[WARNING] Can't keep up!

Tired of seeing that same old message over and over? No, your system time didn't change, and yes, your server is overloaded. Continue reading below to see what you can (and can't) do about it.

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Improving Minecraft Server Performance

No, you aren't the only server owner who gets the infamous message.

[WARNING] Can't keep up! Did the system time change, or is the server overloaded?

But what does it mean, and what can you do to fix it?

What is Lag?

Our Can't keep up! message is caused by the dreaded Lag. Basically, minecraft does everything in Ticks. Each tick is intended to last 50ms and must handle everything in game during that 50ms window. During each tick, your minecraft server must make mobs move, handle all player actions that occurred within the last 50ms, notify all players of all changes, and many more things. Adding plugins to the mix means that even more things must be done each tick. When each tick starts to take more than 50ms, you start experienceing Lag.

Lag can have wide-reaching consequences. Severe Lag can cause the sun and moon to slow down, and make days and nights take longer. Mobs move and react more slowly. Instead of moving smoothly, you'll notice players jump from place to place. Cooking items in a furnace will take longer. Lag significantly deteriorates your players' gaming experience.

What DOESN'T work!

I've read over and over various accounts of how you can simply solve your lag problem by using one java configuration or another. Bullshit. Here's a nice little list of things I've heard people give as an end-all solution to fixing lag - and none of them work.

  • Use my special Java configuration to launch your server! Nope. This is probably the most common thing I hear. Use this configuration, use that configuration. I've seen configurations with 10 different java runtime options spelled out. They are all bullshit. Here's what I like to use, and it works as good as any:
    This assumes you are using bukkit, and you want to start your server with 2GB of ram.
    java -jar -Xincgc -Xmx2G craftbukkit.jar
    There are 10,000 different ways to do what I just did, and they all pretty much do the same thing, performance wise.
  • Give 'er more RAM, Wash! RAM is not a significant bottleneck for a Minecraft server. 2GB to 4GB is generally more than enough for even large servers. My 95-player server (mcpvp.com) runs on 4GB of RAM without a hitch. I could give it 128GB and I still wouldn't be able to up my player cap to more than 95 players without causing lag.

What DOES work!

There ARE some limited things you can do to eek more power out of your existing system. Check it out:

  • MUST... SEE... closer?? Easiest thing to do to is lower your view-distance setting in your server.properties file. The furthest view-distance you can have is 15, while the shortest is 3. If you really want to reduce lag, try the 3 setting and see if you can tolerate it. If that's too short, keep increasing the view distance until you can see far enough that it doesn't bother you.
  • MORE POWER Ok, so be cautions here. This is a bit of a confusing topic. Something you have to realize is that Minecraft Servers can only run on ONE CORE of a processor. Upgrading to a 6-core monster won't do you any good. However, if you get a processor that is faster per core, then you can really up your player capacity without adding any lag. The absolute best processor you can get at the time of this writing is the Intel Core i7-2700K as determined by this website. You'll see there that there are several other processors that performed better, but closer inspection will reveal that they are all 6-cores. That means that even though the 3960X has a passmark of 14,900, your server will only be able to access 2,480 of that power, compared with 2,575 with the 2700K.

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