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

[Alexey Kardashevskiy]

On 21/09/17 03:13, Paolo Bonzini wrote:
> 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.


Yeah, this or just allocate every new FlatView on the stack first, I
thought about this a second after I posted this.


> 
> 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 =
>>
> 


-- 
Alexey