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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

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 <aik@ozlabs.ru>
---
 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);
+        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 =
-- 
2.11.0