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Source Multicore 1 November 2006
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New Source Features Dynamic Shadow Mapping New Foliage System Cinematic Physics Version 2 of Facial Animation System Dynamic Scripted Sequences Particle System 64-Bit Source Engine Reworked Character Lighting Model Companion AI (Alyx in Episode One) Real-Time statistics gathering Automatic game cache defragmentation Multicore
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Multiple Approches to multi-core games
New Features in Source Multiple Approches to multi-core games Source is implementing Hybrid-Threading Close to linear improvements Pretty but dumb era is ending, scalability we've seen in graphics will now apply to rest of the game Hybrid Threading Source will ship before Episode Two You'll get a version of this engine to take home to test with
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Most significant development since 3D cards
Multicore Most significant development since 3D cards Huge potential Huge challenge
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Most significant development since 3D cards
Multicore Most significant development since 3D cards Huge potential Huge challenge The decisions faced with multiple cores How we are using multiple cores
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Most significant development since 3D cards
Multicore Most significant development since 3D cards Huge potential Huge challenge The decisions faced with multiple cores How we are using multiple cores Four cores is more than twice as interesting as two cores
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Challenges
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Games always want 100% CPU utilization
Challenges Games always want 100% CPU utilization
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Games always want 100% CPU utilization Games are inherently serial
Challenges Games always want 100% CPU utilization Games are inherently serial
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Games always want 100% CPU utilization Games are inherently serial
Challenges Games always want 100% CPU utilization Games are inherently serial Decades of experience in single threaded optimization
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Games always want 100% CPU utilization Games are inherently serial
Challenges Games always want 100% CPU utilization Games are inherently serial Decades of experience in single threaded optimization Millions of lines of code written for single threading
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Strategies Threading model Threading framework Application of cores
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Fine grained threading Hybrid threading
Theading Models Single threading Coarse threading Fine grained threading Hybrid threading
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Single threading Easy Obsolete
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Put whole systems on cores Pretty easy, “multiple single threads”
Coarse threading Put whole systems on cores Pretty easy, “multiple single threads” Stay partially serialized, or double buffer
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Coarse threading: Early experimentation
Client User input Rendering Graphics simulation Server AI Physics Game logic
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Coarse threading: Early experimentation
Experiment: run client and server each on own core
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Coarse threading: Early experimentation
Experiment: run client and server each on own core Benefits: forced to confront systems that are not thread safe or not thread efficient
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Coarse threading: Early experimentation
Experiment: run client and server each on own core Benefits: forced to confront systems that are not thread safe or not thread efficient Outcome: Can approach 2x in contrived maps
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Coarse threading: Early experimentation
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Coarse threading: Early experimentation
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Coarse threading: Early experimentation
Experiment: run client and server each on own core Benefits: forced to confront systems that are not thread safe or not thread efficient Outcome: Can approach 2x in contrived maps More like 1.2x in real single player Added latency to single player game
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Coarse threading: Early experimentation
Experiment: run client and server each on own core Benefits: forced to confront systems that are not thread safe or not thread efficient Outcome: Can approach 2x in contrived maps More like 1.2x in real single player Added latency to single player game Opened door to improved listen servers
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Put whole systems on cores Pretty easy, “multiple single threads”
Coarse threading Put whole systems on cores Pretty easy, “multiple single threads” Stay partially serialized, or double buffer Scales poorly Partially idle cores Synchronization, or lag Entirely idle cores
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Fine grained threading
Divide many small identical tasks across cores E.g., take a loop that updates state of 1000 objects and perform 1000/N on each core for N cores Moderate difficulty Scales well… Tricky if cost of each unit is variable Memory bandwidth Limited problem domains
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Fine grained threading
Leverage multicore in production tools: VMPI
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Fine grained threading
VVIS – Visibility calculations VRAD – Lighting calculations
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Fine grained threading
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Performance tuned for mid-level work sharing
Hybrid threading Performance tuned for mid-level work sharing Not splitting sets of very small operations over cores Not putting whole systems onto cores
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Use the appropriate tool for the job
Hybrid threading Use the appropriate tool for the job Some systems on cores (e.g. sound) Some systems split internally similar to coarse Split expensive iterations across cores fine grained Queue some work to run when a core goes idle Most difficult Scales well Maximum core utilization
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Hybrid threading: Rendering
Rough pipeline Build world lists Build object lists Graphical simulation (particles, ropes, sprites) Update animations Compute shadows Draw Once for every “view” Player’s POV TV monitors Water reflections Many times CPU bound
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Hybrid threading: Rendering
Revised pipeline Construct scene rendering lists for multiple scenes in parallel (e.g., the world and its reflection in water) Overlap graphics simulation Compute character bone transformations for all characters in all scenes in parallel Compute shadows for all characters Allow multiple threads to draw in parallel Serialize drawing operations on another core
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Implementing Hybrid Threading Operating system: pools, synchronization
Threading Tools Implementing Hybrid Threading Operating system: pools, synchronization Compiler extensions: OpenMP, fine threading Tailored tools Programmers solve game development problems, not threading problems
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Operating system Too low level Prone to error Lots of stalling Unpredictable scheduling Unpredictable cost
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Focused on fine threading Lack of control Implementation interferes
Compiler extensions OpenMP Focused on fine threading Lack of control Implementation interferes
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Tailored tools: Game Threading Infrastructure
Custom work management system Aimed at gaming problems, intuitive for game programmers Focus on keeping cores busy Thread pool: N-1 threads for N cores Support hybrid threading Function threading Array parallelism Multiple work modes Opportunistic core utilization Queued core utilization
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Tailored tools: Game Threading Infrastructure
The simple thing is the worst thing “Lock-free” algorithms Never leave cores idle waiting on other cores Leverages atomic write primitives of the CPU Under the hood of all services and data structures See: free_algorithms Example: the spatial partition
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Application of cores
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Dual core CPUs: framerate
Application of cores Dual core CPUs: framerate
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Dual core CPUs: framerate Quad core CPUs: new experiences
Application of cores Dual core CPUs: framerate Quad core CPUs: new experiences
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Dual core CPUs: framerate Quad core CPUs: new experiences
Application of cores Dual core CPUs: framerate Quad core CPUs: new experiences Richer visuals Improved simulation Richer AI
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Application of cores: Particle Simulation
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Application of cores: Particle Simulation
Not simply a GPU issue Interactivity Presence
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Application of cores: Particle Simulation
Use cores to run multiple particle systems in parallel Individual particle systems using multiple cores
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Application of cores: Particle Simulation
More complicated systems Interactive particle systems Particles with gameplay implications Like rigid body physics, reinforce consistency of world
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Particle Simulation Benchmark
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Traditionally strict CPU limits
AI Traditionally strict CPU limits Interesting combinations of minimalist algorithms “What could we do if extra CPU were given to AI?”
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Better framerate AI Run AI in parallel with other systems
Parallel agent execution
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Example: Parallel Animation
AI Example: Parallel Animation
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Increased sophistication
AI Better framerate Increased sophistication Without hitching by asynchronously running on secondary cores
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Increased sophistication
AI Better framerate Increased sophistication Without hitching by asynchronously running on secondary cores Richer path finding: better world interactivity
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Increased sophistication
AI Better framerate Increased sophistication Without hitching by asynchronously running on secondary cores Richer path finding: more analysis for better world awareness Deeper tactical analysis Increased world examination “Out of band” AI
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Increased sophistication New kinds of AIs
Better framerate Increased sophistication New kinds of AIs Experimental creatures built with multicore in mind
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Integrate multicore across Valve’s business
Broader Goals Integrate multicore across Valve’s business
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Integrate multicore across Valve’s business
Broader Goals Integrate multicore across Valve’s business Boost developer effectiveness
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Integrate multicore across Valve’s business
Broader Goals Integrate multicore across Valve’s business Boost developer effectiveness Expose to game programmers, licensees and MOD authors
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Integrate multicore across Valve’s business
Broader Goals Integrate multicore across Valve’s business Boost developer effectiveness Expose to game programmers, licensees and MOD authors Transparently scale to cores without recompile
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Integrate multicore across Valve’s business
Broader Goals Integrate multicore across Valve’s business Boost developer effectiveness Expose to game programmers, licensees and MOD authors Transparently scale to cores without recompile Leverage to 360
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Integrate multicore across Valve’s business
Broader Goals Integrate multicore across Valve’s business Boost developer effectiveness Expose to game programmers, licensees and MOD authors Transparently scale to cores without recompile Leverage to 360 Provide value to our customers beyond framerate
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Integrate multicore across Valve’s business
Broader Goals Integrate multicore across Valve’s business Boost developer effectiveness Expose to game programmers, licensees and MOD authors Transparently scale to cores without recompile Leverage to 360 Provide value to our customers beyond framerate Pre-Episode 2 Steam to deliver to customers, MOD authors, and licensees
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Multicore is exciting and scary Accessible solutions Hybrid Threading
Closing thoughts Multicore is exciting and scary Accessible solutions Hybrid Threading Scalable More broadly applicable Game Threading Infrastructure Closely controlled core usage Tools framed in terms of game problems Apply cores to new visual gameplay opportunities
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Experiments and Benchmarks
Particle System Multicore AI Provided to take home: VRAD benchmark, particle simulation benchmark
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