Spawn chunks are chunks found at the world spawn point which are always loaded in memory, unlike normal chunks which unload when no players are nearby.
Normally, old chunks beyond player's chunk loading distance get unloaded from the memory and new chunks get loaded into the memory. Because of this, events such as redstone mechanisms and farms in unloaded chunks stop processing. Since spawn chunks remain loaded, events occurring in them continue to process, even if no players are nearby. If the world spawn is set to a laggy area using commands, such as an area with many redstone clocks, lag is present wherever the player is in the world.
The so-called "spawn chunks" behavior arises from a start ticket which is created for the world spawn chunk. This ticket has a level of 22 which "propagates," or flows, to neighboring chunks. Each time the level flows, it increases its level by 1 until the maximum of 34 is reached. This behavior is illustrated by the picture to the right.
Different levels of propagation have different behaviors. These levels are divided into 4 load types.
|Entity Ticking||31 and below||In the spawn chunks, this consists of a 19×19 chunk area. All game aspects are active.|
|Ticking||32||The margin of the 21×21 area. All game aspects are active except that entities are not processed (do not move) and chunk ticks aren't either.|
|Border||33||The margin of the 23×23 area. Only some game aspects are active (redstone and command blocks do not work).|
|Inaccessible||34 and above||No game aspects are active, but world generation still occurs.|
There are some limitations which may affect the behavior of spawn chunks.
- Idle timeout
Each dimension has its own "idle timeout." Without a player or forceloaded chunks in the overworld, this timeout eventually expires. When the timeout expires, some behaviors such as entity processing stop for spawn chunks and the rest of the dimension. The timeout can be reset by frequently sending entities through a portal.
- Chunk tick
Only chunks with horizontal distance between its center and a player less than 128 blocks are ticked on every game tick. This stops events caused by random ticking, such as crop growth.
- Hostile mobs more than 128 blocks from a player despawn.
- Passive mobs only spawn inside a 240×240 block area around the player.
- Passive mobs present in the spawn chunks count toward the mob cap. This often prevents friendly mobs from naturally spawning elsewhere in the world.
A new world spawn point can be set using
/setworldspawn. If no coordinates are provided, the block position that the player is currently standing on becomes the spawn point. The chunk where the world spawn is located is then given a start ticket with a level of 22 and the ticket for the old spawn chunks is cleared.