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.
> 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.