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

error compiling committee.c: too many arguments to function

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