Commit 729633c2 authored Mar 01, 2016 by Fam Zheng Committed by Paolo Bonzini Mar 07, 2016

exec: Introduce AddressSpaceDispatch.mru_section



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 reduces 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, because phys_page_find is
not called.  Before:

   2.35%  qemu-system-x86_64       [.] phys_page_find
   0.97%  qemu-system-x86_64       [.] address_space_translate_internal
   0.95%  qemu-system-x86_64       [.] address_space_translate
   0.55%  qemu-system-x86_64       [.] address_space_lookup_region

After:

   0.97%  qemu-system-x86_64       [.] address_space_translate_internal
   0.97%  qemu-system-x86_64       [.] address_space_lookup_region
   0.84%  qemu-system-x86_64       [.] address_space_translate

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1456813104-25902-8-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent 29cb533d

exec.c

+14 −2

Original line number	Diff line number	Diff line
		@@ -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.
		*/
		@@ -351,14 +352,25 @@ 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] &&
		section_covers_addr(section, 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;
		}