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ALICE TPC Online Tracking on GPU
David Rohrfor the ALICE Corporation
25.5.2010Lisbon
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Large Hadron Collider
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
The ALICE Experiment
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Tracking
Clusters Tracks
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Real TPC pp-Event in Online Event Display
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Simulated Heavy Ion Event
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Tracks found in simulated Heavy Ion Event
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Tracks in simulated Central Heavy Ion Event
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
TPC Clusters divited into Slices
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
The ALICE TPC
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
TPC Tracker and Merger
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
One ALICE TPC Sector(Traking is performed row by row)
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Tracking Algorithm
Category of Task Name of Task Description on Task
(Initialization)
Combinatorial Part(Cellular Automation)
I: Neighbors FindingConstruct Seeds(Track Candidates)II: Evolution
Kalman Filter PartIII: Tracklet Construction
Fit SeedExtrapolate TrackletFind New Clusters
IV: Tracklet Selection Select good Tracklets
(Tracklet Output)
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Step I: Neighbors Finder(Fit best straight lines)
dx+
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Step II: Evolution(Keep coinciding links)
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Step III: Tracklet Construction
Green: Seed Red: ExtrapolationClusters close to the extraplation point are searched
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Neighbours Finder on Real Data
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Evolution Step on Real Data
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Tracklet Construction on Real Data
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Tracklet Selection on Real Data
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
NVIDIA CUDA GPU
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Parallel Tracklet Construction
Current Row
Tracklets are independent and can be processed simultaneouslyBecause of data locality the tracklets are processed for a common RowSingle Instruction Decoder Either parallel fitting or extrapolationRed: Initial Seed Green: Extrapolation
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Initial GPU Tracker Performance (Peripheral Pb-PB)
(8 threads on Nehalem CPU with 8 virtual / 4 physical cores) Focus on Tracklet Construction when optimizing
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Active GPU Threads for the First Implementation
Active GPU Threads: 19%
Colors represent Tracklet Constructor Steps:Black: IdleBlue: Track FitGreen: Track Extrapolation
x-axis: threads y-axis: time
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Active GPU Threads using Dynamic Scheduling
Active GPU Threads: 62%Colors represent Tracklet Constructor Steps:Black: IdleBlue: Track FitGreen: Track Extrapolation
x-axis: threads y-axis: time
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Pipelining
Initialization / Output on CPU, Tracking on GPU and DMA Transfer can overlap
Time
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Final Speedup (Central Heavy Ion)
(CPU performance was doubled as a side effect)
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Final Speedup in Contrast to Event Size
(PP Mode: Special variant optimized for small scale events)
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Tracking Efficiency
CPU
GPU
ALICE TPC Online Tracking on GPU
IntroductionTracking AlgorithmNVIDIA CUDATracking on GPUResultsSummary
Summary
Twofold performance increase on CPU2.5-fold performance increase compared to CPUTracking efficiency matches the CPU tracker‘s efficiencyA common source code ensures maintainabilityCPU still available during the GPU tracking
Perspective for the Future
Next NVIDIA GPU generation Fermi may result in another boostTrack Merger may be adapted to run on the GPU
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