minecraft_server is locking up and being killed by the watchdog.
In spite of the hardware being adequately spec'd for running the server, the server reports "Can't keep up! Did the system time change, or is the server overloaded?" a number of times. There is little competing activity for CPU; the program is executing from a RAM drive so is not disk-bound, and certainly isn't so for a 60-second period. There are few players.

laptop@IcePlanet /tmp/minecraft $ ./server.sh
[09:36:31] [Server thread/INFO]: Starting minecraft server version 1.8.1
[09:36:32] [Server thread/INFO]: Loading properties
[09:36:32] [Server thread/INFO]: Default game type: SURVIVAL
[09:36:32] [Server thread/INFO]: Generating keypair
[09:36:32] [Server thread/INFO]: Starting Minecraft server on *:25565
[09:36:32] [Server thread/INFO]: Using epoll channel type
[09:36:32] [Server thread/INFO]: Preparing level "world"
[09:36:33] [Server thread/INFO]: Preparing start region for level 0
[09:36:34] [Server thread/INFO]: Preparing spawn area: 6%
[09:36:35] [Server thread/INFO]: Preparing spawn area: 61%
[09:36:35] [Server thread/INFO]: Done (2.907s)! For help, type "help" or "?"
[09:36:55] User Authenticator #1/INFO: UUID of player josh_parris is 7cfb3435-ede1-47ca-bc2c-c4bd402a8257
[09:36:55] [Server thread/INFO]: josh_parris[/192.168.1.6:45770] logged in with entity id 274 at (-184.1399098428389, 66.0, -209.55632427859882)
[09:36:55] [Server thread/INFO]: josh_parris joined the game
[09:38:34] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2289ms behind, skipping 45 tick(s)
[09:39:32] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2694ms behind, skipping 53 tick(s)
[09:52:57] User Authenticator #2/INFO: UUID of player anonymous8085 is 86976552-78be-467c-9456-28e2b2199d58
[09:52:57] [Server thread/INFO]: anonymous8085[/192.168.1.9:47971] logged in with entity id 68889 at (-570.3755975857089, 88.0, -293.02484949784923)
[09:52:57] [Server thread/INFO]: anonymous8085 joined the game
[09:53:59] [Server thread/INFO]: anonymous8085 lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}

<snip>

[17:06:03] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3947ms behind, skipping 78 tick(s)
[17:06:25] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 6428ms behind, skipping 128 tick(s)
[17:06:44] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 10052ms behind, skipping 201 tick(s)
[17:07:04] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 10456ms behind, skipping 209 tick(s)
[17:07:19] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 4824ms behind, skipping 96 tick(s)
[17:07:58] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2596ms behind, skipping 51 tick(s)
[17:09:00] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2300ms behind, skipping 46 tick(s)
[17:09:26] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 9012ms behind, skipping 180 tick(s)
[17:09:41] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 4040ms behind, skipping 80 tick(s)
[17:10:08] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2870ms behind, skipping 57 tick(s)
[17:10:11] [Server thread/WARN]: SmallRiskRapier moved wrongly!
[17:10:40] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 11805ms behind, skipping 236 tick(s)
[17:10:53] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 6294ms behind, skipping 125 tick(s)
[17:11:14] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 11539ms behind, skipping 230 tick(s)
[17:11:25] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 6708ms behind, skipping 134 tick(s)
[17:11:47] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 12029ms behind, skipping 240 tick(s)
[17:12:10] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2794ms behind, skipping 55 tick(s)
[17:12:50] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 19154ms behind, skipping 383 tick(s)
[17:13:06] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2389ms behind, skipping 47 tick(s)
[17:13:38] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 14131ms behind, skipping 282 tick(s)
[17:13:46] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 5256ms behind, skipping 105 tick(s)
[17:14:20] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2324ms behind, skipping 46 tick(s)
[17:14:49] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 11596ms behind, skipping 231 tick(s)
[17:24:53] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2591ms behind, skipping 51 tick(s)
[17:25:40] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 24524ms behind, skipping 490 tick(s)
[17:25:47] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 6344ms behind, skipping 126 tick(s)
[17:26:39] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 31597ms behind, skipping 631 tick(s)
[17:26:44] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 5739ms behind, skipping 114 tick(s)
[17:28:13] [Server Watchdog/FATAL]: A single server tick took 60.00 seconds (should be max 0.05)
[17:28:13] [Server Watchdog/FATAL]: Considering it to be crashed, server will forcibly shutdown.
[17:28:13] [Server Watchdog/ERROR]: This crash report has been saved to: /tmp/minecraft/./crash-reports/crash-2015-02-08_17.28.13-server.txt
[17:28:13] [Server Shutdown Thread/INFO]: Stopping server
[17:28:13] [Server Shutdown Thread/INFO]: Saving players
java.util.ConcurrentModificationException
at java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1115)
at java.util.TreeMap$KeyIterator.next(TreeMap.java:1169)
at qu.a(SourceFile:574)
at qu.a(SourceFile:554)
at bfw.a(SourceFile:259)
at bfw.a(SourceFile:114)
at qt.b(SourceFile:160)
at qt.a(SourceFile:204)
at qu.a(SourceFile:766)
at net.minecraft.server.MinecraftServer.a(SourceFile:361)
at net.minecraft.server.MinecraftServer.s(SourceFile:384)
at pf.run(SourceFile:716)
laptop@IcePlanet /tmp/minecraft $