
    1. private struct BulletHitHandleJob : IJobForEachWithEntity<BulletMoveData>
    2.         {
    3.             [ReadOnly] [DeallocateOnJobCompletion] public NativeArray<RaycastHit> RaycastHits;
    4.             [ReadOnly] public EntityCommandBuffer.Concurrent CommandBuffer;
    5.             [ReadOnly] public uint BaseSeed;
    6.  
    7.             public void Execute(Entity bulletEntity, int index, ref BulletMoveData bulletSpeed)
    8.             {
    9.                 var seed = (uint)(BaseSeed + index);
    10.                 var rnd = new Unity.Mathematics.Random(seed);
    11.  
    12.                 if (RaycastHits[index].distance > 0)
    13.                 {
    14.                     var bulletHitData = new BulletHitData
    15.                     {
    16.                         Hit = RaycastHits[index],
    17.                         Damage = rnd.NextInt(1,50),
    18.                         Attacker = bulletSpeed.Shooter,
    19.                         Impact = -bulletSpeed.Direction * bulletSpeed.MetersPerSecond,
    20.                 };
    21.                  
    22.                     CommandBuffer.AddComponent(index, bulletEntity, bulletHitData);
    23.                     CommandBuffer.RemoveComponent<BulletMoveData>(index, bulletEntity);
    24.                 }
    25.             }
    26.         }
     
    Last edited: Aug 8, 2019
    pakfront likes this.
  7. Mikael-H

    Mikael-H

    Joined:
    Apr 26, 2013
    Posts:
    281
    Ahh I should have read your previous post more carefully! :) Seems very strange to me that similar seeds produce similar values, but my observations are the same as yours!
     
unityunity