[MC-127630] Server: Out of memory

Not sure if this is that I need to assign more memory, a java issue or a minecraft one. Also not sure if this has been picked up, I did a search but couldn't find anything recent.
The server became unresponsive and the CPU stayed up at 77% for about 10 mins till I finally gave up and restarted the machine.

java -version
openjdk version "1.8.0_151"
OpenJDK Runtime Environment (build 1.8.0_151-8u151-b12-0ubuntu0.16.04.2-b12)
OpenJDK 64-Bit Server VM (build 25.151-b12, mixed mode)

Start options java -Xms512M -Xmx1536M -jar minecraft_server.18w11a.jar nogui
Was run in a screen on a limited user account.

[21:09:03] [Server Watchdog/FATAL]: A single server tick took 66.02 seconds (should be max 0.05)
[21:09:32] [Server Watchdog/FATAL]: Considering it to be crashed, server will forcibly shutdown.
[21:10:37] [User Authenticator #31/ERROR]: Caught previously unhandled exception :
[21:10:58] [Server Watchdog/ERROR]: Caught previously unhandled exception :
[21:11:15] [User Authenticator #31/ERROR]: java.lang.OutOfMemoryError: Java heap space
[21:11:09] [Server Watchdog/ERROR]: Server Watchdog
java.lang.OutOfMemoryError: Java heap space
	at java.util.Arrays.copyOf(Arrays.java:3332) ~[?:?]
	at java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:124) ~[?:?]
	at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:448) ~[?:?]
	at java.lang.StringBuilder.append(StringBuilder.java:136) ~[?:?]
	at java.lang.StringBuilder.append(StringBuilder.java:131) ~[?:?]
	at rq.run(SourceFile:53) ~[minecraft_server.18w11a.jar:?]
	at java.lang.Thread.run(Thread.java:748) [?:?]
[21:11:03] [Server thread/ERROR]: Encountered an unexpected exception
java.lang.OutOfMemoryError: Java heap space