Which application languages and frameworks support this kind of dynamic memory allocation? For predictability in performance and throughput reasons we benchmark our Java applications on specific cpu and memory constraints and specific heap and memory settings. How would an app in a container suddenly give back ram? A garbage collected application may be able to do that by collecting garbage. Possibly. But others?

No idea about Java, but any C program will request memory using mmap, and may give it back using munmap. This doesn't work when the program is running inside a VM, but does work for containers (which are basically just regular processes).

How does this work in practice? If the application didn’t need this memory anymore because it is done with the work/data, shouldn’t it already have freed or munmapped it? Is there a signal that can be send to a process to free up and return memory?

With VM balloning, VMs are also able to claim and release memory to the hypervisor/host OS.

Not driven by the guest application they don't. It's frustrating that I actually work in this area and know what I'm talking about. I worked on Katacontainers back when it was Intel's Clear Containers in the late 2010s. So many people in this thread do not have a clue.

i couldn't understand the comment that "the application running in user space cannot request more memory" - can someone explain whats the point of memorybalooning anywhere if an application cannot signal when the system should actually provision physical memory from the 'baloon'

There isn't a point, that's the problem.

The systems administrator can use ballooning to give more memory to a VM before launching a new application. This avoids the need to shut down the VM to give it a new role.

There is still a benefit to ballooning support even if it's not exposed to userspace within the VM, because VMs aren't always used purely to host a single infinitely-long-lived application without outside intervention.