Re: [RFC PATCH 0/3] Weight-balanced binary tree + KVM growable memory slots using wbtree

[Avi Kivity <avi@redhat.com>]

On 02/23/2011 08:06 PM, Alex Williamson wrote:
> On Wed, 2011-02-23 at 15:12 +0200, Avi Kivity wrote:
> >  On 02/22/2011 08:54 PM, Alex Williamson wrote:
> >  >  This series introduces a new weight-balanced binary tree (wbtree) for
> >  >  general use.  It's largely leveraged from the rbtree, copying it's
> >  >  rotate functions, while introducing different rebalance and erase
> >  >  functions.  This tree is particularly useful for managing memory
> >  >  ranges, where it's desirable to have the most likely targets (the
> >  >  largest ranges) at the top of each subtree.
> >  >
> >  >  Patches 2&   3 go on to convert the KVM memory slots to a growable
> >  >  array and make use of wbtree for efficient managment.  Trying to
> >  >  exercise the worst case for this data structure, I ran netperf
> >  >  TCP_RR on an emulated rtl8139 NIC connected directly to the host
> >  >  via a tap.  Both qemu-kvm and the netserver on the host were
> >  >  pinned to optimal CPUs with taskset.  This series resulted in
> >  >  a 3% improvement for this test.
> >  >
> >
> >  In this case, I think most of the faults (at least after the guest was
> >  warmed up) missed the tree completely.
>
> Except for the mmio faults for the NIC, which will traverse the entire
> depth of that branch of the tree for every access.

That is exactly what I meant: most of the faults cause the search to 
fail.  What we want here is to cache the success/fail result of the 
search so we don't do it in the first place.

> >  In this case a weight balanced
> >  tree is hardly optimal (it is optimized for hits), so I think you'll see
> >  a bigger gain from the mmio fault optimization.  You'll probably see
> >  most of the gain running mmu intensive tests with ept=0.
>
> Right, the gain expected by this test is that we're only traversing 6-7
> tree nodes until we don't find a match, versus the full 32 entries of
> the original memslot array.  So it's effectively comparing worst case
> scenarios for both data structures.

If we optimized the linear list we'd sort it by size, descending, and 
limit it by the number of instantiated slots, which I think would beat 
the tree.

-- 
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