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

[Igor Mammedov]

On Fri, 14 Nov 2014 10:57:10 +0100
Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:

> 
> 
> 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 <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;
> > +
> 
> 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).
yes, slot must be updated even if number of pages haven't changed,
since other fields could hold updated values.

> 
> 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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