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Programming multiple cores

Programming multiple cores. Andrew Williams http://www.bolton.ac.uk/staff/adw1. Introduction. Cores Processes Threads Symmetric Multiprocessing (SMP) Asymmetric Multiprocessing. Processes versus threads. Processes MS Word Powerpoint Firefox Threads (within Firefox)

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Programming multiple cores

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  1. Games at Bolton Programming multiple cores Andrew Williams http://www.bolton.ac.uk/staff/adw1

  2. Games at Bolton Introduction • Cores • Processes • Threads • Symmetric Multiprocessing (SMP) • Asymmetric Multiprocessing

  3. Games at Bolton Processes versus threads • Processes • MS Word • Powerpoint • Firefox • Threads (within Firefox) • Read data from a website • Render page to screen

  4. Games at Bolton 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

  5. Games at Bolton 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

  6. Games at Bolton 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;

  7. Games at Bolton 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

  8. Games at Bolton 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

  9. Games at Bolton 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

  10. Games at Bolton 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?

  11. Games at Bolton Xbox360’s Xenon CPU Image reproduced from Ars Technica, op cit

  12. Games at Bolton 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?

  13. Games at Bolton Making use of Multiple Cores A game-tree in a two-person, perfect information game +16 α=+16 β=+inf –6 α=-6 β=+inf -9 α=-9 β=-6 -16 α=-16 β=-9 +12 α=+12 β=+inf +9 α=+6 β=+12 +16 α=+16 β=+inf +36 α=+36 β=+16 +12 +2 +5 +9 +3 +16 +36

  14. Games at Bolton Making use of Multiple Cores Q: How do we parallelize this process? Give me some options! +16 α=+16 β=+inf –6 α=-6 β=+inf -9 α=-9 β=-6 -16 α=-16 β=-9 +12 α=+12 β=+inf +9 α=+6 β=+12 +16 α=+16 β=+inf +36 α=+36 β=+16 +12 +2 +5 +9 +3 +16 +36

  15. 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! Games at Bolton Making use of Multiple Cores

  16. Games at Bolton 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?

  17. Games at Bolton 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?

  18. Games at Bolton 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?

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