Pitfalls of Object Oriented Programming by SONY

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Sony Computer Entertainment Europe
Research & Development Division

Pitfalls of Object Oriented Programming
Tony Albrecht ...

What I will be covering

• A quick look at Object Oriented (OO)
programming
• A common example
• Optimisation of that exam...

Object Oriented (OO) Programming
•

What is OO programming?
–

•

a programming paradigm that uses "objects" – data struct...

What’s OOP for?
• OO programming allows you to think about
problems in terms of objects and their
interactions.
• Each obj...

Objects
• If objects are self contained then they can be
– Reused.
– Maintained without side effects.
– Used without under...

Are Objects Good?
• Well, yes
• And no.
• First some history.

Slide 6

A Brief History of C++

C++ development started

1979

2009

Slide 7

A Brief History of C++

Named “C++”

1979 1983

2009

Slide 8

A Brief History of C++

First Commercial release

1979

1985

2009

Slide 9

A Brief History of C++

Release of v2.0

1979

1989

2009

Slide 10

A Brief History of C++
Added
• multiple inheritance,
• abstract classes,
• static member functions,
Release of v2.0
• cons...

A Brief History of C++

Standardised

1979

1998

Slide 12

2009

A Brief History of C++

Updated

1979

2003

Slide 13

2009

A Brief History of C++

C++0x

1979

2009

Slide 14

?

So what has changed since 1979?
• Many more features have
been added to C++
• CPUs have become much
faster.
• Transition t...

CPU performance

Slide 16

Computer architecture: a quantitative approach
By John L. Hennessy, David A. Patterson, Andrea ...

CPU/Memory performance

Slide 17

Computer architecture: a quantitative approach
By John L. Hennessy, David A. Patterson, ...

What has changed since 1979?
• One of the biggest changes is that memory
access speeds are far slower (relatively)
– 1980:...

What has this to do with OO?

• OO classes encapsulate code and data.
• So, an instantiated object will generally
contain ...

My Claim

• With modern HW (particularly consoles),
excessive encapsulation is BAD.
• Data flow should be fundamental to y...

Consider a simple OO Scene Tree
•

Base Object class
– Contains general data

•

Node
– Container class

•

Modifier
– Upd...

Object

• Each object
–
–
–
–

Maintains bounding sphere for culling
Has transform (local and world)
Dirty flag (optimisat...

Objects
Class Definition

Each square is
4 bytes

Slide 23

Memory Layout

Nodes

• Each Node is an object, plus
– Has a container of other objects
– Has a visibility flag.

Slide 24

Nodes
Memory Layout

Class Definition

Slide 25

Consider the following code…

• Update the world transform and world
space bounding sphere for each object.

Slide 26

Consider the following code…

• Leaf nodes (objects) return transformed
bounding spheres

Slide 27

Consider the following code…

• Leaf nodes (objects)this
What‟s wrong with return transformed
bounding code?
spheres

Slid...

Consider the following code…

• Leaf nodesm_Dirty=false thenreturn transformed
If (objects) we get branch
bounding mispred...

Consider the following code…

• Leaf nodes (objects) return transformed
of
bounding Calculation12 cycles. bounding sphere
...

Consider the following code…

• Leaf nodes (objects) return transformed
bounding So using a dirty using oneis actually
sph...

Lets illustrate cache usage
Main Memory

Each cache line is
128 bytes

Slide 32

L2 Cache

Cache usage
Main Memory

L2 Cache
parentTransform is already
in the cache (somewhere)

Slide 33

Cache usage
Main Memory

Assume this is a 128byte
boundary (start of cacheline)

Slide 34

L2 Cache

Cache usage
Main Memory

L2 Cache

Load m_Transform into cache

Slide 35

Cache usage
Main Memory

L2 Cache

m_WorldTransform is stored via
cache (write-back)
Slide 36

Cache usage
Main Memory

L2 Cache

Next it loads m_Objects
Slide 37

Cache usage
Main Memory

L2 Cache

Then a pointer is pulled from
somewhere else (Memory
managed by std::vector)

Slide 38

Cache usage
Main Memory

L2 Cache

vtbl ptr loaded into Cache

Slide 39

Cache usage
Main Memory

L2 Cache

Look up virtual function

Slide 40

Cache usage
Main Memory

L2 Cache

Then branch to that code
(load in instructions)

Slide 41

Cache usage
Main Memory

L2 Cache

New code checks dirty flag then
sets world bounding sphere

Slide 42

Cache usage
Main Memory

L2 Cache

Node‟s World Bounding Sphere
is then Expanded

Slide 43

Cache usage
Main Memory

L2 Cache

Then the next Object is
processed
Slide 44

Cache usage
Main Memory

L2 Cache

First object costs at least 7
cache misses
Slide 45

Cache usage
Main Memory

L2 Cache

Subsequent objects cost at least
2 cache misses each

Slide 46

The Test
• 11,111 nodes/objects in a
tree 5 levels deep
• Every node being
transformed
• Hierarchical culling of tree
• Re...

Performance
This is the time
taken just to
traverse the tree!

Slide 48

Why is it so slow?
~22ms

Slide 49

Look at GetWorldBoundingSphere()

Slide 50

Samples can be a little
misleading at the source
code level

Slide 51

if(!m_Dirty) comparison

Slide 52

Stalls due to the load 2
instructions earlier

Slide 53

Similarly with the matrix
multiply

Slide 54

Some rough calculations
Branch Mispredictions: 50,421 @ 23 cycles each ~= 0.36ms

Slide 55

Some rough calculations
Branch Mispredictions: 50,421 @ 23 cycles each ~= 0.36ms
L2 Cache misses: 36,345 @ 400 cycles each...

• From Tuner, ~ 3 L2 cache misses per
object
– These cache misses are mostly sequential
(more than 1 fetch from main memor...

Slow memory is the problem here

• How can we fix it?
• And still keep the same functionality and
interface?

Slide 58

The first step

• Use homogenous, sequential sets of data

Slide 59

Homogeneous Sequential Data

Slide 60

Generating Contiguous Data
• Use custom allocators
– Minimal impact on existing code

• Allocate contiguous
– Nodes
– Matr...

Performance
19.6ms -> 12.9ms
35% faster just by
moving things around in
memory!

Slide 62

What next?
• Process data in order
• Use implicit structure for hierarchy
– Minimise to and fro from nodes.

• Group logic...

We start with
a parent Node

Hierarchy
Node

Slide 64

Hierarchy
Node

Node

Which has children
nodes

Slide 65

Node

Hierarchy
Node

Node

Node

And they have a
parent

Slide 66

Hierarchy
Node

Node

Node

Node

Node

Node

Node

Node

And they have
children
Slide 67

Node

Node

Node

Hierarchy
Node

Node

Node

Node

Node

Node

Node

Node

And they all have
parents
Slide 68

Node

Node

Node

Hierarchy
Node

Node

Node

Node Node Node
Node
Node

Node

Node

Node NodeNode
Node
Node

A lot of this
information can b...

Hierarchy
Node

Node

Node

Node

Node

Use a set of arrays,
one per hierarchy
level

Node

Node

Node

Slide 70

Node

No...

Hierarchy
Node

Parent has 2
children

:2

Node

Node
:4

:4

Node

Children have 4
children

Node

Node

Node

Node

Slid...

Hierarchy
Ensure nodes and their
data are contiguous in
memory

Node
:2

Node

Node
:4

:4

Node

Node

Node

Node

Node

...

• Make the processing global rather than
local
– Pull the updates out of the objects.
• No more virtuals

– Easier to unde...

Need to change some things…

• OO version
– Update transform top down and expand WBS
bottom up

Slide 74

Update
transform

Node

Node

Node

Node

Node

Node

Node

Slide 75

Node

Node

Node

Node

Update
transform

Node

Node

Node

Node

Node

Node

Node

Slide 76

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Update transform and
world bounding sphere
Slide 77

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Add bounding sphere
of child
Slide 78

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Update transform and
world bounding sphere
Slide 79

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Add bounding sphere
of child
Slide 80

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Update transform and
world bounding sphere
Slide 81

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Node

Add bounding sphere
of child
Slide 82

Node

Node

Node

Node

• Hierarchical bounding spheres pass info up
• Transforms cascade down
• Data use and code is „striped‟.
– Processing is a...

Conversion to linear

• To do this with a „flat‟ hierarchy, break it
into 2 passes
– Update the transforms and bounding
sp...

Transform and BS updates
Node
:2

Node

Node
:4

:4

Node

Node

Node

For each node at each level (top down)
{
multiply w...

Update bounding sphere hierarchies
Node
:2

Node

Node
:4

:4

Node

Node

Node

For each node at each level (bottom up)
{...

Update Transform and Bounding Sphere
How many children nodes
to process

Slide 87

Update Transform and Bounding Sphere

For each child, update
transform and bounding
sphere
Slide 88

Update Transform and Bounding Sphere

Note the contiguous arrays

Slide 89

So, what’s happening in the cache?
Parent

Unified L2 Cache
Children node
Children‟s
data not needed
Parent Data

Children...

Load parent and its transform
Parent

Unified L2 Cache

Parent Data

Childrens‟ Data
Slide 91

Load child transform and set world transform
Parent

Unified L2 Cache

Parent Data

Childrens‟ Data
Slide 92

Load child BS and set WBS
Parent

Unified L2 Cache

Parent Data

Childrens‟ Data
Slide 93

Load child BS and set WBS
Parent
Next child is calculated with
no extra cache misses !

Parent Data

Childrens‟ Data
Slide...

Load child BS and set WBS
Parent next 2 children incur 2
The
cache misses in total

Parent Data

Childrens‟ Data
Slide 95
...

Prefetching
Parent
Because all data is linear, we
can predict what memory will
be needed in ~400 cycles
and prefetch
Paren...

• Tuner scans show about 1.7 cache misses
per node.
• But, these misses are much more frequent
– Code/cache miss/cache mis...

Performance
19.6 -> 12.9 -> 4.8ms

Slide 98

Prefetching
• Data accesses are now predictable
• Can use prefetch (dcbt) to warm the cache
– Data streams can be tricky
–...

Prefetching example

• Prefetch a predetermined number of
iterations ahead
• Ignore incorrect prefetches

Slide 100

Performance
19.6 -> 12.9 -> 4.8 -> 3.3ms

Slide 101

A Warning on Prefetching

• This example makes very heavy use of the
cache
• This can affect other threads‟ use of the
cac...

The old scan
~22ms

Slide 103

The new scan
~16.6ms

Slide 104

Up close
~16.6ms

Slide 105

Looking at the code (samples)

Slide 106

Performance counters
Branch mispredictions: 2,867 (cf. 47,000)
L2 cache misses: 16,064
(cf 36,000)

Slide 107

In Summary

• Just reorganising data locations was a win
• Data + code reorganisation= dramatic
improvement.
• + prefetchi...

OO is not necessarily EVIL
• Be careful not to design yourself into a corner
• Consider data in your design
– Can you deco...

Its all about the memory

• Optimise for data first, then code.
– Memory access is probably going to be your
biggest bottl...

Homogeneity

• Keep code and data homogenous
– Avoid introducing variations
– Don‟t test for exceptions – sort by them.

•...

Remember

• You are writing a GAME
– You have control over the input data
– Don‟t be afraid to preformat it – drastically ...

Data Oriented Design Delivers

•
•
•
•

Better performance
Better realisation of code optimisations
Often simpler code
Mor...

The END

• Questions?

Slide 114

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