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Exploring the Impact of Caches on Computer Systems

Understand the CPU-Memory Gap and the importance of caches in improving system performance. Explore memory hierarchy, processor trends, and memory locality principles. Discover how cache levels optimize data access efficiency and overall system speed.

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Exploring the Impact of Caches on Computer Systems

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  1. Computer Systems the impact of caches Computer Systems – the impact of caches

  2. Introduction Different sorts of memory • On-die 0/1/10 cycles • On-board 100 • On-disk 10.000 • Off-machine 1.000.000 Computer Systems – the impact of caches

  3. The CPU-Memory Gap • The increasing gap between disk, DRAM and SRAM, CPU speeds. Computer Systems – the impact of caches

  4. metric 1980 1985 1990 1995 2000 2000:1980 $/MB 8,000 880 100 30 1 8,000 access (ns) 375 200 100 70 60 6 typical size (MB) 0.064 0.256 4 16 64 1,000 DRAM metric 1980 1985 1990 1995 2000 2000:1980 $/MB 500 100 8 0.30 0.05 10,000 access (ms) 87 75 28 10 8 11 typical size (MB) 1 10 160 1,000 9,000 9,000 Disk Storage Trendsbigger, not faster (Culled from back issues of Byte and PC Magazine) Computer Systems – the impact of caches

  5. Processor trendsfaster metric 1980 1985 1990 1995 2000 2000:1980 $/MB 19,200 2,900 320 256 100 190 access (ns) 300 150 35 15 2 100 typical size (MB)0.008 0.016 0.032 SRAM 1980 1985 1990 1995 2000 2000:1980 processor 8080 286 386 Pent P-III clock rate (MHz) 1 6 20 150 750 750 cycle time (ns) 1,000 166 50 6 1.6 750 Computer Systems – the impact of caches

  6. Intel Processors CacheSRAM http://www.intel.com/pressroom/kits/quickreffam.htm Computer Systems – the impact of caches

  7. L0: Smaller, faster, and costlier (per byte) storage devices Registers CPU registers hold words retrieved from cache memory. On-chip L1 cache (SRAM) L1: L1 cache holds cache lines retrieved from the L2 cache. Off-chip L2 cache (SRAM) L2: L2 cache holds cache lines retrieved from memory. Main memory (DRAM) L3: Larger, slower, and cheaper (per byte) storage devices Main memory holds disk blocks retrieved from local disks. Local secondary storage (local disks) L4: Local disks hold files retrieved from disks on remote network servers. Remote secondary storage (distributed file systems, Web servers) L5: Memory Hierarchy Computer Systems – the impact of caches

  8. 80 GB: ~$110 1GB: ~$200 4 MB: ~$500 SRAM DRAM Disk Pay the price • To access large amounts of data in a cost-effective manner, the bulk of the data must be stored on disk Computer Systems – the impact of caches

  9. Locality • Principle of Locality: • Programs tend to reuse data and instructions near those they have used recently, or that were recently referenced themselves. • Temporal locality: Recently referenced items are likely to be referenced in the near future. • Spatial locality: Items with nearby addresses tend to be referenced close together in time. Computer Systems – the impact of caches

  10. Computer Systems – the impact of caches

  11. Locality Example sum = 0; for (i = 0; i < n; i++) sum += a[i]; return sum; • Data • Reference array elements in succession (stride-1 reference pattern): • Reference sum each iteration: • Instructions • Reference instructions in sequence: • Cycle through loop repeatedly: Spatial locality Temporal locality Spatial locality Temporal locality Computer Systems – the impact of caches

  12. Power Programmer • Claim: Being able to look at code and get a qualitative sense of its locality is a key skill for a professional programmer. • Good locality? int sumarrayrows(int a[M][N]) { int i, j, sum = 0; for (i = 0; i < M; i++) for (j = 0; j < N; j++) sum += a[i][j]; return sum } Computer Systems – the impact of caches

  13. Stride-M example • Question: Does this function have good locality? int sumarraycols(int a[M][N]) { int i, j, sum = 0; for (j = 0; j < N; j++) for (i = 0; i < M; i++) sum += a[i][j]; return sum } Computer Systems – the impact of caches

  14. Matrix M=2,N=3 int sumarrowrows() int sumarrowcols() Computer Systems – the impact of caches

  15. Expect: Stride-1 is better! • int A[2][4] Computer Systems – the impact of caches

  16. Reality: small matrices fit in cache • int A[32][32] Computer Systems – the impact of caches

  17. Reality: Performance-drop cache L2 / L1 not dramatic • int A[180][180] Computer Systems – the impact of caches

  18. Reality: Only when DRAM is accessed,the penalty can be seen • int A[512][512] Computer Systems – the impact of caches

  19. Memory Mountain Computer Systems – the impact of caches

  20. Summary • As long as your data fits in the cache, and your program shows good locality, good performance is guaranteed. Computer Systems – the impact of caches

  21. Assignment • Practice Problem 6.9 (p. 624): 'Order three functions to the spatial locality enjoyed by each.' • Practice Problem 6.22 (p. 659): 'Estimate the time, in CPU cycles, to read a 8-byte word, from the different L1-d of a i7 processor Computer Systems – the impact of caches

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