Motivation Memory is a shared resource Core Core Memory Core - PowerPoint PPT Presentation

Thread Cluster Memory Scheduling : Exploiting Differences in Memory Access Behavior Yoongu Kim Michael Papamichael Onur Mutlu Mor Harchol-Balter

Motivation • Memory is a shared resource Core Core Memory Core Core • Threads’ requests contend for memory – Degradation in single thread performance – Can even lead to starvation • How to schedule memory requests to increase both system throughput and fairness? 2

Previous Scheduling Algorithms are Biased 17 System throughput Maximum Slowdown 15 Better fairness bias 13 FRFCFS 11 STFM 9 PAR-BS 7 ATLAS Fairness 5 bias 3 1 8 8.2 8.4 8.6 8.8 9 Weighted Speedup Better system throughput No previous memory scheduling algorithm provides both the best fairness and system throughput 3

Why do Previous Algorithms Fail? Throughput biased approach Fairness biased approach Prioritize less memory-intensive threads Take turns accessing memory Good for throughput Does not starve thread A less memory higher thread B thread C thread B thread A intensive priority thread C not prioritized  starvation  unfairness reduced throughput Single policy for all threads is insufficient 4

Insight: Achieving Best of Both Worlds For Throughput higher priority Prioritize memory-non-intensive threads thread thread thread For Fairness thread Unfairness caused by memory-intensive thread being prioritized over each other thread • Shuffle threads thread Memory-intensive threads have thread different vulnerability to interference • Shuffle asymmetrically 5

Outline  Motivation & Insights  Overview  Algorithm  Bringing it All Together  Evaluation  Conclusion 6

Overview: Thread Cluster Memory Scheduling 1. Group threads into two clusters 2. Prioritize non-intensive cluster higher 3. Different policies for each cluster priority Non-intensive Memory-non-intensive cluster Throughput thread thread thread thread thread thread higher Prioritized priority thread thread thread thread Threads in the system Memory-intensive Intensive cluster Fairness 7

Outline  Motivation & Insights  Overview  Algorithm  Bringing it All Together  Evaluation  Conclusion 8

TCM Outline 1. Clustering 9

Clustering Threads Step1 Sort threads by MPKI (misses per kiloinstruction) higher thread thread MPKI thread thread thread thread Non-intensive Intensive α T cluster cluster T α < 10% T = Total memory bandwidth usage ClusterThreshold Step2 Memory bandwidth usage α T divides clusters 10

TCM Outline 1. Clustering 2. Between Clusters 11

Prioritization Between Clusters Prioritize non-intensive cluster > priority • Increases system throughput – Non-intensive threads have greater potential for making progress • Does not degrade fairness – Non-intensive threads are “light” – Rarely interfere with intensive threads 12

TCM Outline 3. Non-Intensive Cluster 1. Clustering Throughput 2. Between Clusters 13

Non-Intensive Cluster Prioritize threads according to MPKI higher lowest MPKI priority thread thread thread thread highest MPKI • Increases system throughput – Least intensive thread has the greatest potential for making progress in the processor 14

TCM Outline 3. Non-Intensive Cluster 1. Clustering Throughput 2. Between 4. Intensive Clusters Cluster Fairness 15

Intensive Cluster Periodically shuffle the priority of threads higher Most prioritized priority thread thread Increases fairness thread • Is treating all threads equally good enough? • BUT: Equal turns ≠ Same slowdown 16

Case Study: A Tale of Two Threads Case Study: Two intensive threads contending 1. random-access Which is slowed down more easily? 2. streaming Prioritize random-access Prioritize streaming 14 14 11x 12 12 Slowdown Slowdown 10 10 7x 8 8 6 6 prioritized prioritized 4 4 1x 1x 2 2 0 0 random-access streaming random-access streaming random-access thread is more easily slowed down 17

Why are Threads Different? rows Memory Bank 1 Bank 2 Bank 3 Bank 4 18

Why are Threads Different? random-access activated row req rows req req req Memory Bank 1 Bank 2 Bank 3 Bank 4 • All requests parallel • High bank-level parallelism 19

Why are Threads Different? random-access streaming activated row req req req rows req Memory Bank 1 Bank 2 Bank 3 Bank 4 • All requests parallel • All requests  Same row • High bank-level parallelism • High row-buffer locality 20

Why are Threads Different? random-access streaming stuck req req req req req rows req req req Memory Bank 1 Bank 2 Bank 3 Bank 4 • All requests parallel • All requests  Same row • High bank-level parallelism • High row-buffer locality Vulnerable to interference 21

TCM Outline 3. Non-Intensive Cluster 1. Clustering Throughput 2. Between 4. Intensive Clusters Cluster Fairness 22

Niceness How to quantify difference between threads? Niceness High Low Bank-level parallelism Row-buffer locality Vulnerability to interference Causes interference Niceness + - 23

Shuffling: Round-Robin vs. Niceness-Aware  What can go wrong? 1. Round-Robin shuffling 2. Niceness-Aware shuffling 24

Shuffling: Round-Robin vs. Niceness-Aware  What can go wrong? 1. Round-Robin shuffling 2. Niceness-Aware shuffling GOOD: Each thread prioritized once Most prioritized D A B C D Priority D A B C D Nice thread C D A B C B C D A B Least nice thread A B C D A Time ShuffleInterval 25

Shuffling: Round-Robin vs. Niceness-Aware  What can go wrong? 1. Round-Robin shuffling 2. Niceness-Aware shuffling GOOD: Each thread prioritized once Most prioritized D A B C D Priority D A B C D Nice thread C D A B C B C D A B Least nice thread A B C D A Time BAD: Nice threads receive ShuffleInterval lots of interference 26

Shuffling: Round-Robin vs. Niceness-Aware 1. Round-Robin shuffling 2. Niceness-Aware shuffling 27

Shuffling: Round-Robin vs. Niceness-Aware 1. Round-Robin shuffling 2. Niceness-Aware shuffling GOOD: Each thread prioritized once Most prioritized D C B A D Priority D D B A D Nice thread C C C B C B B D C B Least nice thread A A A D A Time ShuffleInterval 28

Shuffling: Round-Robin vs. Niceness-Aware 1. Round-Robin shuffling 2. Niceness-Aware shuffling GOOD: Each thread prioritized once Most prioritized D C B A D Priority D D B A D Nice thread C C C B C B B D C B Least nice thread A A A D A Time GOOD: Least nice thread stays ShuffleInterval mostly deprioritized 29

TCM Outline 3. Non-Intensive Cluster 1. Clustering Throughput 2. Between 4. Intensive Clusters Cluster Fairness 30

Outline  Motivation & Insights  Overview  Algorithm  Bringing it All Together  Evaluation  Conclusion 31

Quantum-Based Operation Previous quantum Current quantum (~1M cycles) (~1M cycles) Time Shuffle interval During quantum: (~1K cycles) • Monitor thread behavior 1. Memory intensity Beginning of quantum : 2. Bank-level parallelism • Perform clustering 3. Row-buffer locality • Compute niceness of intensive threads 32

TCM Scheduling Algorithm 1. Highest-rank : Requests from higher ranked threads prioritized • Non-Intensive cluster > Intensive cluster • Non-Intensive cluster: l ower intensity  higher rank • Intensive cluster: r ank shuffling 2.Row-hit : Row-buffer hit requests are prioritized 3.Oldest : Older requests are prioritized 33

Implementation Costs Required storage at memory controller (24 cores) Thread memory behavior Storage MPKI ~0.2kb Bank-level parallelism ~0.6kb Row-buffer locality ~2.9kb Total < 4kbits • No computation is on the critical path 34

Outline  Motivation & Insights  Overview  Algorithm Throughput Fairness  Bringing it All Together  Evaluation  Conclusion 35

Metrics & Methodology • Metrics System throughput Unfairness shared alone IPC IPC    i i Weighted Speedup Maximum Slowdown max i IPC alone shared IPC i i i • Methodology – Core model • 4 GHz processor, 128-entry instruction window • 512 KB/core L2 cache – Memory model: DDR2 – 96 multiprogrammed SPEC CPU2006 workloads 36

Previous Work FRFCFS [Rixner et al., ISCA00]: Prioritizes row-buffer hits – Thread-oblivious  Low throughput & Low fairness STFM [Mutlu et al., MICRO07]: Equalizes thread slowdowns – Non-intensive threads not prioritized  Low throughput PAR-BS [Mutlu et al., ISCA08]: Prioritizes oldest batch of requests while preserving bank-level parallelism – Non-intensive threads not always prioritized  Low throughput ATLAS [Kim et al., HPCA10]: Prioritizes threads with less memory service – Most intensive thread starves  Low fairness 37

Results: Fairness vs. Throughput Averaged over 96 workloads 16 FRFCFS Better fairness Maximum Slowdown 14 ATLAS 5% 12 STFM 10 39% PAR-BS 8 TCM 5% 6 8% 4 7.5 8 8.5 9 9.5 10 Weighted Speedup Better system throughput TCM provides best fairness and system throughput 38