On 14/11/2014 00:00, Igor Mammedov wrote:
> 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 <imamm...@redhat.com>
> ---
> 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;
> +
It is important to leave the "*old = *new" assignment here, see the
comment in __kvm_set_memory_region:
/*
* We can re-use the old_memslots from above, the only difference
* from the currently installed memslots is the invalid flag. This
* will get overwritten by update_memslots anyway.
*/
This small problem was already present in v1, but I didn't notice it
yesterday. With the new code, we can add it inside this if:
> + if (new->npages == old->npages)
> + return;
Do you agree? (No need to send v3).
Paolo
> - 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);
> }
> }
>
>
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