Subject: Re: [RFC PATCH 0/3] Weight-balanced binary tree + KVM growable
 memory slots using wbtree
From: Alex Williamson <alex.williamson@redhat.com>
To: Avi Kivity <avi@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, mtosatti@redhat.com,
        xiaoguangrong@cn.fujitsu.com
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Date: Wed, 23 Feb 2011 12:28:52 -0700
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On Wed, 2011-02-23 at 11:06 -0700, 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.
> 
> > 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,

I had forgotten about <1M mem, so actually the slot configuration was:

0: <1M
1: 1M - 3.5G
2: 4G+

I stacked the deck in favor of the static array (0: 4G+, 1: 1M-3.5G, 2:
<1M), and got these kernbench results:

            base (stdev)    reorder (stdev)   wbtree (stdev)
--------+-----------------+----------------+----------------+
Elapsed |  42.809 (0.19)  |  42.160 (0.22) |  42.305 (0.23) |
User    | 115.709 (0.22)  | 114.358 (0.40) | 114.720 (0.31) |
System  |  41.605 (0.14)  |  40.741 (0.22) |  40.924 (0.20) |
%cpu    |   366.9 (1.45)  |   367.4 (1.17) |   367.6 (1.51) |
context |  7272.3 (68.6)  |  7248.1 (89.7) |  7249.5 (97.8) |
sleeps  | 14826.2 (110.6) | 14780.7 (86.9) | 14798.5 (63.0) |

So, wbtree is only slightly behind reordering, and the standard
deviation suggests the runs are mostly within the noise of each other.
Thanks,

Alex


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