Understanding Computer Memory Hierarchy: A Comprehensive Overview

1. CS-447– Computer Architecture M,W 10-11:20amLecture 19Memory Hierarchy November 7th, 2007 Majd F. Sakrmsakr@qatar.cmu.edu www.qatar.cmu.edu/~msakr/15447-f07/

2. During This Lecture • Introduction to the Memory Hierarchy • Processor Memory Gap • Locality • Latency Hiding

3. Keyboard, Mouse Computer Processor (active) Devices Memory (passive) (where programs, data live when running) Input Disk, Network Control (“brain”) Output Datapath (“brawn”) Display, Printer The Big Picture

4. Processor-DRAM Memory Gap (latency) µProc 60%/yr. (2X/1.5yr) 1000 CPU “Moore’s Law” 100 Processor-Memory Performance Gap:(grows 50% / year) 10 DRAM 9%/yr. (2X/10 yrs) Performance DRAM 1 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 Time

5. Memories: • SRAM: • value is stored on a pair of inverting gates • very fast but takes up more space than DRAM (4 to 6 transistors)

6. Memories: DRAM: • value is stored as a charge on capacitor (must be refreshed) • very small but slower than SRAM (factor of 5 to 10) Word line Pass Transistor Capacitor Bit line

7. Memory • Users want large and fast memories! • SRAM access times are .5 – 5ns at cost of $4000 to $10,000 per GB. • DRAM access times are 50-70ns at cost of $100 to $200 per GB. • Disk access times are 5 to 20 million ns at cost of $.50 to $2 per GB. 2004

8. Storage Trends SRAM metric 1980 1985 1990 1995 2000 2005 2005:1980 $/MB 19,200 2,900 320 256 100 75 256 access (ns) 300 150 35 15 12 10 30 DRAM metric 1980 1985 1990 1995 2000 2005 2005:1980 $/MB 8,000 880 100 30 1 0.20 40,000 access (ns) 375 200 100 70 60 50 8 typical size(MB) 0.064 0.256 4 16 64 1,000 15,000 Disk metric 1980 1985 1990 1995 2000 2005 2005:1980 $/MB 500 100 8 0.30 0.05 0.001 10,000 access (ms) 87 75 28 10 8 4 22 typical size(MB) 1 10 160 1,000 9,000 400,000 400,000

9. CPU Clock Rates 1980 1985 1990 1995 2000 2005 2005:1980 processor 8080 286 386 Pentium P-III P-4 clock rate(MHz) 1 6 20 150 750 3,000 3,000 cycle time(ns) 1,000 166 50 6 1.3 0.3 3,333

10. The CPU-Memory Gap The gap widens between DRAM, disk, and CPU speeds.

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

12. Locality Example • Claim: Being able to look at code and get a qualitative sense of its locality is a key skill for a professional programmer. • Question: Does this function have good locality? int sum_array_rows(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; }

13. Locality Example • Question: Does this function have good locality? int sum_array_cols(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; }

14. Memory Hierarchy (1/3) • Processor • executes instructions on order of nanoseconds to picoseconds • holds a small amount of code and data in registers • Memory • More capacity than registers, still limited • Access time ~50-100 ns • Disk • HUGE capacity (virtually limitless) • VERY slow: runs ~milliseconds

15. Processor Increasing Distance from Proc.,Decreasing speed Levels in memory hierarchy Level 1 Level 2 Level 3 . . . Level n Size of memory at each level Memory Hierarchy (2/3) Higher Lower As we move to deeper levels the latency goes up and price per bit goes down.

16. Memory Hierarchy (3/3) • If level closer to Processor, it must be: • smaller • faster • subset of lower levels (contains most recently used data) • Lowest Level (usually disk) contains all available data • Other levels?

17. Memory Caching • We’ve discussed three levels in the hierarchy: processor, memory, disk • Mismatch between processor and memory speeds leads us to add a new level: a memory cache • Implemented with SRAM technology

18. Memory Hierarchy Analogy: Library (1/2) • You’re writing a term paper (Processor) at a table in Library • Library is equivalent to disk • essentially limitless capacity • very slow to retrieve a book • Table is memory • smaller capacity: means you must return book when table fills up • easier and faster to find a book there once you’ve already retrieved it

19. Memory Hierarchy Analogy: Library (2/2) • Open books on table are cache • smaller capacity: can have very few open books fit on table; again, when table fills up, you must close a book • much, much faster to retrieve data • Illusion created: whole library open on the tabletop • Keep as many recently used books open on table as possible since likely to use again • Also keep as many books on table as possible, since faster than going to library

20. Memory Hierarchy Basis • Disk contains everything. • When Processor needs something, bring it into to all higher levels of memory. • Cache contains copies of data in memory that are being used. • Memory contains copies of data on disk that are being used. • Entire idea is based on Temporal Locality: if we use it now, we’ll want to use it again soon (a Big Idea)

21. A View of the Memory Hierarchy Regs Upper Level Instr. Operands Faster Cache Blocks L2 Cache Blocks Memory Pages Disk Files Larger Tape Lower Level