Commit 063584d4 authored by Igor Mammedov's avatar Igor Mammedov Committed by Paolo Bonzini
Browse files

kvm: memslots: replace heap sort with an insertion sort pass 



memslots is a sorted array.  When a slot is changed, heapsort (lib/sort.c)
would take O(n log n) time to update it; an optimized insertion sort will
only cost O(n) on an array with just one item out of order.

Replace sort() with a custom sort that takes advantage of memslots usage
pattern and the known position of the changed slot.

performance change of 128 memslots insertions 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: default avatarIgor Mammedov <imammedo@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 1d4e7e3c

virt/kvm/kvm_main.c

+28 −28
Original line number Diff line number Diff line
@@ -668,31 +668,37 @@ 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;



	sort(slots->memslots, KVM_MEM_SLOTS_NUM,

	      sizeof(struct kvm_memory_slot), cmp_memslot, NULL);

	if (new->npages == old->npages) {

		*old = *new;

		return;

	}



	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 +708,7 @@ 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);

	}

}

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