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

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Date:	Wed, 23 Feb 2011 11:06:35 -0700
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	mtosatti@...hat.com, xiaoguangrong@...fujitsu.com
Subject: Re: [RFC PATCH 0/3] Weight-balanced binary tree + KVM growable
 memory slots using wbtree

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.

Hopefully the followup with kernbench run with ept=0 show that there's
also still a benefit in the data match scenario.  The existing array
ends up being nearly optimal for memory hits since it registers memory
from 1M - 3.5G in slot0 and 4G - 10.5G in slot1.  For the tree, we jump
straight to the bigger slot.  I'll run one more set of kernbench tests
with the original code, just reversing slots 0&1 to see if we take much
of a hit from the tree overhead.  Thanks,

Alex

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