Optimization for the virtio-balloon feature on the ARM platform

[Yangming]

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.

 

Best regards,

Qi