Re: [Qemu-devel] [PATCH qemu v4 15/18] memory: Share special empty FlatView

[Paolo Bonzini]

On 20/09/2017 13:46, Alexey Kardashevskiy wrote:
> This shares an cached empty FlatView among address spaces. The empty
> FV is used every time when a root MR renders into a FV without memory
> sections which happens when MR or its children are not enabled or
> zero-sized. The empty_view is not NULL to keep the rest of memory
> API intact; it also has a dispatch tree for the same reason.
> 
> On POWER8 with 255 CPUs, 255 virtio-net, 40 PCI bridges guest this halves
> the amount of FlatView's in use (557 -> 260) and dispatch tables
> (~800000 -> ~370000), however the total memory footprint is pretty much
> the same as RCU is holding all these temporary FVs which are created
> (and then released) to make sure that they are empty and can be replaced
> with @empty_view.
> 
> Signed-off-by: Alexey Kardashevskiy <address@hidden>
> ---
>  memory.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/memory.c b/memory.c
> index 4add0fd030..92b1304a20 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -48,6 +48,7 @@ static QTAILQ_HEAD(, AddressSpace) address_spaces
>      = QTAILQ_HEAD_INITIALIZER(address_spaces);
>  
>  static GHashTable *flat_views;
> +static FlatView *empty_view;
>  
>  typedef struct AddrRange AddrRange;
>  
> @@ -755,6 +756,19 @@ static FlatView *generate_memory_topology(MemoryRegion 
> *mr)
>      }
>      flatview_simplify(view);
>  
> +    if (!view->nr) {
> +        flatview_unref(view);

This can be changed to flatview_destroy directly to avoid overloading
RCU with all these temporary FlatViews.

Paolo

> +        if (!empty_view) {
> +            empty_view = flatview_new(NULL);
> +        }
> +        view = empty_view;
> +        flatview_ref(view);
> +    }
> +
> +    if (view->dispatch) {
> +        return view;
> +    }
> +
>      view->dispatch = address_space_dispatch_new(view);
>      for (i = 0; i < view->nr; i++) {
>          MemoryRegionSection mrs =
>