On Mon, Feb 20, 2023 at 10:15:37AM +0100, David Hildenbrand wrote: > 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.
And as any optimization, patches are welcome accompanied by a measurement showing it's a net improvement. > -- > Thanks, > > David / dhildenb
