Minecraft Server Out of Memory: Fix "Java heap space" and Watchdog Crashes

"Out of memory" covers three crashes with three different fixes. Find the line in the console or in the newest file under crash-reports/, then jump to the matching section.

Every start script has a maximum heap, set with -Xmx. The world's loaded chunks, every entity, every plugin's data, and every player's inventory live inside it. When the next allocation does not fit and a full garbage collection cannot free enough, Java throws OutOfMemoryError: Java heap space and the server dies with it.

Two questions decide the fix.

Was the heap simply too small?

Look at the start script. A 26.x server on -Xmx1G or -Xmx1024M, which older tutorials still show, does not have enough to load a world with a few players. Raise it to at least 2 GB, and set -Xms to the same value:

java -Xms4G -Xmx4G -jar server.jar nogui

Use the RAM calculator for a figure based on your player count, plugins, and view distance, and read how much RAM to give it below for the limits.

Or did something fill a heap that used to be enough?

If the server ran for weeks on the same heap and now crashes, the load changed. The usual suspects, in order:

• An entity build-up: a mob farm that stopped killing, a breeder that never culls, thousands of dropped items. Run /paper entity list on Paper, or teleport around with F3 watching the entity count.
• Players spreading out. Each player loads their own set of chunks; ten explorers load far more than ten people in one base.
• A new plugin that caches heavily, such as a map renderer (Dynmap, BlueMap) or a large logging plugin with a growing database.
• A raised view-distance or simulation-distance. Memory grows with the square of the distance.

Raise the heap to stop the crashes today, then reduce the load so it does not just happen again at the new limit.

This one appears before the server even starts. Java asked the operating system for the heap you set and was refused. Three causes:

• -Xmx is bigger than free RAM. A machine with 8 GB cannot give Java 8 GB; the OS needs 2 GB or so and other programs need theirs. Lower -Xmx.
• 32-bit Java. It cannot address more than about 1.5 GB no matter how much RAM the machine has. Run java -version; if it says 32-Bit or lacks 64-Bit, install the 64-bit build. The Java 25 guide installs the right one.
• A typo in the flag. -Xmx4GB is invalid; it is -Xmx4G or -Xmx4096M. A missing unit is read as bytes.

People search for this as an out of memory error because it often follows one, but it is a different mechanism. The server watchdog kills the server when one game tick takes longer than max-tick-time in server.properties, which defaults to 60000 milliseconds. A tick should take 50. The crash report begins:

The server has stopped responding! This is (probably) not a Paper bug.
A single server tick took 60.00 seconds (should be max 0.05)
Considering it to be crashed, server will forcibly shutdown.

What stalled the tick is in the stack trace below that line. Common ones: a huge redstone machine, a chunk generation burst when a player flies with elytra, a plugin doing database work on the main thread, or the garbage collector pausing for seconds because the heap is nearly full. That last case is where memory comes in: a heap that is 95 percent used makes every collection slow, and the watchdog fires before the OutOfMemoryError does.

Three rules cover nearly every setup.

1. Floor: 2 GB for any 26.x server. About 4 GB for ten players on Paper with a few plugins. 6 to 8 GB for a modpack.
2. Ceiling: physical RAM minus 2 GB on a dedicated machine, and minus whatever else runs on it otherwise. Java also uses memory outside the heap, roughly another 10 to 20 percent, so -Xmx6G really costs about 7 GB.
3. Xms equals Xmx on a dedicated server. It avoids heap resizing and makes garbage collection predictable.

More is not automatically better. Past the point of need, a larger heap means the collector has more to scan and pauses get longer. If the server lags with plenty of free heap, the bottleneck is the CPU or a slow tick, not memory.

These changes cut memory use without buying anything, roughly in order of effect.

• Lower view-distance and simulation-distance. 10 is the default. 8 halves the loaded area with little visible difference; 6 is fine for a small survival server.
• Switch to PaperMC. It uses less memory per chunk and entity than vanilla and adds limits for mob and item build-up. The PaperMC guide covers the switch.
• Cull entities. Kill dropped items with kill @e[type=item], fix farms that hoard mobs, and lower max-entity-cramming via the game rule.
• Pre-generate the world. Chunk generation is the single most memory- and CPU-hungry thing the server does. The Chunky plugin generates the area inside your world border once, ahead of time.
• Set a world border. worldborder set 10000 stops explorers loading endless new terrain.
• Audit plugins. On Paper, /spark healthreport and /spark heapsummary (Paper bundles spark) show which plugin holds the memory.

A plain start line is enough for most servers:

java -Xms4G -Xmx4G -jar server.jar nogui

For a server that stays up for days with many players, the garbage collector settings known as Aikar's flags reduce pauses. They are long, so copy them from the PaperMC documentation rather than retyping. They work with vanilla and Paper alike, on Java 25 as on 21. The one thing they do not do is reduce how much memory the server needs; size the heap first.