Games at Bolton

Programming multiple cores

Andrew Williams

http://www.bolton.ac.uk/staff/adw1

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Introduction

Cores Processes Threads Symmetric Multiprocessing (SMP) Asymmetric Multiprocessing

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Processes versus threads

Processes– MS Word– Powerpoint– Firefox

Threads (within Firefox)– Read data from a website– Render page to screen

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Processes versus Threads

Processes usually independent of each other

Every process contains at least one thread (… of execution)

Threads tend to be doing different aspects of the same task

Processes have separate, protected address spaces and call stacks– Context switches tend to be slow

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Processes versus Threads

Processes communicate through operating system calls– Inter-process communication– Typically a socket or a pipe– My chess program communicates with

its GUI like this

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Inter-process communication - Xboard Launch command (shell command on Unix):

xboard -fcp "./postmodernist xboard" -size Middling -animateMoving False -tc 1400 -inc 0 -td 0.6 -showCoords True -showThinking True $1 $2 $3 $4 $5

Checking for input from Xboard (from xbd.c):retValue = PeekNamedPipe(stdInputHandle,

NULL, 0, NULL, &dw, NULL);if(!retValue || dw == 0)

return 0;else

return 1;

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Multiple CPU motherboards

“Back in the day” approach Multiple CPUs on the motherboard Two, four, even eight CPU boards Very popular in computer chess

circles!

Dual-Celeron MB from http://tinyurl.com/392zwu

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Multiple CPU motherboards

These were SMP Problems with this approach:

– Communication between CPUs• Information transferred over slower, external

bus

– Complex, expensive motherboards– Needed to buy two (or four or eight)

CPUs

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Modern approaches

On 26/9/2007, Dell’s cheapest model for home customers was the Inspiron 531 (£269 without display)– This features an AMD® Athlon™ 64 X2

Dual Core Processor 3800+ Modern consumer CPUs have

multiple cores:– Intel Core 2 Duo– AMD X2

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Multiple Cores in Consoles

We are used to this PS2 used the Emotion engine

– Q: Symmetric or Asymmetric? XBox360’s CPU is an IBM-designed

triple-core product– See Ars Technica’s coverage

PS3 uses the CELL chip– Q: Symmetric or Asymmetric?

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Xbox360’s Xenon CPU

Image reproduced from Ars Technica, op cit

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Xbox360’s Xenon CPU

Questions:– Is the Xenon symmetric or asymmetric?– The marketing material for the Xenon

says: “Two hardware threads per core; six

hardware threads total”

– How is this achieved?

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Making use of Multiple Cores

–6α=-6

β=+inf

+16α=+16β=+inf

-9α=-9β=-6

+12α=+12β=+inf

+12 +2

+9α=+6β=+12

+5 +9

-16α=-16β=-9

+16α=+16β=+inf

+3 +16

+36α=+36β=+16

+36

A game-tree in a two-person, perfect information game

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Making use of Multiple Cores

–6α=-6

β=+inf

+16α=+16β=+inf

-9α=-9β=-6

+12α=+12β=+inf

+12 +2

+9α=+6β=+12

+5 +9

-16α=-16β=-9

+16α=+16β=+inf

+3 +16

+36α=+36β=+16

+36

Q: How do we parallelize this process? Give me some options!

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Making use of Multiple Cores

Typical game loop (pseudocode):Initialize_game();

while(!quit) {Get_user_input();

Do_ai();

Do_physics();

Render();

}

Shutdown_game();

Q: How do we parallelize that?

Give me some options!

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Task Parallelism

The question is, to what extent are (eg) physics and AI separate things?

Can we sensibly do them both at once?

What if one of them needs data produced by the other?– which is pretty likely if you think about it

even for a moment How up-to-date does this information

really need to be?

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Data parallelism

Suppose you have 200 NPCs running around the screen (eg in an RTS)

Divide them up among the available cores?

So four “spare” cores do the AI and physics for 50 NPCs each?– Simultaneously?– What if two of them bump into each other?

• Fine if they are both “on” the same core?

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Mixed approaches

What about a mixture between task parallelism and data parallelism?

We have six cores:– Two for our NPCs, so give each core 100– Two for rendering?– Two for game logic and user input?

Note that if you think like this, you’re committing yourself to the Xbox360 implementation– What about PS3 or PC or Wii?