Re: Optimization for the virtio-balloon feature on the ARM platform

[David Hildenbrand]

On 20.02.23 02:33, Yangming via wrote:
> Dear QEMU maintainers,
>
> I am writing to discuss a possible optimization for the virtio-balloon 
> feature on the ARM platform. The ‘virtio_balloon_set_config’ function is 
> called frequently during the balloon inflation process, and its 
> subfunction ‘get_current_ram_size’ needs to traverse the virtual 
> machine's memory modules in order to count the current virtual machine's 
> memory (i.e initial ram size + hotplugged memory). This can be very time 
> consuming on the ARM platform, as the ARM virtual machine has much more 
> complex memory modules than the x86 virtual machine.
>
> Therefore, I suggest introducing a global variable, ‘total_ram_size’, 
> that would be updated only when the balloon is initialized and hotplug 
> memory has completed. This would increase the efficiency of balloon 
> inflation by more than 60% on the ARM platform.
>
> The following code is part of the optimization for balloon:
>
> --- a/qemu/hw/virtio/virtio-balloon.c
>
> +++ b/qemu/hw/virtio/virtio-balloon.c
>
> static void virtio_balloon_set_config(…)
>
> …
>
> -    ram_addr_t vm_ram_size = get_current_ram_size();
>
> +   ram_addr_t vm_ram_size = total_ram_size;
>
> …
>
> I hope this suggestion could be considered or discussed by QEMU 
> developers. I would love to seeing this improvement added to QEMU in the 
> future.

I'd suggest keeping track of the plugged DIMM size inside 
ms->device_memory->dimm_size. We can update it from 
pc_dimm_plug/pc_dimm_unplug. We just have to make sure to exclude NVDIMMs.

We can then optimize get_current_ram_size() to return "ms->ram_size + 
ms->device_memory->dimm_size", of course taking care of 
ms->device_memory == NULL on some machines.

--
Thanks,

David / dhildenb