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
