IJobFor

Description

An interface that represents a job that performs the same independent operation for each element of a native container or for a fixed number of iterations.

This job type has the following options to schedule work:

• IJobForExtensions.Run runs the job on the main thread and finishes immediately.
• IJobForExtensions.Schedule schedules the job to run on a worker thread or the main thread, but indicates that the work should happen in a single thread. This option allows for work to be done off the main thread, but the work is performed sequentially.
• IJobForExtensions.ScheduleParallel schedules the job to run on multiple worker threads concurrently. This scheduling option can give the best performance, but conflicts might happen when accessing the same data from multiple worker threads simultaneously.
• Execute(int index) is executed once for each index from 0 to the provided length.

Run and Schedule guarantees that the the job's Execute(int index) method is invoked sequentially. ScheduleParallel doesn't invoke the job's Execute method sequentially because it's called from multiple worker threads in parallel to each other.

Each iteration must be independent from other iterations and the safety system enforces this rule for you. The indices have no guaranteed order and are executed on multiple cores in parallel.

Unity automatically splits the work into chunks of no less than the provided batchSize, and schedules an appropriate number of jobs based on the number of worker threads, the length of the array and the batch size. You should choose the batch size based on the amount of work performed in the job. A simple job, for example adding a couple of Vector3 to each other should have a batch size of 32 to 128. However, if the work performed is very expensive then it's best practice to use a small batch size, for example, a batch size of 1. IJobFor performs work stealing using atomic operations. Batch sizes can be small but they are not for free.

You can use the returned JobHandle to check that the job has completed, or pass it to other jobs as a dependency. When you pass a JobHandle as a dependency, it ensures that the jobs are executed one after another on the worker threads.

Additional resources: IJobForExtensions

using UnityEngine;
using Unity.Collections;
using Unity.Jobs;

class ApplyVelocityParallelForSample : MonoBehaviour
{
    struct VelocityJob : IJobFor
    {
        // Jobs declare all data that will be accessed in the job
        // By declaring it as read only, multiple jobs are allowed to access the data in parallel
        [ReadOnly]
        public NativeArray<Vector3> velocity;

        // By default containers are assumed to be read & write
        public NativeArray<Vector3> position;

        // Delta time must be copied to the job since jobs generally don't have concept of a frame.
        // The main thread waits for the job same frame or next frame, but the job should do work deterministically
        // independent on when the job happens to run on the worker threads.
        public float deltaTime;

        // The code actually running on the job
        public void Execute(int i)
        {
            // Move the positions based on delta time and velocity
            position[i] = position[i] + velocity[i] * deltaTime;
        }
    }

    public void Update()
    {
        var position = new NativeArray<Vector3>(500, Allocator.Persistent);

        var velocity = new NativeArray<Vector3>(500, Allocator.Persistent);
        for (var i = 0; i < velocity.Length; i++)
            velocity[i] = new Vector3(0, 10, 0);

        // Initialize the job data
        var job = new VelocityJob()
        {
            deltaTime = Time.deltaTime,
            position = position,
            velocity = velocity
        };

        // Schedule job to run immediately on main thread. First parameter is how many for-each iterations to perform.
        job.Run(position.Length);

        // Schedule job to run at a later point on a single worker thread.
        // First parameter is how many for-each iterations to perform.
        // The second parameter is a JobHandle to use for this job's dependencies.
        //   Dependencies are used to ensure that a job executes on worker threads after the dependency has completed execution.
        //   In this case we don't need our job to depend on anything so we can use a default one.
        JobHandle scheduleJobDependency = new JobHandle();
        JobHandle scheduleJobHandle = job.Schedule(position.Length, scheduleJobDependency);

        // Schedule job to run on parallel worker threads.
        // First parameter is how many for-each iterations to perform.
        // The second parameter is the batch size,
        //   essentially the no-overhead innerloop that just invokes Execute(i) in a loop.
        //   When there is a lot of work in each iteration then a value of 1 can be sensible.
        //   When there is very little work values of 32 or 64 can make sense.
        // The third parameter is a JobHandle to use for this job's dependencies.
        //   Dependencies are used to ensure that a job executes on worker threads after the dependency has completed execution.
        JobHandle scheduleParallelJobHandle = job.ScheduleParallel(position.Length, 64, scheduleJobHandle);

        // Ensure the job has completed.
        // It is not recommended to Complete a job immediately,
        // since that reduces the chance of having other jobs run in parallel with this one.
        // You optimally want to schedule a job early in a frame and then wait for it later in the frame.
        scheduleParallelJobHandle.Complete();

        Debug.Log(job.position[0]);

        // Native arrays must be disposed manually.
        position.Dispose();
        velocity.Dispose();
    }
}

Public Methods