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Multithreading correctness


TheGreyGhost
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Howdy folks

 

Are there any guidelines I should follow to ensure that my mod code is not going to cause multithreading problems?

 

Most of the time it is obvious because I know whether my code is in a render thread, client tick thread, server thread etc, and the method I'm overriding or implementing gives me the (thread-safe) object I need.

 

But I think there are some other places where there might be risk of concurrent threads colliding if I call vanilla objects; especially static registration methods of vanilla classes during mod initialisation.

 

Are ModEventBus events called concurrently if multiple mods are present?  If so, any calls I make to vanilla code from a ModEventBus event is likely to be risky I think.  I hear rumours that DeferredWordQueue::enqueueWork is the method to ensure that registrations using vanilla classes are executed single-threaded, is this the intended method?

 

What about the server thread, client thread?  Eg are there ever concurrent server threads which may cause my mod to access non-thread-safe objects or methods?  I know there are multiple render threads, which is perfectly safe provided I stick to the objects provided in the calling method or event and don't try to access server or client objects via 'sneaky' methods.

 

What assumptions is it safe to make?

 

The worst bugs I've ever had to deal with have all been caused by multithread collisions or race conditions so I'm very keen to understand where the high risk areas are...

 

Cheers

 TGG

 

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During mod loading any event extending ParallelDispatchEvent (mostly FMLCommonSetupEvent and FMLClientSetupEvent) are run in parallel on a threadpool. If you access non-threadsafe things (e.g. BrewingRecipeRegistry) you have to use enqueueWork, which will run the given code synchronously on the main thread after the parallel dispatch.

 

As for client and server thread: There is mainly just the server and client main thread, although the server uses separate threads for world generation (if I remember correctly) and the client uses separate threads (sometimes) for chunk rendering - but you will rarely find problems here.

99% of Minecraft is still single threaded.

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7 minutes ago, diesieben07 said:

although the server uses separate threads for world generation (if I remember correctly)

This is what I believe happens as well. Each chunk may (or may not!) be on its own thread, so you can't reach out too far. I think/hope/assume that the +X/+Z chunks are accessible, due to how vanilla has always handled generation across chunk boundaries before, but I haven't had the time to actually find out. Its possible that there's some precompute (eg. structures) with the structure part bounding boxes and vanilla goes "ok, cool, you want to generate in this volume, that'd be this list of chunks...pooling chunks together...spawning thread...here you go, do what you need."

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Apparently I'm a complete and utter jerk and come to this forum just like to make fun of people, be confrontational, and make your personal life miserable.  If you think this is the case, JUST REPORT ME.  Otherwise you're just going to get reported when you reply to my posts and point it out, because odds are, I was trying to be nice.

 

Exception: If you do not understand Java, I WILL NOT HELP YOU and your thread will get locked.

 

DO NOT PM ME WITH PROBLEMS. No help will be given.

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