Why doesn't VirtualBox allocate memory from the host's swap?

[CutterX]

Hi,

When I launch a VM and there isn't enough free memory on the host, VirtualBox pauses the VM and gives an error. I've noticed that the host's swap file isn't used to allocate the missing memory. Why?

[socratis]

Because actual, physical, non-swappable memory must be used.

[ChipMcK]

so that the guest OS may do its own paging/swapping if it is capable

[CutterX]

Because actual, physical, non-swappable memory must be used.

Please could you elaborate? I would be interested in knowing why.

[socratis]

There's nothing to elaborate on. The memory needs to be locked in order to reduce latency and to prevent paging and swapping, which are really, really slow processes.

If your host runs out of memory, close some applications on the host, buy more RAM for the host, or reduce the amount of RAM assigned to the guest.

Read the following threads. They actually deal with the same problem as yours:

• Reasons of vm to get into "aborted" state,
• Virtual box and memory management, and most importantly
• What happens if I exceed my RAM? (especially Ramshankar's answer).

All three of them are a great reading.

[CutterX]

Thank you both for your answers.
The threads you've linked discuss immediate versus gradual memory allocation, but they don't touch the subject of swapping. Sure, swapping is slower but isn't it better than crashing the VM?

so that the guest OS may do its own paging/swapping if it is capable

It shouldn't prevent the guest from swapping, even though I assume it could be very slow to swap on both the host and the guest simultaneously. Just a hypothesis.

[socratis]

The threads that I linked to, talk about memory allocation in general, do not simply focus on the "when", but focus on the "why", the kind of memory that needs to be allocated.

What I was hoping that you'd get from those threads is that if you want to prevent your VM crashing mid-running, would be to force the allocation of the RAM when the VM starts, and not rely on the lazy memory allocation algorithm. That way you will know from the get-go that you don't have enough RAM on your host.

And no, swapping is not an option. There are some processes that require specific features from the allocated memory; VirtualBox requires the memory to be available and non-swappable.