[PATCH v2] kvm: memslots: replace heap sort with insertion sort

[Igor Mammedov]

memslots is a sorted array, when slot changes in it
with current heapsort it would take O(n log n) time
to update array, while using insertion sort like
algorithm on array with 1 item out of order will
take only O(n) time.

Replace current heapsort with custom sort that
takes advantage of memslots usage pattern and known
position of changed slot.

performance change of 128 memslots insersions with
gradually increasing size (the worst case):
      heap sort   custom sort
max:  249747      2500 cycles
with custom sort alg taking ~98% less then original
update time.

Signed-off-by: Igor Mammedov <imammedo@xxxxxxxxxx>
---
v2:
  - replace swap with slot shift, improves result 2x
  - reprofile original/swap based and swapless 15 times
      discarding spikes swap based takes ~5900 cycles max
      and swapless ~2500 cycles.
---
 virt/kvm/kvm_main.c | 54 ++++++++++++++++++++++++++---------------------------
 1 file changed, 26 insertions(+), 28 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 25ffac9..49f896a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -668,31 +668,35 @@ static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
 	return 0;
 }
 
-static int cmp_memslot(const void *slot1, const void *slot2)
-{
-	struct kvm_memory_slot *s1, *s2;
-
-	s1 = (struct kvm_memory_slot *)slot1;
-	s2 = (struct kvm_memory_slot *)slot2;
-
-	if (s1->npages < s2->npages)
-		return 1;
-	if (s1->npages > s2->npages)
-		return -1;
-
-	return 0;
-}
-
 /*
- * Sort the memslots base on its size, so the larger slots
- * will get better fit.
+ * Insert memslot and re-sort memslots based on their size,
+ * so the larger slots will get better fit. Sorting algorithm
+ * takes advantage of having initially sorted array and
+ * known changed memslot position.
  */
-static void sort_memslots(struct kvm_memslots *slots)
+static void insert_memslot(struct kvm_memslots *slots,
+			   struct kvm_memory_slot *new)
 {
-	int i;
+	int i = slots->id_to_index[new->id];
+	struct kvm_memory_slot *old = id_to_memslot(slots, new->id);
+	struct kvm_memory_slot *mslots = slots->memslots;
+
+	if (new->npages == old->npages)
+		return;
 
-	sort(slots->memslots, KVM_MEM_SLOTS_NUM,
-	      sizeof(struct kvm_memory_slot), cmp_memslot, NULL);
+	while (1) {
+		if (i < (KVM_MEM_SLOTS_NUM - 1) &&
+			new->npages < mslots[i + 1].npages) {
+			mslots[i] = mslots[i + 1];
+			i++;
+		} else if (i > 0 && new->npages > mslots[i - 1].npages) {
+			mslots[i] = mslots[i - 1];
+			i--;
+		} else {
+			mslots[i] = *new;
+			break;
+		}
+	}
 
 	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
 		slots->id_to_index[slots->memslots[i].id] = i;
@@ -702,13 +706,7 @@ static void update_memslots(struct kvm_memslots *slots,
 			    struct kvm_memory_slot *new)
 {
 	if (new) {
-		int id = new->id;
-		struct kvm_memory_slot *old = id_to_memslot(slots, id);
-		unsigned long npages = old->npages;
-
-		*old = *new;
-		if (new->npages != npages)
-			sort_memslots(slots);
+		insert_memslot(slots, new);
 	}
 }
 
-- 
1.8.3.1

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