Question Some questions about IJobChunk determinism

If I schedule an IJobChunk, what kind of determinism can be expected?

1. If I schedule the job twice, will the exact same chunks be processed in the exact same order, both times?

Will the same chunkIndex always map to the same chunk, in both of those jobs?

2. if I create two different queries, but with the same component types, and then use them to schedule two different jobs of the same type, will those jobs iterate over the same chunks on an identical order?

3. Reality check: Can change filters have any impact on the order in which chunks are processed in an IJobChunk? (What I’ve found in the source so far points to no. Order is the same - the unchanged chunks are just skipped.)

4. Reality check: Is it correct to assume there is no expectation of consistency about which chunks end up being batched together on a thread? That seems like it would be entirely up to which threads are available, and what the scheduler thinks is the most efficient split between them.

5. Is there any correlation between thread index and chunk index? Are all the chunk indices processed on thread index ‘0’ guaranteed to lower than all the chunk indices processed on thread index ‘1’?

My guess is - no. Unity will steal indices from one thread and reassign them to another, idle thread - in the middle of a job’s execution - if the first thread is taking too long.

6. I’m curious how Unity maintains a deterministic order of chunks, internally. ArchetypeChunks don’t seem to have any intrinsic data that could be used to sort them, so I’m guessing they are stored in the order that they’re created?

Thank you very much for any advice.

 

Thank you, these are awesome answers.

How is each Archetype’s chunk list ordered? Is that sorted in any way?

Or is it just based on the order in which new chunks with that archetype are created and destroyed?

Would an Archetype’s chunk list have the same order in two different instances of the game?

I should probably say that I’m not doing anything crazy with this info. It’s to better understand what is and isn’t deterministic, to stay informed and sane.

Thanks again!