Xmx512m

Xmx512m refers to a Java Virtual Machine (JVM) option that sets the maximum size of the Java heap to 512 megabytes. The Java heap is the area of memory used for object allocation and garbage collection. The suffix m indicates megabytes, with 1 m equal to 1024*1024 bytes. Common alternatives include k for kilobytes and g for gigabytes, and these suffixes are generally case-insensitive.

Usage and implications. The option is provided to the JVM at startup, for example: java -Xmx512m -jar myapp.jar. It is often paired with -Xms to fix the initial heap size (for example, -Xms512m). The Oracle/OpenJDK JVM will attempt to allocate the specified heap at launch and will not grow beyond this limit. Actual availability can be affected by the operating system, hardware, other processes, and, in containerized environments, restrictions imposed by cgroups.

Considerations for tuning. A larger heap reduces the frequency of garbage collection but can increase pause times when GC does occur, while a smaller heap may lead to more frequent collection and potential pauses. Setting -Xms close to -Xmx can reduce heap expansion overhead. Availability of 512 MB may also be limited on 32-bit JVMs or systems with tight address space, and on modern containers, memory limits can influence the effective heap size.

Relation to other memory areas. -Xmx specifically controls the Java heap. Other memory spaces used by the JVM, such as the metaspace (or permanent generation in older versions) and native memory, are not directly constrained by -Xmx.