> And it’s straightforward to switch to jemalloc, which often improves on the system allocator.
It is true that jemalloc is much better than the standard malloc, but it still suffers from memory fragmentation, although typically a program can run for much longer with it.
> It’s the operating system’s job to manage fragmentation in the allocator, right?
No, not really. The operating system does nothing with the memory space of the process, other than providing syscalls for giving the process more pages.
> Wouldn’t this be a criticism of any long-running program which uses dynamic allocation, not only Rust?
What some thicker runtimes such as JVM do is that 1) they track all pointers present in the data used by the program and do not allow programs to cast them to raw integers (or back); 2) they can defragment the heap by moving the memory chunks around to fill in the gaps and at the same time they rewrite all pointers that point to moved chunks so that they still point to the same data.
It is true that jemalloc is much better than the standard malloc, but it still suffers from memory fragmentation, although typically a program can run for much longer with it.
> It’s the operating system’s job to manage fragmentation in the allocator, right?
No, not really. The operating system does nothing with the memory space of the process, other than providing syscalls for giving the process more pages.
> Wouldn’t this be a criticism of any long-running program which uses dynamic allocation, not only Rust?
What some thicker runtimes such as JVM do is that 1) they track all pointers present in the data used by the program and do not allow programs to cast them to raw integers (or back); 2) they can defragment the heap by moving the memory chunks around to fill in the gaps and at the same time they rewrite all pointers that point to moved chunks so that they still point to the same data.