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Could Not Reserve Enough Space for Heap

The server will not even begin to start, and Java reports it cannot reserve space for the heap. This is a JVM startup failure caused by asking for more memory than is available, or by running a 32-bit Java. It happens before any Minecraft code runs.

What does this error mean?
At launch the JVM tries to reserve the maximum heap set by -Xmx. If that amount exceeds what the system can give (or a 32-bit Java caps you near ~1.5 GB), the JVM aborts immediately, this is distinct from running out of heap later during play. 
Could not reserve enough space for object heap / Invalid maximum heap size
Most Common Causes
  • An -Xmx larger than the machine's available RAM.
  • A 32-bit Java, which cannot address large heaps.
  • Other processes already consuming most of the RAM.
  • A container/host memory limit lower than the requested heap.
  • A typo in the heap flag (e.g. wrong unit).
How To Diagnose
  1. Check your -Xmx value against the machine's actual RAM.
  2. Run 'java -version', a '32-Bit' line means you must switch to 64-bit.
  3. Look at free memory ('free -h' on Linux).
  4. Confirm any host/container memory cap.
Recommended Fixes
  • Lower -Xmx
    Set a heap that fits available RAM, leaving room for the OS (e.g. do not allocate all of it).
  • Use 64-bit Java
    Install a 64-bit JDK so large heaps can be addressed.
  • Free or add memory
    Close other processes or increase the machine/container RAM.
  • Respect host limits
    Keep -Xmx under your hosting plan's memory cap.
Analyze My Log
Frequently Asked Questions

This fails at startup because the heap cannot be reserved. A runtime OOM happens later when live data exceeds the heap.

32-bit processes can only address a small amount of memory, so large -Xmx values are rejected. Use 64-bit Java.

Leave headroom for the OS, never assign 100% of RAM to -Xmx.