CS 7960-4 Lecture 20

1. CS 7960-4 Lecture 20 The Case for a Single-Chip Multiprocessor K. Olukotun, B.A. Nayfeh, L. Hammond, K. Wilson, K-Y. Chang Proceedings of ASPLOS-VII October 1996

2. CMP vs. Wide-Issue Superscalar • What is the best use of on-chip real estate? • wide-issue processor (complex design/clock, diminishing ILP returns) • CMP (simple design, high TLP, lower ILP) • Contributions: • Takes area and latencies into account • Attempts fine-grain parallelization

3. Scalability of Superscalars • Properties of large-window processors: • Requires good branch prediction and fetch • High rename complexity • High issue queue complexity (grows with issue • width and window size) • High bypassing complexity • High port requirements in the register file and cache •  Necessitates partitioned architectures

4. Application Requirements • Low-ILP programs (SPEC-Int) benefit little from • wide-issue superscalar machines (1-wide R5000 • is within 30% of 4-wide R10000) • High-ILP programs (SPEC-FP) benefit from large • windows – typically, loop-level parallelism that • might be easy to extract

5. The CMP Argument • Build many small CPU cores • The small cores are enough to optimize low-ILP • programs (high thruput with multiprogramming) • For high-ILP programs, the compiler parallelizes • the application into multiple threads – since the • cores are on a single die, cost of communication • is affordable • Low communication cost  even integer programs • with moderate ILP could be parallelized

6. The CMP Approach • Wide-issue superscalar  the brute force method • that extracts parallelism by blindly increasing • in-flight window size and using more hardware • CMP  extract parallelism by static analysis; • minimum hardware complexity and maximum • compiler smarts • CMP can exploit far-flung ILP, has low hw cost • Far-flung ILP and SPEC-Int threads are hard to • automatically extract  memory disam, control flow

7. Area Extrapolations

8. Processor Parameters

9. Applications

10. 2-Wide  6-Wide • No change in branch prediction accuracy  area penalty for 6-wide? • More speculation  more cache misses • IPC improvements of at least 30% for all programs

11. CMP Statistics

12. Results

13. Clustered SMT vs. CMP CMP Single-Thread Performance Fetch Fetch Fetch Fetch Proc Proc Proc Proc Clustered SMT Fetch Fetch Fetch Fetch DL1 DL1 DL1 DL1 Cluster Cluster Cluster Cluster Interconnect for register traffic DL1 DL1 DL1 DL1 Interconnect for cache coherence traffic

14. Clustered SMT vs. CMP CMP Multi-Program Performance Fetch Fetch Fetch Fetch Proc Proc Proc Proc Clustered SMT Fetch Fetch Fetch Fetch DL1 DL1 DL1 DL1 Cluster Cluster Cluster Interconnect for register traffic DL1 DL1 DL1 Interconnect for cache coherence traffic

15. Clustered SMT vs. CMP CMP Multi-thread Performance Fetch Fetch Fetch Fetch Proc Proc Proc Proc Clustered SMT Fetch Fetch Fetch Fetch DL1 DL1 DL1 DL1 Cluster Cluster Cluster Interconnect for register traffic DL1 DL1 DL1 Interconnect for cache coherence traffic

16. Clustered SMT vs. CMP CMP Multi-thread Performance Fetch Fetch Fetch Fetch Proc Proc Proc Proc Clustered SMT Fetch Fetch Fetch Fetch DL1 DL1 DL1 DL1 Cluster Cluster Cluster Cluster Interconnect for register traffic DL1 DL1 DL1 DL1 Interconnect for cache coherence traffic

17. Conclusions • CMP reduces hardware/power overhead • Clustered SMT can yield better single-thread • and multi-programmed performance (at high cost) • CMP can improve application performance if the • compiler can extract thread-level parallelism • What is the most effective use of on-chip real • estate? • Depends on the workload • Depends on compiler technology

18. Next Class’ Paper • “The Potential for Using Thread-Level Data • Speculation to Facilitate Automatic Parallelization”, • J.G. Steffan and T.C. Mowry, Proceedings of • HPCA-4, February 1998

19. Title • Bullet