Zombie pathfinding to unreachable targets causes server lag

As some new commenters do not bother to read older comments, a link back to the buried earlier comment with my analysis (in short, a possible reason is already known): Summed up in this comment.

Workaround: Using Forge (files.minecraftforge.net) or the snapshot 13w39a+

For clarity, let me rewrite this:

Now:

This bug appears when a large number of zombies cannot successfully calculate a safe path to their target. Severe lag in either single or multiplayer results as well as time moving jumping forward/backward. The only work-around is to kill the zombies, create a path for them, or switch to peaceful mode. Avoiding the night by sleeping may also work. This is especially noticeable around villages due to the number of zombie that appear and their intended targets (villagers) not being reachable.

Was:

While protecting a village (building walls and putting torches up) a zombie siege event occurs, very often, causing extreme lag.

Could be conflicting code between zombie generation and villager reaction exacerbating each other. The more villagers, the more zombies, the more lag.

----------- From MC-18865 ----------------------------------------------------------------

I am not sure if all these factors are actually relevant, but here we go:

/gamerule mobGriefing false
Hard difficulty
Villagers inside a house
Zombies trying to break in (difficulty is set to hard, so they would try to break the door, but /gamerule doesn't let them)

These things MIGHT be related to the issue. I get really bad delay on picking items, killing mobs and even brewing.

NOTE: this is on SINGLE PLAYER, clearly an internal server problem.
NOTE2: when I kill all the zombies, the delay is gone. There is probably something wrong with the new zombie system as well.

---------------------------------------------------------------- ----------------------------------------------------------------
Emphasized the analysis of Markku:

As some new commenters do not bother to read older comments, a link back to the buried earlier comment with my analysis (in short, the reason is already known):
https://bugs.mojang.com/browse/MC-17630?focusedCommentId=92590&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-92590