java - Do the -Xms and -Xmx flags reserve the machine's resources?

Question

I know that the -Xms flag of JVM process is to allow the JVM process to use a specific amount of memory to initialize its process. And in regard to performance of a Java application, it is often recommended to set the same values to both -Xms and -Xmx when starting the application, like -Xms2048M -Xmx2048M.

I'm curious whether the -Xms and -Xmx flags mean that the JVM process makes a reservation for the specific amount of memory to prevent other processes in the same machine from using it.

Is this right?

Answer 1

Xmx merely reserves virtual address space. Xms actually allocates (commits) it but does not necessarily prefault it.

How operating systems respond to allocations varies.

Windows does allow you to reserve very large chunks of address space (Xmx) but will not allow overcommit (Xms). The limit is defined by swap + physical. The exception are large pages (which need to be enabled with a group policy setting), which will limit it by physical ram.

Linux behavior is more complicated, it depends on the vm.overcommit_memory and related sysctls and various flags passed to the mmap syscall, which to some extent can be controlled by JVM configuration flags. The behavior can range from a) Xms can exceed total ram + swap to b) Xmx is capped by available physical ram.