Commit 092aa2fc authored by Alexey Kardashevskiy's avatar Alexey Kardashevskiy Committed by Paolo Bonzini
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memory: Share special empty FlatView 



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).  In an unrelated experiment with 112 non-virtio
devices on x86 ("-M pc"), only 4 FlatViews are alive, and about ~2000
are created at startup.

Signed-off-by: default avatarAlexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20170921085110.25598-16-aik@ozlabs.ru>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent e673ba9a

memory.c

+16 −2
Original line number Diff line number Diff line
@@ -317,6 +317,7 @@ static void flatview_unref(FlatView *view) 
{

    if (atomic_fetch_dec(&view->ref) == 1) {

        trace_flatview_destroy_rcu(view, view->root);

        assert(view->root);

        call_rcu(view, flatview_destroy, rcu);

    }

}
@@ -761,16 +762,19 @@ static MemoryRegion *memory_region_get_flatview_root(MemoryRegion *mr) 
                    }

                }

            }

            if (found == 0) {

                return NULL;

            }

            if (next) {

                mr = next;

                continue;

            }

        }



        break;

        return mr;

    }



    return mr;

    return NULL;

}



/* Render a memory topology into a list of disjoint absolute ranges. */
@@ -966,12 +970,22 @@ static void address_space_update_topology_pass(AddressSpace *as, 


static void flatviews_init(void)

{

    static FlatView *empty_view;



    if (flat_views) {

        return;

    }



    flat_views = g_hash_table_new_full(g_direct_hash, g_direct_equal, NULL,

                                       (GDestroyNotify) flatview_unref);

    if (!empty_view) {

        empty_view = generate_memory_topology(NULL);

        /* We keep it alive forever in the global variable.  */

        flatview_ref(empty_view);

    } else {

        g_hash_table_replace(flat_views, NULL, empty_view);

        flatview_ref(empty_view);

    }

}



static void flatviews_reset(void)