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Technical ArchitectUbisoft Montreal
Michael LAVAIRETechnical LeadUbisoft Montreal
Remi QUENIN
Squeeze the Juice out of CPUsPost Mortem of a Data-Driven Scheduler
Table of Contents
1. Issues on common MT Archi.
2. “Shears” Solution
3. Tips and Tricks
1. COMMON ARCHITECTURAL DESIGNS Usual multithreading patterns
Folded loop
Gameplay
Engine Loop
Graphic
Smaller Engine Loop
Thread 1
Thread 2
Background LoadingThread 3
Frame NN+1N+2
Sync point
Sync
Sync
Sync
Sync
Applying modifications
Applying modifications
Applying modifications
Gameplay
Graphic
Background Loading
Applying modifications
Tasks SchedulingStage 1
Waiting
Stage 2 Stage 3
Gate
Task A
Task D
Task C
Task B
Task E
Task F
2. « SHEARS » SOLUTION60 FPS for everyone
Objectives
2.1 DATA DRIVEN SCHEDULINGFocus on data, not on the code
Task A
Task A
Task A
Data driven scheduling
Task A
Task A
Task A
Task A
Task B
Task A
Task A
Task A
Task C
Data D1
Data D0
Data D0
Data D1
D0 D1
Data driven scheduling
Task A
Task A
Task A
Task A
Task A
Task A
Task A
Task B
Task A
Task A
Task A
Task C
Data D1
Data D0
Data driven schedulingD0 D1
Task A
Task B
Task C
Data driven scheduling
D0 D1Task
A
Task B
Task C
Data driven scheduling
Data driven scheduling
Data driven scheduling
2.2 WORKLOADSNo locks, be scalable
Workload
Lock-free : Internal
Container
State
Thread 2Container::Remove
()
State State
Thread 1Container::Add()
State StateState State
Test &Set Test &Set
SUCCESS ! FAILED !SUCCESS !
Lock-free
1 2 3 40
2000
4000
6000
8000
10000
12000
Lock-free - Op/msecCrit.Sec. - Op/msec
Lock-free : Comparison Q6600
Thread Count 1 2 3 4 Lock-free Mean Op Time (cycles) 233 678 2145 3278
Op/msec 10300 3540 1119 732Race count 0 9571 91205 161371
Crit.Sec. Mean Op Time (cycles) 771 24862 36835 49079
Op/msec 3113 97 65 49Idle % 57% 74% 83% 87%
Lock-free / Crit.Sec. ratio 3.31 36.67 17.17 14.97
Lock-free : Comparison X360
Thread Count 1 2 3 4 5 6 Lock-free Mean Op Time (usec) 0.29 0.5 0.6 0.83 0.91 1.2
Op/msec 3448 2000 1667 1205 1099 833Race count 0 10447 72716 152283 218379 329422
Crit. Sec. Mean Op Time (usec) 0.96 10.06 13.76 19.16 24.66 29.88
Op/msec 1042 99 73 52 41 33Idle % 23% 75% 85% 89% 92% 93%
Lock-free / Crit.Sec. ratio 3.31 20.12 22.93 23.08 27.10 24.90
1 2 3 4 5 60
500
1000
1500
2000
2500
3000
3500
4000
Lock-free - Op/msec
Crit.Sec. - Op/msec
1 20
500
1000
1500
2000
2500
3000
3500
4000
Lock-free - Op/msec
Crit.Sec. - Op/msec
Lock-free : Comparison PS3
Thread Count 1 2 Lock-free Mean Op Time (usec) 0.27 0.46
Op/msec 3704 2174Race count 0 253
Crit.Sec. Mean Op Time (usec) 1.15 5.15
Op/msec 870 194Idle % 25% 60%
Lock-free / Crit.Sec. ratio 4.26 11.20
2.3 WORKING WITH SPUEasy cross-platforming, easy debugging
Working with SPUCross Platform API
?
Working with SPUCross Platform API
Main Memory Impl DMA Impl
Memory Access Interface
SatelliteTask
Working with SPU Easy Debugging
Main Memory Impl DMA Impl
Memory Access Interface
SatelliteTask
Working with SPUEasy Debugging
Working with SPU Easy Debugging
Main Memory Impl DMA Impl
Memory Access Interface
SatelliteTask
Named Pipe
3. TIPS & TRICKSMultithreading & peace of mind
Tip 1: Clever profiling
Tip 2: Watchdog
Tip 3: Unit Tests
Trick 1: Perturbation
Trick 1: Perturbation
Test A
Test C
Test B
Test D
Test A
Test C
Test B
Test D
Loop n Loop n+1
Thread A
Thread B
New thread synchronization
Trick 2: State validation
Trick 2: State validationSt
ate
1
Process A Process B Process C
Process X
Stat
e 2
Stat
e 3
Stat
e X
Assert !
Trick 2: State validationclass StateChecker{public:
enum State { State1, State2, State3 };
StateChecker() { m_state = State1; }
bool SetState( State oldState, State newState ){
return Atomic::TestAndSet( &m_state, oldState, newState ) == oldState;
}
private:volatile State m_state;
};
Trick 3: Access verification
Trick 3: Access verificationclass AccessChecker{public:
AccessChecker() { m_access = 0; }
bool StartReadAccess() { return Atomic::Inc( &m_access ) > 0; }bool EndReadAccess() { return Atomic::Dec( &m_access ) >= 0; }
bool StartWriteAccess() { return Atomic::Dec( &m_access ) == -1; }bool EndWriteAccess() { return Atomic::Inc( &m_access ) == 0; }
private:volatile int m_access;
};
Trick 4: Multithreaded Assert
Trick 4: Multithreaded Assert
extern volatile bool g_waitOnAssert = false;
#define ASSERT( condition ) \while(g_waitOnAssert) {} \if( !(condition) ) \{ \
g_waitOnAssert = true; \DoAssert(); \g_waitOnAssert = false; \
}
Squeeze the Juice !
Inspiration• Game Programming Gems 6: Lock-free Algorithms
by Toby Jones• Design and Implementation of Multi-Threaded Games
by Bruce Dawson• Floodgate: Maximizing SPU parallelism without
sacrificing cross platform developmentby David Asbell & Michael Noland
• SPU Shadersby Mike Acton
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