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

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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>
This commit is contained in:
Igor Mammedov 2014-11-13 23:00:13 +00:00 committed by Paolo Bonzini
parent 1d4e7e3c0b
commit 063584d443
1 changed files with 28 additions and 28 deletions
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56
virt/kvm/kvm_main.c
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@ -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);
}
}