I am trying to tune my production environment and I am confused about the default memory allocation. If I don't specify any -Xms or -Xmx flags in my startup script, how much memory does the JVM actually claim from the system by default? Does it depend on the total RAM available on the machine?
3 answers
For most modern 64-bit JVMs (like OpenJDK or Oracle JDK 8+), the default values are calculated based on the physical memory of the host. The default maximum heap size (Xmx) is typically 1/4 of the physical memory, while the initial heap size (Xms) is 1/64 of the physical memory. For example, if your server has 32GB of RAM, your Xmx will default to 8GB. However, these ergonomics can change if you are running inside a Docker container, where the JVM looks at the container's memory limits instead of the host's total RAM. This is a vital consideration for cloud technology deployments where resource cost is a factor.
Are you seeing high garbage collection overhead with the default settings? Sometimes the 1/4 rule for Xmx is too aggressive for small microservices or too conservative for big data apps.
You can check your specific defaults by running java -XX:+PrintFlagsFinal -version | findstr /i "HeapSize". It will show you the exact bytes assigned to your environment.
This command is exactly what I needed. It confirmed that my Xmx was capping at 4GB on my 16GB laptop, which explains the bottleneck I was facing.
I am seeing frequent 'Out of Memory' errors during peak loads. Michael, would you recommend setting Xms and Xmx to the same value to avoid the overhead of the heap resizing itself dynamically during execution? I've heard this is a best practice for high-performance software development environments.