[RFC] memory pressure detection in VMs using PSI mechanism for dynamically inflating/deflating VM memory

[David Hildenbrand]

>>>
>>> 1. This will be a native userspace daemon that will be running only in
>>> the Linux VM which will use virtio-mem driver that uses memory hotplug
>>> to add/remove memory. The VM (aka Secondary VM, SVM) will request for
>>> memory from the host which is Primary VM, PVM via the backend hypervisor
>>> which takes care of cross-VM communication.
>>>
>>> 2. This will be guest driver. This daemon will use PSI mechanism to
>>> monitor memory pressure to keep track of memory demands in the system.
>>> It will register to few memory pressure events and make an educated
>>> guess on when demand for memory in system is increasing.
>>
>> Is that running in the primary or the secondary VM?
> 
> The userspace PSI daemon will be running on secondary VM. It will talk
> to a kernel driver (running on secondary VM itself) via ioctl. This
> kernel driver will talk to slightly modified version of virtio-mem
> driver where it can call the virtio_mem_config_changed(virtiomem_device)
> function for resizing the secondary VM. So its mainly "guest driven" now.

Okay, thanks.

[...]

>>>
>>> This daemon is currently in just Beta stage now and we have basic
>>> functionality running. We are yet to add more flesh to this scheme to
>>
>> Good to hear that the basics are running with virtio-mem (I assume :) ).
>>
>>> make sure any potential risks or security concerns are taken care as
>>> well.
>>
>> It would be great to draw/explain the architecture in more detail.
> 
> We will be looking into solving any potential security concerns where
> hypervisor would restrict few actions of resizing of memory. Right now,
> we are experimenting to see if PSI mechanism itself can be used for ways
> of detecting memory pressure in the system and add memory to secondary
> VM when memory is in need. Taking into account all the latencies
> involved in the PSI scheme (i.e. time when one does malloc call till
> when extra memory gets added to SVM system). And wanted to know
> upstream's opinion on such a scheme using PSI mechanism for detecting
> memory pressure and resizing SVM accordingly.

One problematic thing is that adding memory to Linux by virtio-mem 
eventually consumes memory (e.g., the memmap), especially when having to 
to add a completely new memory block to Linux.

So if you're already under severe memory pressure, these allocations to 
bring up new memory can fail. The question is, if PSI can notify "early" 
enough such that this barely happens in practice.

There are some possible ways to mitigate:

1) Always keep spare memory blocks by virtio-mem added to Linux, that
    don't expose any memory yet. Memory from these block can be handed
    over to Linux without additional Linux allocations. Of course, they
    consume metadata, so one might want to limit them.

2) Implement memmap_on_memory support for virtio-mem. This might help in
    some setups, where the device block size is suitable.

Did you run into that scenario already during your experiments, and how 
did you deal with that?

-- 
Thanks,

David / dhildenb