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THE MEMORY HIERARCHY

THE MEMORY HIERARCHY. Jehan-François Pâris jfparis@uh.edu. Chapter Organization. Technology overview Caches Cache associativity, write through and write back, … Virtual memory Page table organization, the translation lookaside buffer (TLB), page fault handling, memory protection

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THE MEMORY HIERARCHY

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  1. THE MEMORY HIERARCHY Jehan-François Pâris jfparis@uh.edu

  2. Chapter Organization • Technology overview • Caches • Cache associativity, write through andwrite back, … • Virtual memory • Page table organization, the translation lookaside buffer (TLB), page fault handling, memory protection • Virtual machines • Cache consistency

  3. TECHNOLOGY OVERVIEW

  4. Dynamic RAM • Standard solution for main memory since 70's • Replaced magnetic core memory • Bits represented stored on capacitors • Charged state represents a one • Capacitors discharge • Must be dynamically refreshed • Achieved by accessing each cell several thousand times each second

  5. Dynamic RAM Row select nMOS transistor ColumnSelect Capacitor Ground

  6. Row select (gate) ColumnSelect (source) drain Not on the exam The role of the nMOS transistor • Normally, no current can go from the source to the drain • When the gate is positive with respect to the ground, electrons are attracted to the gate (the "field effect")and current can go through

  7. Magnetic disks Servo Platter Arm R/W head

  8. Magnetic disk (I) • Data are stored into circular tracks • Tracks are partitioned into a variable number of fixed-size sectors • Ifdisk drive has more than one platter, all tracks corresponding to the same position of the R/ W head form a cylinder

  9. Magnetic disk (II) • Disk spins at a speed varying between • 5,400 rpm (laptops) and • 15,000 rpm (Seagate Cheetah X15, …) • Accessing data requires • Positioning the head on the right track: • Seek time • Waiting for the data to reach the R/W head • On the average half a rotation

  10. Disk access times • Dominated by seek time and rotational delay • We try to reduce seek times by placing all data that are likely to be accessed together on nearby tracks or same cylinder • Cannot do as much for rotational delay • On the average half a rotation

  11. Average rotational delay

  12. Overall performance • Disk access times are still dominated by rotational latency • Were 8-10 ms in the late 70's when rotational speeds were 3,000 to 3,600 RPM • Disk capacities and maximum transfer rates have done much better • Pack many more tracks per platter • Pack many more bits per track

  13. The internal disk controller • Printed circuit board attached to disk drive • As powerful as the CPU of a personal computer of the early 80's • Functions include • Speed buffering • Disk scheduling • …

  14. Reliability issues • Disk drives have more reliability issues than most other computer components • Moving parts eventually wear • Infant mortality • Would be too costly to produceperfect magnetic surfaces • Disks have bad blocks

  15. Disk failure rates • Failure rates follow a bathtub curve • High infantile mortality • Low failure rate during useful life • Higher failure rates as disks wear out

  16. Disk failure rates (II) Failurerate Wearout Infantilemortality Useful life Time

  17. Disk failure rates (III) • Infant mortality effect can last for months for disk drives • Cheap ATA disk drives seem to age less gracefully than SCSI drives

  18. MTTF • Disk manufacturers advertise very highMean Times To Fail (MTTF) for their products • 500,000 to 1,000,000 hours, that is,57 to 114 years • Does not mean that disk will last that long! • Means that disks will fail at an average rate of one failure per 500,000 to 100,000 hours duringtheir useful life

  19. More MTTF Issues (I) • Manufacturers' claims are not supported by solid experimental evidence • Obtained by submitting disks to a stress test at high temperature and extrapolating results to ideal conditions • Procedure raises many issues

  20. More MTTF Issues (II) • Failure rates observed in the field aremuch higher • Can go up to 8 to 9 percent per year • Corresponding MTTFs are 11 to 12.5 years • If we have 100 disks and a MTTF of 12.5 years, we can expect an average of 8 disk failures per year

  21. Bad blocks (I) • Also known as • Irrecoverable read errors • Latent sector errors • Can be caused by • Defects in magnetic substrate • Problems during last write

  22. Bad blocks (II) • Disk controller uses redundant encoding that can detect and correct many errors • When internal disk controller detects a bad block • Marks it as unusable • Remaps logical block address of bad block tospare sectors • Each disk is extensively tested duringburn in period before being released

  23. The memory hierarchy (I)

  24. The memory hierarchy (II) • To make sense of these numbers, let us consider an analogy

  25. Writing a paper (I)

  26. Writing a paper (II)

  27. Writing a paper (III)

  28. Writing a paper (IV)

  29. Major issues • Huge gaps between • CPU speeds and SDRAM access times • SDRAM access times and disk access times • Both problems have very different solutions • Gap between CPU speeds and SDRAM access times handled by hardware • Gap between SDRAM access times and disk access times handled by combination of software and hardware

  30. Why? • Having hardware handle an issue • Complicates hardware design • Offers a very fast solution • Standard approach for very frequent actions • Letting software handle an issue • Cheaper • Has a much higher overhead • Standard approach for less frequent actions

  31. Will the problem go away? • It will become worse • RAM access times are not improving as fast as CPU power • Disk access times are limited by rotational speed of disk drive

  32. What are the solutions? • To bridge the CPU/DRAM gap: • Interposing between the CPU and the DRAM smaller, faster memories that cache the data that the CPU currently needs • Cache memories • Managed by the hardware and invisible to the software (OS included)

  33. What are the solutions? • To bridge the DRAM/disk drive gap: • Storing in main memory the data blocks that are currently accessed (I/O buffer) • Managing memory space and disk space as a single resource (Virtual memory) • I/O buffer and virtual memory are managed by the OS and invisible to the user processes

  34. Why do these solutions work? • Locality principle: • Spatial locality:at any time a process only accesses asmall portion of its address space • Temporal locality:this subset does not change too frequently

  35. Can we think of examples? • The way we write programs • The way we act in everyday life • …

  36. CACHING

  37. The technology • Caches use faster static RAM (SRAM) • Similar organization as that of D flipflops • Can have • Separate caches for instructions and data • Great for pipelining • A unified cache

  38. A little story (I) • Consider a closed-stack library • Customers bring book requests to circulation desk • Librarians go to stack to fetch requested book • Solution is used in national libraries • Costlier than open-stack approach • Much better control of assets

  39. A little story (II) • Librarians have noted that some books get asked again and again • Want to put them closer to the circulation desk • Would result in much faster service • The problem is how to locate these books • They will not be at the right location!

  40. A little story (III) • Librarians come with a great solution • They put behind the circulation desk shelves with 100 book slots numbered from 00 to 99 • Each slot is a home for the most recently requested book that has a call number whose last two digits match the slot number • 3141593 can only go in slot 93 • 1234567 can only go in slot 67

  41. A little story (IV) Let me see if it's in bin 93 The call number of the book I need is 3141593

  42. A little story (V) • To let the librarian do her job each slot much contain either • Nothing or • A book and its reference number • There are many books whose reference number ends in 93or any two given digits

  43. A little story (VI) Sure Could I get this time the book whose call number 4444493?

  44. A little story (VII) • This time the librarian will • Go bin 93 • Find it contains a book with a different call number • She will • Bring back that book to the stacks • Fetch the new book

  45. Basic principles • Assume we want to store in a faster memory 2n words that are currently accessed by the CPU • Can be instructions or data or even both • When the CPU will need to fetch an instruction or load a word into a register • It will look first into the cache • Can have a hit or a miss

  46. Cache hits • Occur when the requested word is found in the cache • Cache avoided a memory access • CPU can proceed

  47. Cache misses • Occur when the requested word is not found in the cache • Will need to access the main memory • Will bring the new word into the cache • Must make space for it by expelling one of the cache entries • Need to decide which one

  48. Handling writes (I) • When CPU has to store the contents of a register into main memory • Write will update the cache • If the modified word is already in the cache • Everything is fine • Otherwise • Must make space for it by expelling one of the cache entries

  49. Handling writes (II) • Two ways to handle writes • Write through: • Each write updates both the cache and the main memory • Write back: • Writes are not propagated to the main memory until the updated word is expelled from the cache

  50. Write through Write back CPU Cache RAM Handling writes (II) CPU Cache later RAM

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