CPU hotplug has been supported for a long time. I once managed some Sun boxes that allowed replacing/upgrading CPUs without shutting down... They don't build em like that anymore.
Yes, but most workloads are fairly unprepared for this sadly. And they're really not ready for memory unplug. (I also miss the days of my multi socket boxes and plugging in CPUs and memory).
What do VM-guest memory-ballon drivers do right now when the host suddenly attempts to reserve more memory than the guest has free? I'd presume the kernel would just consider itself to be in an OOM condition, and start killing processes to free up the memory until it can return OK to the balloon driver, no?
Because, from what I understand, that's closer to the scenario we're talking about here: you're not abruptly yanking DIMMs (like physical memory hotplug); rather, you (the hypervisor) are gracefully letting the guest know that some memory is about to go away, and since you (the hypervisor) have your own virtual TLB, you can let the guest OS decide which "physical" memory (from its perspective) is going away, before it happens.
Linux and Windows have both supported it, but use tends to be at the fringes on mainframe/datacenter machines that are validated for it and so those paths aren't tested on a very wide variety of hardware and running applications. And adding CPUs and memory is one thing but removing is another.
CPU cores being hotplugged on & off was actually super common for a few years, and still is in a lot more devices than you'd expect.
It used to be a corner stone of power management on mobile devices. The Nexus 5, for example, would regularly run with just a single core online, hotplugging the other 3 off until hit with a load and then brought cores back online 1 by 1 as needed.
That behavior still is in some corners of the mobile world, but increasingly less so.
So the CPU hotplug path is as a result actually a lot more battle hardened than you'd expect, and a lot more consumer software than you'd think ran just fine in that setup without noticing.
Yes, but most workloads are fairly unprepared for this sadly. And they're really not ready for memory unplug. (I also miss the days of my multi socket boxes and plugging in CPUs and memory).