From what Linux kernel/libc version is Java Runtime.exec() safe with regards to memory?

Question

At work one of our target platforms is a resource constrained mini-server running Linux (kernel 2.6.13, custom distribution based on an old Fedora Core). The application is writ

Answer 1

This is pretty much the way *nix (and linux) have worked since the dawn of time(or atleat the dawn of mmus).

To create a new process on *nixes you call fork(). fork() creates a copy of the calling process with all its memory mappings, file descriptors, etc. The memory mappings are done copy-on-write so (in optimal cases) no memory is actually copied, only the mappings. A following exec() call replaces the current memory mapping with that of the new executable. So, fork()/exec() is the way you create a new process and that's what the JVM uses.

The caveat is with huge processes on a busy system, the parent might continue to run for a little while before the child exec()'s causing a huge amount of memory to be copied cause of the copy-on-write. In VMs , memory can be moved around a lot to facilitate a garbage collector which produces even more copying.

The "workaround" is to do what you've already done, create an external lightweight process that takes care of spawning new processes - or use a more lightweight approach than fork/exec to spawn processes (Which linux does not have - and would anyway require a change in the jvm itself). Posix specifies the posix_spawn() function, which in theory can be implemented without copying the memory mapping of the calling process - but on linux it isn't.