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A STEP TOWARDS DATA ORIENTATIONJOHAN TORP <[email protected]>STHLM GAME DEVELOPER FORUM 5/5 2011
› M.Sc. Computer Science. OO (Java) and functional programming (Haskell)› Worked ~5 years outside game industry. C++, generic programming &
boost, DbC› AI coder at DICE ~2½ years
MY BACKGROUND
Optimal game dev credentials?
NEW TRADE-OFFS
• OOP• GP• FP• DbC• TMP
PC & normal sized apps = cache schmache
Games on consoles 5000 L2 misses = ~1ms
- data-oriented design ftw!
› A lot of OO code and knowledge out there› Incrementally moving from OO to cache-friendlier code
THIS TALK
› Facts needed before looking at code› Cache-friendly pathfinding› Async vs sync code› Questions
AGENDA
› Visual domain specific scripting language› Gameplay / AI code in C++› NavPower pathfinding middleware› EASTL containers› We love to blow up parts of our game worlds – and call this destruction
FROSTBITE
› PS3 has 1 core: 32KB data and 32KB instruction L1 cache › 512KB L2 I+D cache› 360 has 3 cores: 32KB data and 32KB instruction L1 cache for each core, › 1Mb L2 I+D shared by all cores
› 1 L1 cache miss ~= 40 cycles
“You miss L1 so much that you cry yourself to sleep every night with a picture of it under your pillow” @okonomiyonda
› 1 L2 cache miss ~= 600 cycles › 1 L2 cache miss ~= 20 matrix multiplications› Other than heavy calculations: CPU performance ~= cache misses
PS3 (PPU) / XBOX 360 CACHE ECONOMY
Keep copy of common data nearby … in a compact representation
Pointer chasing thrashes both I-cache and D-cache
Often better to copy frequently accessed data once each frame, access copy instead
KEEP HOT DATA NEARBY
getBot()->getPlayer()->getControllable()->getWorldPosition()
EXAMPLE DATA-ORIENTED INPUT
/// Temporary struct containing information about a single sensor. /// Never stored between updates. struct VisionInfo {
VisionInfo(const AiSettings& settings, EntryComponent& owner, ...);
Vec3 eyePos; Vec2 eyeForwardXz; uint playerId;
// Extracted from settings float centralAngle; float peripheralAngle; float seeingDistance; bool seeThroughTerrain;
};
› Temporary data structures common in Data-Oriented Design› Stack variables or alloca()› Not suited for large amounts or large edge cases
TEMPORARIES – STACK SPACE
› Put aside 8x128kb blocks for ”scratch pad calculations”› Linear allocator – doesn’t free within block› Return whole block when done – zero fragmentation
TEMPORARIES – SCRATCH PAD
Find a good slot in fragmented memory space Expensive!Container of new:ed objects scattered in memory Poor cache locality!Mix short/long lived allocations -> fragmention Lose memory over time!
You should prefer pre-allocated flat vectors and try to minimize new/malloc
NEW / MALLOC
› Find path› Load / unload nav mesh section› Add / remove obstacles› Path invalidation detection› Can go-tests› Line- / can go straight-tests, circle tests, triangle tests
NAVPOWER OPERATIONS NEEDED
› Find path› Load / unload nav mesh section› Add / remove obstacles› Path invalidation detection› Can go-tests› Line- / can go straight-tests, circle tests, triangle tests
Collect and batch process for good cache locality
NAVPOWER OPERATIONS NEEDED
› Pathfinder - find path, path invalidation, circle/line tests› Random position generator - can go-tests› Manager - load nav mesh, obstacles, destruction, updates
Let some line tests in AI decision making remain synchronous
ABSTRACTIONS
class Pathfinder { virtual PathHandle* findPath(const PathfindingPosition& start, const PathfindingPosition& end, float corridorRadius, PathHandle::StateListener* listener) = 0;
virtual void releasePath(PathHandle* path) = 0;
virtual bool canGoStraight(Vec3Ref start, Vec3Ref end, Vec3* collision = nullptr) const = 0; };
PATHFINDER INTERFACE
PATH HANDLE
typedef eastl::fixed_vector<Vec3, 8> WaypointVector; typedef eastl::fixed_vector<float, 8> WaypointRadiusVector;
struct PathHandle { enum State {ComputingPath, ValidPath, NoPathAvailable, RepathingRequired};
class StateListener { virtual void onStateChanged(PathHandle* handle) = 0; };
PathHandle():waypoints(pathfindingArena()), radii(pathfindingArena()) {}
WaypointVector waypoints; WaypointRadiusVector radii; State state;
};
PATH HANDLE
typedef eastl::fixed_vector<Vec3, 8> WaypointVector; typedef eastl::fixed_vector<float, 8> WaypointRadiusVector;
struct PathHandle { enum State {ComputingPath, ValidPath, NoPathAvailable, RepathingRequired};
class StateListener { virtual void onStateChanged(PathHandle* handle) = 0; };
PathHandle():waypoints(pathfindingArena()), radii(pathfindingArena()) {}
WaypointVector waypoints; WaypointRadiusVector radii; State state;
};
› class NavPowerPathfinder : public Pathfinder {› public:
virtual PathHandle* findPath(...) override;› virtual PathHandle* findPathFromDestination(...) override;› virtual void releasePath(...) override;› virtual bool canGoStraight(...) const override;
void updatePaths();› void notifyPathListeners();
› private:› bfx::PolylinePathRCPtr m_paths[MaxPaths];
PathHandle m_pathHandles[MaxPaths];› PathHandle::StateListener* m_pathHandleListeners[MaxPaths];› u64 m_usedPaths, m_updatedPaths, m_updatedValidPaths; };
NAVPOWER PATHFINDER
CORRIDOR STEP1. Copy all new NavPower paths -> temporary representation2. Drop unnecessary points for all paths3. Corridor adjust all paths 4. Copy temporaries -> PathHandles
typedef eastl::vector<CorridorNode> Corridor;
ScratchPadArena scratch; Corridor corridor(scratch); corridor.resize(navPowerPath.size()); // Will allocate memory using scratch pad
CORRIDOR STEP1. Copy all new NavPower paths -> temporary representation2. Drop unnecessary points for all paths3. Corridor adjust all paths 4. Copy temporaries -> PathHandles
for (...) { // Loop through all paths in their corridor representation dropUnnecessaryPoints(it->corridor, scratchPad);
for (...) shrinkEndPoints(it->corridor);
for (...) calculateCornerDisplacements(it->corridor);
for (...) displaceCorners(it->corridor);
for (...) shrinkSections(it->corridor);
for (...) copyCorridorToHandle(it->corridor, it->pathHandle);
}
CORRIDOR STEP 2-4
NAVPOWER MANAGER
void NavPowerManager::update(float frameTime) { m_streamingManager.update(); m_destructionManager.update(); m_obstacleManager.update();
for (PositionGeneratorVector::const_iterator it= ...) (**it).update();
bfx::SystemSimulate(frameTime);
for (PathfinderVector::const_iterator it=m_pathfinders.begin(), ...) (**it).updatePaths(); for (PathfinderVector::const_iterator it=m_pathfinders.begin(), ...) (**it).notifyPathListeners(); }
AI Decision Making Code
NAIVE OO CALL PATTERN
Pathfinding Runtime Code
NavPower Code
Animation Code
EXECUTION
Animation Code
Animation Code
AI Decision Making Code
AI Decision Making Code
NavPower Code
CURRENT CALL PATTERN
Pathfinding Runtime Code
EXECUTION
HOT HOT HOT!!!!!
› Keep pathfinding code/data cache hot› Avoid call sites cache running cold› Easier to jobify / SPUify› Easy to schedule and avoid spikes
BATCHING BENEFITS
LESS SIMPLIFIED ARCHITECTURE
LOCOMOTION
PATHFINDING
DRIVING LOCOMOTION
ANIMATION
SCRIPTINGSERVER CLIENT
VEHICLE INPUT
PATH FOLLOWING
AI DECISION MAKING
NAVPOWER
Waypoint DataCorridor Radii
Waypoint Positions
Each server update
1. Each AI decision making2. Pathfinding manager update
All pathfinding requestsAll corridor adjustmentsAll PathHandle notifications -> path following -> server locomotion
3. Network pulse. Server locomotion -> client locomotion4. ...rest of update
No extra latency added
MY PRECIOUS LATENCY
› Callbacks. Delay? Fire in batch?› Handle+poll instead of callbacks. Poll in batch?› Record messages/events, act on them later.. in batch?› Assume success, recover from failure next update
DELAYING/BATCHING
+ Cache friendly & parallelizable+ Easy to profile & schedule+ Avoid bugs with long synchronous callback chains+ Modular
- More glue code managers, handles, polling update calls, multiple representations of the same data
- More bugsindex fiddling, life time handling, latency, representations drifting out of sync
- Callstack won’t tell you everythingbreak point in sync code gives easy-to-debug vertical slice...
...but can we afford vertical deep dives?
DELAYING/BATCHING - PROS & CONS
› Do not have to abandon OO nor rewrite the world› Start small, batch a bit, cut worst pointer chasing, avoid deep dives, grow
from there› Much easer to rewrite a system in a DO fashion afterwards
Existing code is crystallized knowledge, refactor incrementally to learn!
INCREMENTAL GAINS
›Background: Console caches, heap allocations expensive, temporary memory›AI decision making – pathfinding – animation›Code: Async abstractions, handles, scratch pad, fixed_vector, batch processing›Latency analysis, pros&cons sync vs async
Think about depth/width of calls, try stay within your system, keep hot data nearby
SUMMARY