C# Job System tips and troubleshooting

When using the Unity C# Job System, make sure you adhere to the following:

Do not access static data from a job

Accessing static data from a job circumvents all safety systems. If you access the wrong data, you might crash Unity, often in unexpected ways. For example, accessing MonoBehaviour can cause crashes on domain reloads.

Note: Because of this risk, future versions of Unity will prevent global variable access from jobs using static analysis. If you do access static data inside a job, you should expect your code to break in future versions of Unity.

Flush scheduled batches

When you want your jobs to start executing, then you can flush the scheduled batch with JobHandle.ScheduleBatchedJobs. Note that calling this method can negatively impact performance. Not flushing the batch delays the scheduling until the main thread waits for the result. In all other cases use JobHandle.Complete to start the execution process.

Note: In the Entity Component System (ECS) the batch is implicitly flushed for you, so calling JobHandle.ScheduleBatchedJobs is not necessary.

Don’t try to update NativeContainer contents

Due to the lack of ref returns, it is not possible to directly change the content of a NativeContainer. For example, nativeArray[0]++; is the same as writing var temp = nativeArray[0]; temp++; which does not update the value in nativeArray.

Instead, you must copy the data from the index into a local temporary copy, modify that copy, and save it back, like so:

MyStruct temp = myNativeArray[i];
temp.memberVariable = 0;
myNativeArray[i] = temp;

Call JobHandle.Complete to regain ownership

Tracing data ownership requires dependencies to complete before the main thread can use them again. It is not enough to check JobHandle.IsCompleted. You must call the method JobHandle.Complete to regain ownership of the NativeContainer types to the main thread. Calling Complete also cleans up the state in the safety system. Not doing so introduces a memory leak. This process also applies if you schedule new jobs every frame that has a dependency on the previous frame’s job.

Use Schedule and Complete in the main thread

You can only call Schedule and Complete from the main thread. If one job depends on another, use JobHandle to manage dependencies rather than trying to schedule jobs within jobs.

Use Schedule and Complete at the right time

Call Schedule on a job as soon as you have the data it needs, and don’t call Complete on it until you need the results. It is good practice to schedule a job that you do not need to wait for when it is not competing with any other jobs that are running. For example, if you have a period between the end of one frame and the beginning of the next frame where no jobs are running, and a one frame latency is acceptable, you can schedule the job towards the end of a frame and use its results in the following frame. Alternatively, if your game is saturating that changeover period with other jobs, and there is a big under-utilized period somewhere else in the frame, it is more efficient to schedule your job there instead.

Mark NativeContainer types as read-only

Remember that jobs have read and write access to NativeContainer types by default. Use the [ReadOnly] attribute when appropriate to improve performance.

Check for data dependencies

In the Unity Profiler window, the marker “WaitForJobGroup” on the main thread indicates that Unity is waiting for a job on a worker thread to complete. This marker could mean that you have introduced a data dependency somewhere that you should resolve. Look for JobHandle.Complete to track down where you have data dependencies that are forcing the main thread to wait.

Debugging jobs

Jobs have a Run function that you can use in place of Schedule to immediately execute the job on the main thread. You can use this for debugging purposes.

Do not allocate managed memory in jobs

Allocating managed memory in jobs is incredibly slow, and the job is not able to make use of the Unity Burst compiler to improve performance. Burst is a new LLVM based backend compiler technology that makes things easier for you. It takes C# jobs and produces highly-optimized machine code taking advantage of the particular capabilities of your platform.

Further Information

• Watch the Unity GDC 2018: C# Job System playlist of clips.
• For more advanced information on how the C# Job System relates to ECS, see the ECS package documentation on GitHub.

• 2018–06–15 Page published with editorial review

• C# Job System exposed in 2018.1 NewIn20181