linux-kernel - Re: [PATCH 1/3] kvm: memslots: track id_to_index changes during the insertion sort

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Date:	Fri, 14 Nov 2014 15:17:25 +0100
From:	Igor Mammedov <imammedo@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, yoshikawa_takuya_b1@....ntt.co.jp
Subject: Re: [PATCH 1/3] kvm: memslots: track id_to_index changes during the
 insertion sort

On Fri, 14 Nov 2014 14:35:00 +0100
Radim Krčmář <rkrcmar@...hat.com> wrote:

> 2014-11-14 12:12+0100, Paolo Bonzini:
> > This completes the optimization from the previous patch, by
> > removing the KVM_MEM_SLOTS_NUM-iteration loop from insert_memslot.
> > 
> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> > ---
> >  virt/kvm/kvm_main.c | 39 +++++++++++++++++++--------------------
> >  1 file changed, 19 insertions(+), 20 deletions(-)
> > 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index c0c2202e6c4f..c8ff99cc0ccb 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -677,31 +677,30 @@ static int kvm_create_dirty_bitmap(struct
> > kvm_memory_slot *memslot) static void insert_memslot(struct
> > kvm_memslots *slots, struct kvm_memory_slot *new)
> >  {
> > -	int i = slots->id_to_index[new->id];
> > -	struct kvm_memory_slot *old = id_to_memslot(slots,
> > new->id);
> > +	int id = new->id;
> > +	int i = slots->id_to_index[id];
> >  	struct kvm_memory_slot *mslots = slots->memslots;
> >  
> > -	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;
> > +	WARN_ON(mslots[i].id != id);
> > +	if (new->npages != mslots[i].npages) {
> > +		while (1) {
> > +			if (i < (KVM_MEM_SLOTS_NUM - 1) &&
> > +    			    new->npages < mslots[i +
> > 1].npages) {
>   (^^^^ whitespace error)
> > +				mslots[i] = mslots[i + 1];
> > +				slots->id_to_index[mslots[i].id] =
> > i;
> > +				i++;
> > +			} else if (i > 0 &&
> > +				   new->npages > mslots[i -
> > 1].npages) {
> > +				mslots[i] = mslots[i - 1];
> > +				slots->id_to_index[mslots[i].id] =
> > i;
> > +				i--;
> > +			} else
> > +				break;
> 
> We are replacing in a sorted array, so the the direction of our
> traversal doesn't change, (and we could lose one tab level here,)
> 
> 	if (new->npages < mslots[i].npages) {
> 		while (i < (KVM_MEM_SLOTS_NUM - 1) &&
> 		       new->npages < mslots[i + 1].npages) {
> 			mslots[i] = mslots[i + 1];
> 			slots->id_to_index[mslots[i].id] = i;
> 			i++;
> 		}
> 	else if (new->npages > mslots[i].npages)
> 		while (i > 0 &&
> 		       new->npages > mslots[i - 1].npages) {
> 			mslots[i] = mslots[i - 1];
> 			slots->id_to_index[mslots[i].id] = i;
> 			i--;
> 		}
> 
> (I guess you don't want me to abstract these two loops further :)
> 
> If the probability of slots with same npages was high, we could also
> move just the last one from each group, but I think that the current
> algorithm is already faster than we need.
> 
> (We'll have to change it into an interval tree, or something, if the
>  number of slots rises anyway.)
Only if it rises to huge amount, I've played with proposed 512 memslots
and it takes ~10000 cycles which is 5% of current heapsort overhead.
Taking in account that it's slow path anyway, it's unlikely that there
would be need to speedup this case even more.

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