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memory: Share special empty FlatView

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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: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20170921085110.25598-16-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Alexey Kardashevskiy 2017-09-21 18:51:07 +10:00 committed by Paolo Bonzini
parent e673ba9af9
commit 092aa2fc65
1 changed files with 16 additions and 2 deletions
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18
memory.c
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@ -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)