[Qemu-devel] [PATCH 6/6] exec: Introduce AddressSpaceDispatch.mru_section

[Fam Zheng <famz@redhat.com>]

Under heavy workloads the lookup will likely end up with the same
MemoryRegionSection from last time. Using a pointer to cache the result, like
ram_list.mru_block, significantly reduce computation cost of
address_space_translate.

During address space topology update, as->dispatch will be reallocated
so the pointer is invalidated automatically.

Perf reports a visible drop on the cpu usage.  Before:

   + 2.06% phys_page_find
   + 0.95% address_space_translate_internal
   + 0.80% address_space_translate

After:
   + 0.78% address_space_translate
   + 0.77% address_space_translate_internal
   + 0.69% address_space_lookup_region

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 exec.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/exec.c b/exec.c
index ad8b826..3232861 100644
--- a/exec.c
+++ b/exec.c
@@ -135,6 +135,7 @@ typedef struct PhysPageMap {
 struct AddressSpaceDispatch {
     struct rcu_head rcu;
 
+    MemoryRegionSection *mru_section;
     /* This is a multi-level map on the physical address space.
      * The bottom level has pointers to MemoryRegionSections.
      */
@@ -342,14 +343,26 @@ static MemoryRegionSection *address_space_lookup_region(AddressSpaceDispatch *d,
                                                         hwaddr addr,
                                                         bool resolve_subpage)
 {
-    MemoryRegionSection *section;
+    MemoryRegionSection *section = atomic_read(&d->mru_section);
     subpage_t *subpage;
+    bool update;
 
-    section = phys_page_find(d->phys_map, addr, d->map.nodes, d->map.sections);
+    if (section && section != &d->map.sections[PHYS_SECTION_UNASSIGNED] &&
+        range_covers_byte(section->offset_within_address_space,
+                          section->size.lo, addr)) {
+        update = false;
+    } else {
+        section = phys_page_find(d->phys_map, addr, d->map.nodes,
+                                 d->map.sections);
+        update = true;
+    }
     if (resolve_subpage && section->mr->subpage) {
         subpage = container_of(section->mr, subpage_t, iomem);
         section = &d->map.sections[subpage->sub_section[SUBPAGE_IDX(addr)]];
     }
+    if (update) {
+        atomic_set(&d->mru_section, section);
+    }
     return section;
 }
 
-- 
2.4.3