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authorIgor Mammedov <imammedo@redhat.com>2014-11-13 23:00:13 +0000
committerPaolo Bonzini <pbonzini@redhat.com>2014-11-14 10:49:04 +0100
commit063584d44377ebde5ebc6e99cedc1bc6561939d7 (patch)
treec7ce69f00f8b1edfd89ca621d51a590709b5f14f /virt
parent1d4e7e3c0bca747d0fc54069a6ab8393349431c0 (diff)
downloadlinux-063584d44377ebde5ebc6e99cedc1bc6561939d7.tar.gz
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: Igor Mammedov <imammedo@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'virt')
-rw-r--r--virt/kvm/kvm_main.c56
1 files changed, 28 insertions, 28 deletions
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 3a31ec6e396..c0c2202e6c4 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -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);
}
}