1.14 snapshot server always crashes after slightly more than one minute, fresh start, no users online

Whenever I start up 18w44a on a world that was created fresh for 18w43b, the server will not stay up for more than about 70 seconds. This is freshly started, with no users logged on.

My JVM options are as follows:

java -Xmx4G -Xmx4G -Xmn128M -Xincgc -jar server.jar nogui

Here's the console output:

[12:48:51] [Server thread/WARN]: Keeping entity minecraft:zombie_pigman that already exists with UUID bc2e7a4f-cb60-4b1b-995c-9197c345a2dd
[12:48:51] [Server thread/WARN]: Keeping entity minecraft:cow that already exists with UUID 97196cb7-a069-4e23-8a64-04a95b575928
[12:48:53] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 2044ms or 40 ticks behind
[12:50:04] [Server Watchdog/FATAL]: A single server tick took 60.00 seconds (should be max 0.05)
[12:50:04] [Server Watchdog/FATAL]: Considering it to be crashed, server will forcibly shutdown.
[12:50:05] [Server Watchdog/ERROR]: This crash report has been saved to: /home/minecraft/snapshot_test/./crash-reports/crash-2018-11-01_12.50.05-server.txt
[12:50:05] [Server Shutdown Thread/INFO]: Stopping server

This is the stack trace from the offending thread:

java.lang.Error
    at azt.a(SourceFile:1800)
    at tz.n(SourceFile:700)
    at tz.a(SourceFile:606)
    at ua.a(SourceFile:180)
    at net.minecraft.server.MinecraftServer.b(SourceFile:748)
    at ti.b(SourceFile:360)
    at net.minecraft.server.MinecraftServer.a(SourceFile:685)
    at net.minecraft.server.MinecraftServer.run(SourceFile:573)
    at java.lang.Thread.run(Thread.java:748)

Gnembon told me he's pretty sure that azt.a is related to "mob counting for spawn reasons." My suspicion is that there is mob overload due to some earlier snapshot not implementing a mob cap properly. But if it's spending too much time counting mobs, then it should just stop early. There's also the possibility that the watchdog bug (which Gnembon had provided a fix for) is back.

I enabled profiling (honest profiler), and here's the dominant part of the call tree:

(t 100.0,s 13.5) java.lang.Thread::run
  (t 86.4,s  0.1) net.minecraft.server.MinecraftServer::run
   (t 81.0,s  0.0) net.minecraft.server.MinecraftServer::a
    (t 81.0,s  0.0) ti::b
     (t 81.0,s  0.0) net.minecraft.server.MinecraftServer::b
      (t 66.1,s  0.0) ua::a
       (t 66.1,s  0.0) tz::a
        (t 66.0,s  0.3) tz::n
         (t 55.8,s  2.3) azt::a
          (t 17.7,s  0.0) bnh::b
           (t 15.1,s  0.0) bet::c
            (t 14.9,s  0.1) bjt::a
             (t  4.4,s  0.1) bjt::b
              (t  4.1,s  0.0) azx::D

So basically, azt::a accounted for 55% of the one minute that the server managed to stay up.

Going on the hypothesis that mob overload might be related to the problem, I restarted with a fresh empty world. It's definitely still a huge problem that we have one thread hammering the CPU at 100%. but the server manages to stay up. Now it's back to having Thread::yield wasting CPU time in a tight loop, which is the topic of MC-138071.