Parallel Applications Parallel Hardware Parallel Software IT industry (Silicon Valley) Users Efficient Parallel CKY Parsing on GPUs Youngmin Yi (University of Seoul) Chao-Yue Lai (UC Berkeley) Slav Petrov (Google Research) Kurt Keutzer (UC Berkeley) 1

Outline  Motivation  CUDA Programming Model  Parallel CKY Parsing on GPUs  Experimental Results  Conclusions 2

Outline  Motivation  CUDA Programming Model  Parallel CKY Parsing on GPUs  Experimental Results  Conclusions 3

Why Faster Parsers?  Parsing is the backbone of most NLP applications  Machine translation  Question answering  Information extraction  High-accuracy parsing takes time:  What if we want to parse the web? 4

Great Speedups: GPUs  GPUs: manycore  Hundreds of processing cores, massive parallelism  Allows general-purpose computing  Computer vision: Catanzaro, B. et al Efficient, high-quality image contour detection. In ICCV ‘09.  Speech recognition: Chong, J. et al Scalable HMM based inference engine in large vocabulary continuous speech recognition. In ICME’09.  We want to bring GPUs to the NLP community 5 130x 10.5x

CKY Parsing I love you. love you. you.. you you love love I I love I love you love you (0,0) (0,1) (0,2) (0,3) (1,3) (2,3) (3,3) (2,2) (1,1) (1,2)  Constituency parsing with a weighted CFG  Using Dynamic Programming to iteratively build parse trees with larger spans from smaller spans  In O(|G|n 3 )  n: #words in a sentence, 20 on average  |G|: grammar constant, proportional to #rules High-accuracy grammars have 1,000,000 rules! More impact on speed than n 6

Outline  Motivation  CUDA Programming Model  Computational Model  Memory Model  Parallel CKY Parsing on GPUs  Experimental Results  Conclusions 7

CUDA Computational Model  Two levels of hierarchy  Thread blocks  Threads  Thread blocks (Blocks)  Independent execution units  Max. threads per block: 512 or 1024  Threads in a block  Not independent Work best as if using vectorized units  Communicate via “shared memory” 8

CUDA Memory Model  Global memory  Off-chip, slow but large  Shared memory  On-chip, fast but small  Shared among threads in a thread block  Texture Memory  Fast memory written from CPU  Works best with read- only data 9

CUDA Programming Principles  Mapping computations to blocks and threads  Load balancing among threads in a block saves time  Efficient usage of different types of memory  Reduce global memory accesses 10 block

Outline  Motivation  CUDA Programming Model  Parallel CKY Parsing on GPUs  Mapping Thread-Based vs. Block-Based Sequential Spans vs. Parallel Spans  Atomic Operations vs. Parallel Reduction  Reducing Global Memory Accesses  Experimental Results  Conclusions 11

Parallelism in CKY Parsing  Bottleneck: binary relaxation  Parallelism in spans, symbols and rules 12 I love you. love you. you.. you you love love I I love I love you love you (0,0) (0,1) (0,2) (0,3) (1,3) (2,3) (3,3) (2,2) (1,1) (1,2) spans symbols S 1  S 1 S 2 S 1  S 2 S 30 S 1  S 44 S 53 S 100  S 2 S 26 S 100  S 22 S 3 S1S1 S2S2 S 100 … S 2  S 2 S 4 S 2  S 2 S 5 S 2  S 99 S 52 … S 2  S 3 S 33 … rules

Mapping  A symbol  a thread?  Load imbalance 13 symbols S 1  S 1 S 2 S 1  S 2 S 30 S 1  S 44 S 53 S 100  S 2 S 26 S 100  S 22 S 3 S1S1 S2S2 S 100 … S 2  S 2 S 4 S 2  S 2 S 5 S 2  S 99 S 52 … S 2  S 3 S 33 …

Thread-Based Mapping  A rule  a thread  Flatten out the symbol dimension  (+) 850k rules: great parallelism  (+) load balanced  (-) a block may handle rules with different parent symbols  Harder to get maximum scores for symbols 14 rules S 1  S 1 S 2 S 1  S 2 S 30 S 1  S 44 S 53 S 100  S 2 S 26 S 100  S 22 S 3 … S 2  S 2 S 4 S 2  S 2 S 5 S 2  S 99 S 52 … S 2  S 3 S 33 … …

Block-Based Mapping  A symbol  a block  A rule  a thread  (+) All the threads in the same block have the same parent  (-) What if #rules of a symbol exceeds the limit of #threads per block?  Splitting symbols to virtual symbols 15 symbols S 1  S 1 S 2 S 1  S 2 S 30 S 1  S 44 S 53 S 100  S 2 S 26 S 100  S 22 S 3 … S 2  S 2 S 4 S 2  S 2 S 5 S 2  S 99 S 52 … S 2  S 3 S 33 … … S1S1 S2S2 S 100 …

Sequential Spans 16 symbols S 1  S 1 S 2 S 1  S 2 S 30 S 1  S 44 S 53 S 100  S 2 S 26 S 100  S 22 S 3 … S 2  S 2 S 4 S 2  S 2 S 5 S 2  S 99 S 52 … S 2  S 3 S 33 … … S1S1 S2S2 S 100 … spans … … … … … …

Parallel Spans 17 symbols S 1  S 1 S 2 S 1  S 2 S 30 S 1  S 44 S 53 S 100  S 2 S 26 S 100  S 22 S 3 … S 2  S 2 S 4 S 2  S 2 S 5 S 2  S 99 S 52 … S 2  S 3 S 33 … … S1S1 S2S2 S 100 … spans … … … … … …

Atomic Operations  Multiple threads update the scores of the same parent symbol  Schedule the updates so that they don’t happen simultaneously to ensure correctness  Atomic operations  Guarantee a memory location is accessed by one thread at any time  Serialize operations if necessary 18 S 1  S 1 S 2 S 1  S 2 S 30 S 1  S 44 S 53 S 2  S 2 S 4 scores[S 1 ] scores[S 2 ]

Parallel Reduction  Binary tree reduction  An efficient O(logN) runtime  All the threads in a block must have the same parent symbol  An option only for block-based mapping 19

Reducing Global Memory Accesses  Shared memory: frequently accessed data  Scores of parent symbols  Texture Memory: read-only data  Grammar information such as rule scores  Scores of symbols with smaller spans  Changing the layout of scores  Minimize the overhead of copying data to texture memory 20 span = 1 span = 2 span = 3 span = 4 I love you. love you. you.. you you love love I I love I love you love you (0,0) (0,1) (0,2) (0,3) (1,3) (2,3) (3,3) (2,2) (1,1) (1,2)

Outline  Motivation  CUDA Programming Model  Parallel CKY Parsing on GPUs  Experimental Results  Conclusions 21

Setup  Two GPU architectures  NVIDIA GTX285 (Tesla)  NVIDIA GTX480 (Fermi)  GTX480 better than GTX285 in #cores, support of cache, size of memory  Benchmark  1000 sentences of sec. 22 of WSJ in Penn Treebank  Speedups  Comparing to a serial C implementation of Berkeley Parser 22

GTX285 (Tesla)  No cache memory supported  Lower memory bandwidth speedups 23 serial 1.0 PSpan: Parallel Spans SSpan: Sequential Spans reduce: parallel reduction tex: texture memory thread- atomic- PSpan6.4 block- atomic- PSpan 8.1 block- atomic- SSpan 11.1 block- atomic- SSpan- tex 11.9 block- reduce- PSpan 10.1 block- reduce- SSpan 14.2 block- reduce- SSpan- tex 17.4

GTX480 (Fermi)  Cache memory supported  Higher memory bandwidth speedups 24 PSpan: Parallel Spans SSpan: Sequential Spans reduce: parallel reduction tex: texture memory 1.0 serial thread- atomic- PSpan 13.2 block- atomic- PSpan 14.1 block- atomic- SSpan 15.2 block- atom- SSpan- tex13.9 block- reduce- PSpan 25.8 Block- reduce- SSpan 23.4 block- reduce- SSpan- tex 22.2

Conclusions  We explored the design space for parallelizing CKY parsing on GPUs  Different mappings, synchronization methods  Utilizing different types of memories  We compared two GPU architectures  26X on GTX480, 17X on GTX285  We expect a scalable performance gain as the number of processing cores increases in the future GPUs 25