PowerPoint Presentation

Memory Hierarchies Clip 1 TechnologiesSchool of Computer Science & CommunicationsB. Falsafi (charts), Ph. Janson (commentary)

Information, Computing & CommunicationICC Module 3 Lesson 2 Memory Hierarchies# / 9 2015 Ph. JansonThis video clip is part of the E.P.F.L. introductory course on Information, Computing, and Communication.It is the first in a set of video clips on memory hierarchies and management.

1OutlineClip 1 TechnologiesClip 2 ConceptClip 3 PrincipleClip 4 ImplementationClip 5 Reading memoryClip 6 Writing memoryClip 7 Cache management the Least Recently Used algorithmClip 8 A simulated exampleClip 9 Locality

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ICC Module 3 Lesson 2 Memory Hierarchies# / 9 2015 Ph. JansonIt introduces the topic and then briefly reviews typical memory and storage technologies2

Entertainment Life Business

5,000, 000, 000, 000, 000, 000,000 bytes in 2013(1021 = 1 zetabyte)We live in a digital universe

ICC Module 3 Lesson 2 Memory Hierarchies# / 9 2015 Ph. JansonWhatever we do and wherever we look it is obvious that we are immersed in an increasingly digital universe.Not only science and engineering but also commercial business, entertainment, and everything else swims, even drowns, in digital data.In 2013 the total amount of digital data world-wide was estimated at around 5 zetabytes, where a zetabyte is 10^21 bytes.3How can all that be stored in computer memoryArithm.unitRegister bankData memoryInstruction memoryInstruction pointer+1Decoder

ICC Module 3 Lesson 2 Memory Hierarchies# / 9 2015 Ph. JansonWhere and how can that much data be stored in a way that we can manage to find and retrieve it when we need it?4

1. Technologies

Magnetic tape robotUSB FLASH FLASH Tablet

Primary memory (RAM)

Hard Disk Array 2m2m1.2m0.5m0.5m0.1m

Processor registers

Cache memoryOn-Chip

Off-Chip

Hard diskSSD

ICC Module 3 Lesson 2 Memory Hierarchies# / 9 2015 Ph. JansonTo start answering this question, let us first review what memory and storage technologies are available to store digital information.On this slide one can see a range of technologies, incl. so-called cache memory found inside a processor chip,main or primary memory found inside computers but off the processor chip, flash memory found in USB sticks or tablets,solid-state devices (SSD), hard disks, disk arrays, and magnetic tapes.Let us look in some more details at the properties of and differences between these technologies.5Salient technology characteristicsCapacitySize= number of bytesCost= unit cost (in GB for instance)

PerformanceLatency= time required to access a given byteThroughput= number of consecutive bytes transferred by second RetentionRetention = characteristic of media that retain information when powered offVolatility= characteristic of media that forget information when powered off

ICC Module 3 Lesson 2 Memory Hierarchies# / 9 2015 Ph. JansonAll such memory and storage technologies are characterized by three main properties: their capacity, their performance, and their retention ability.

Their capacity is in turn characterized by how many bytes, more often KB or today GB a typical device of that technology can contain (and what the unit cost of that technology is per GB).

Their performance is characterized by their latency and their throughput, where the latency is the time needed to access a given byte on a device, and the throughput is the number of bytes that can then be transferred in or out of the device every second.

Their retention characteristic specifies whether they are able to retain information when powered off, or whether this information is volatile and erased (lost) when powered off.6CapacitySupport TextImageAudioVideo10KB2 pages100KB1 photo1MBDiskette1 book1 photo HD1 min. MP310MB1 min. hi-fi100MB1GB1h hi-fi1h video10GB2500 MP31h video HD100GBA library1TB1000 films10TBHard disk

US Libraryof Congress in a pocketCDsDVDs

RAMFlashMagnetic tape

ICC Module 3 Lesson 2 Memory Hierarchies# / 9 2015 Ph. JansonThis slide shows the capacity that is typical of a number of storage technologies as well as what capacity is needed to store logical units of information such as text, images, audio or video information.So for instance one can see that a typical book, a HD picture, or an MP3 clip requires roughly 1 MB of space or sowhereas about 1 hour of music or video will fit on a CD or a DVD respectively.Todays high-end disks can however store a whole library or hundreds of hours of movies.7Capacity and latency10 ms

10s

10 ns

0.1 ns

(10 GHz)1 TB

ICC Module 3 Lesson 2 Memory Hierarchies# / 9 2015 Ph. JansonThis slide shows the evolution of device capacity and latency for some of the key storage technologies: hard disks, flash, and main memory.As one can see, hard disks typically offer a much higher capacity but also a longer latency than main memory.One can also see on the latency graph that todays processors have become much faster than main memory,requiring or producing information much faster than main memory can provide or take it.These are key observations to understand the video clips of this lesson.Flash memory lands on these graphics somewhere in between hard disks and main memory.8Technology characteristics in perspectiveLatency ThroughputCost ($/GB)Capacity Retention Processors nsGHzn.a.n.a.n.a.RAM1 - 100 nsGB/s10MB - GBNOFlashsGB/s0.5GB - TBYes Disksms100s MB/s0.05> TBYes Magnetic tapesEven slower100s MB/sEven cheaperEven largerYes

ICC Module 3 Lesson 2 Memory Hierarchies# / 9 2015 Ph. JansonThis last slide summarizes the latency, throughput, unit cost, capacity, and retention properties of the main memory and storage technologies.As can be seen main memory technology, also called RAM for Random Access Memory, is much more expensive than other technologiesand provides much substantially less capacity per typical device.This is another key observation for understanding the video clips of this lesson.9