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Tick guard

Keep a crowded area like spawn smooth for everyone in it, by thinning out the crowd's thinking instead of limiting the players.

Players who stand in the same place share the same region, and a region is ticked by one CPU core. When someone's mob farm or a crowded villager hall at spawn takes more than a tick's worth of time, everyone nearby used to feel it, even players who had nothing to do with it.

The tick guard fixes the cause instead of the symptom. When a region gets too slow, it thins out the most expensive part of the crowd, the mobs' decisions, only where the crowd is. The players in the region are never limited for it.

How it works#

Every region measures its own tick time, continuously.
Once a second it counts mobs per chunk. A chunk with at least crowd-threshold (16) mobs is crowded.
If the region's average tick time is above target-mspt (40 ms, of the 50 ms a tick may take), mobs in crowded chunks re-plan only every 2nd tick. If that is not enough, every 4th, then every 8th tick (max-level).
When the region has headroom again (below 60% of the target), it steps back down to normal.

On the ticks in between, a crowded mob keeps doing what it already decided:

Every tick (unchanged)	Only on full ticks
Movement, gravity, water flow, collisions and pushing	Sensing (who can I see?)
Walking along the current path	Choosing targets and goals
Looking, jumping, running goals already started	Brain updates (villagers, piglins, ...)

Blocks are never touched: redstone, pistons, hoppers, crops and block entities run exactly as before.

Who is never thinned out#

Mobs within player-radius (8) blocks of any player, so fights feel normal.
Mobs that are targeting a player.
Pets and other owned animals, and leashed mobs.
The Ender Dragon and the Wither.

What it changes, honestly#

Only while a region is overloaded, and only in crowded chunks:

Crowded mobs react a little later: a zombie in a packed pen may notice a new target a few ticks later.
Farms that depend on mob decisions (for example villager breeding or iron farms) produce somewhat less during the overload. Farms driven by physics (water streams, drops, crushers) are unaffected.

When the region is not overloaded, nothing is changed at all.

Measured#

A laggy spawn: 600 zombies, 300 villagers and 500 cows in pens, two players standing nearby and one far away.

	Spawn region	TPS	Players standing nearby
Without the tick guard	50–55 ms per tick	18	view distance cut to 6 by the old player budget
With the tick guard	32–35 ms per tick	20.0	not limited

The player far away was unaffected in both cases (about 1 ms per tick).

Finding the cause#

/storia region

2 region(s), busiest first (last 5s):
 world -329, 71: 66% thread, 32.8 MSPT, 20.0 TPS, 2 player(s), 766 chunks
   tick guard: crowded mobs re-plan every 8 ticks (23993 AI updates skipped/s)
   crowd at 8, 8: 572 mobs, mostly zombie
   crowd at 8, 24: 460 mobs, mostly cow
   crowd at 24, 8: 300 mobs, mostly villager

The crowd lines are block coordinates of the chunk center, so you can go and look at what is there.

Settings#

tick-guard:
  enabled: true
  target-mspt: 40.0      # keep regions below this (a tick has 50 ms)
  crowd-threshold: 16    # mobs per chunk that count as a crowd
  player-radius: 8.0     # mobs this close to a player always think every tick
  max-level: 3           # thin out down to 1/2^max-level (3 = every 8th tick)

When the server's tick threads are saturated and a region uses more than its players' fair share, the target for that region is lowered to 60%, so one busy area cannot starve everyone else.