What the warning means
A Minecraft server does its work in ticks, twenty every second, so each tick has 50 milliseconds. When one takes longer, the server is late and writes this to the log:
[Server thread/WARN]: Can't keep up! Is the server overloaded? Running 2345ms or 46 ticks behindThe numbers say how far behind it got: 2345ms is about 46 ticks of 50 ms. While it's late, everything slows down for every player at once. Blocks break late, mobs stutter, players rubber-band back.
You may also see Did the system time change, or is the server overloaded?. Same warning, worded for a clock that jumped, which happens when a laptop wakes from sleep.
When you can ignore it
A line or two while the server is starting, while a world loads, or while a lot of new land is being generated isn't worth chasing. The warning matters when it repeats during normal play, say every few seconds, or when players start complaining about lag.
What usually causes it
- Generating new chunks, for example when several players fly off in different directions with elytra.
- Too many entities: a big mob farm, hundreds of dropped items, forty cows in one pen.
- Large redstone machines and long hopper chains, which tick every single tick.
- A slow processor. The server does most of its work on one thread (the
Server threadin the log), so one fast core beats many slow ones. - A plugin or mod that does heavy work on that same main thread.
- Long memory clean-up pauses, when the server has too little memory or far too much. See OutOfMemoryError.
Find what is causing it
Guessing wastes time, so measure. Install spark, run /spark profiler start, let it record while the lag is happening, then run /spark profiler stop. You get a page that shows which plugin, mod or activity used the most tick time. On Paper and its forks, /tps prints the current ticks per second; a healthy server sits at 20.0.
Ways to fix it
- Lower
view-distanceandsimulation-distanceinserver.properties; both default to 10, and 8 or 6 is a common first step. It's the quickest way to cut the work per tick. - Pre-generate the world with Chunky (
/chunky radius 5000, then/chunky start) so nobody triggers it during play. - Reduce entities: smaller farms, fewer animals per pen, and clear the item piles.
- Remove plugins and mods you don't use, and update the ones you do.
- Check the memory setting. If the profiler shows the server is just busy, the answer is a host with a faster processor.
Questions
Is "Can't keep up" a crash?
No. The server keeps running; it's only behind. It becomes a problem when it repeats and players notice lag.
How many ticks per second should a server have?
Twenty, so each tick gets 50 milliseconds. Below 20 the game slows down.
Will more RAM stop the warning?
Only if the server was short of memory. Usually the limit is processor speed, or one heavy farm, plugin or mod, so profile before you buy more.
Keep reading
- How to find your Minecraft logsFind latest.log and crash reports for the Minecraft client, servers and launchers like Prism and Modrinth App. Paths for Windows, macOS and Linux.
- What "Exception in server tick loop" meansException in server tick loop is only a label on a server crash report. The real cause is the exception under it. Learn how to read it and what usually breaks.
- How to share a log file without leaking private detailsLog files can hold your IP address, your Windows username and even login tokens. This is what to look for and how to share a log without exposing it.