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[Qemu-devel] [PATCH 3/3] memory: Share special empty FlatView
From: |
Paolo Bonzini |
Subject: |
[Qemu-devel] [PATCH 3/3] memory: Share special empty FlatView |
Date: |
Thu, 21 Sep 2017 14:07:51 +0200 |
From: Alexey Kardashevskiy <address@hidden>
This shares an cached empty FlatView among address spaces. The empty
FV is used every time when a root MR would render into a FV without
memory sections, which happens when MR or its children are not enabled
(or zero-sized, which isn't caught yet). 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 while the guest runs.
Signed-off-by: Alexey Kardashevskiy <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
---
memory.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff --git a/memory.c b/memory.c
index 3207ae55e2..cb369223b1 100644
--- a/memory.c
+++ b/memory.c
@@ -316,6 +316,7 @@ static void flatview_ref(FlatView *view)
static void flatview_unref(FlatView *view)
{
if (atomic_fetch_dec(&view->ref) == 1) {
+ 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. */
@@ -962,12 +966,22 @@ 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)
--
2.13.5