[PATCH 3/5] kvm: search_memslots: add simple LRU memslot caching

Igor Mammedov
2014-12-01T17:29:40Z

In typical guest boot workload only 2-3 memslots are used
extensively, and at that it's mostly the same memslot
lookup operation.

Adding LRU cache improves average lookup time from
46 to 28 cycles (~40%) for this workload.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
include/linux/kvm_host.h | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 231dd94..1a37144 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -353,6 +353,7 @@ struct kvm_memslots {
struct kvm_memory_slot memslots[KVM_MEM_SLOTS_NUM];
/* The mapping table from slot id to the index in memslots[]. */
short id_to_index[KVM_MEM_SLOTS_NUM];
+ atomic_t lru_slot;
};

struct kvm {
@@ -790,12 +791,19 @@ static inline void kvm_guest_exit(void)
static inline struct kvm_memory_slot *
search_memslots(struct kvm_memslots *slots, gfn_t gfn)
{
- struct kvm_memory_slot *memslot;
+ int slot = atomic_read(&slots->lru_slot);
+ struct kvm_memory_slot *memslot = &slots->memslots[slot];
+
+ if (gfn >= memslot->base_gfn &&
+ gfn < memslot->base_gfn + memslot->npages)
+ return memslot;

kvm_for_each_memslot(memslot, slots)
if (gfn >= memslot->base_gfn &&
- gfn < memslot->base_gfn + memslot->npages)
+ gfn < memslot->base_gfn + memslot->npages) {
+ atomic_set(&slots->lru_slot, memslot - slots->memslots);
return memslot;
+ }

return NULL;
}
--
1.8.3.1