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CS 161 Chapter 8 - I/O

CS 161 Chapter 8 - I/O Lecture 17 Anatomy: 5 components of any Computer Keyboard, Mouse Computer Processor (active) Devices Memory (passive) (where programs, data live when running) Disk (where programs, data live when not running) Input Control (“brain”) Output

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CS 161 Chapter 8 - I/O

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  1. CS 161 Chapter 8 - I/O Lecture 17

  2. Anatomy: 5 components of any Computer Keyboard, Mouse Computer Processor (active) Devices Memory (passive) (where programs, data live when running) Disk(where programs, data live when not running) Input Control (“brain”) Output Datapath (“brawn”) Display, Printer

  3. I/O Device Examples and Speeds • I/O Speed: bytes transferred per second(from mouse to display: million-to-1) • Device Behavior Partner Data Rate (Mbit/sec) Keyboard Input Human 0.0001 Mouse Input Human 0.0038 Laser Printer Output Human 3.2.000 Magnetic Disk Storage Machine 240-2560 Modem I or O Machine 0.016-0.064 Network-LAN I or O Machine 100-1000 Graphics Display Output Human 800-8000 See Fig. 8.2 Text

  4. Memory bus PCI: Internal(Backplane) I/O bus CPU SCSI: External I/O bus PCI Interface Memory Ethernet Interface SCSI Interface • Data rates • Memory: 133 MHz, 8 bytes 1064 MB/s (peak) • PCI: 33 MHz, 8 bytes wide  264 MB/s (peak) • SCSI: “Ultra3” (80 MHz), “Wide” (2 bytes) 160 MB/s (peak) (1 to 15 disks) Ethernet Local Area Network Ethernet: 12.5 MB/s (peak) Buses in a PC: connect a few devices

  5. Inner Track Outer Track Sector Head Arm Platter Actuator Disk Device Terminology • Several platters, with information recorded magnetically on both surfaces (usually) • Bits recorded in tracks, which in turn divided into sectors (e.g., 512 Bytes) • Actuator moves head (end of arm,1/surface) over track (“seek”), select surface, wait for sector rotate under head, then read or write • “Cylinder”: all tracks under heads

  6. Disk Device Performance Inner Track Outer Track Sector Head Controller Arm Spindle • Disk Latency = Seek Time + Rotation Time + Transfer Time + Controller Overhead • Seek Time? depends no. tracks move arm, seek speed of disk • Rotation Time? depends on speed disk rotates, how far sector is from head • Transfer Time? depends on data rate (bandwidth) of disk (bit density), size of request Platter Actuator

  7. Disk Device Performance • Average distance sector from head? • 1/2 time of a rotation • 7200 Revolutions Per Minute  120 Rev/sec • 1 revolution = 1/120 sec  8.33 milliseconds • 1/2 rotation (revolution)  4.16 ms • Average no. tracks move arm? • Sum all possible seek distances from all possible tracks / # possible • Assumes average seek distance is random • Disk industry standard benchmark

  8. Disk Performance Model /Trends • Capacity + 100%/year (2X / 1.0 yrs) • Transfer rate (BW) + 40%/year (2X / 2.0 yrs) • Rotation + Seek time – 8%/ year (1/2 in 10 yrs) • MB/$ > 100%/year (2X / <1.5 yrs) Fewer chips + areal density

  9. Disk Performance • Calculate time to read 1 sector (512B) for UltraStar 72 using advertised performance; sector is on outer track Disk latency = average seek time + average rotational delay + transfer time + controller overhead = 5.3 ms + 0.5 * 1/(10000 RPM) + 0.5 KB / (50 MB/s) + 0.15 ms = 5.3 ms + 0.5 /(10000 RPM/(60000ms/M)) + 0.5 KB / (50 KB/ms) + 0.15 ms = 5.3 + 3.0 + 0.10 + 0.15 ms = 8.55 ms

  10. Instruction Set Architecture for I/O • Some machines have special input and output instructions • Alternative model (used by MIPS): • Input: ~ reads a sequence of bytes • Output: ~ writes a sequence of bytes • Memory also a sequence of bytes, so use loads for input, stores for output • Called “Memory Mapped Input/Output” • A portion of the address space dedicated to communication paths to Input or Output devices (no memory there)

  11. cmd reg. data reg. Memory Mapped I/O • Certain addresses are not regular memory • Instead, they correspond to registers in I/O devices address 0 0xFFFF0000 0xFFFFFFFF

  12. Processor-I/O Speed Mismatch • 500 MHz microprocessor can execute 500 million load or store instructions per second, or 2,000,000 KB/s data rate • I/O devices from 0.01 KB/s to 30,000 KB/s • Input: device may not be ready to send data as fast as the processor loads it • Also, might be waiting for human to act • Output: device may not be ready to accept data as fast as processor stores it • What to do?

  13. Processor Checks Status before Acting: Polling • Path to device generally has 2 registers: • 1 register says it’s OK to read/write (I/O ready), often called Control Register • 1 register that contains data, often called Data Register • Processor reads from Control Register in loop, waiting for device to set Ready bit in Control reg to say its OK (0  1) • Processor then loads from (input) or writes to (output) data register • Load from device/Store into Data Register resets Ready bit (1  0) of Control Register

  14. Cost of Polling? • Assume for a processor with a 500-MHz clock it takes 400 clock cycles for a polling operation (call polling routine, accessing the device, and returning). Determine % of processor time for polling • Mouse: polled 30 times/sec so as not to miss user movement • Floppy disk: transfers data in 2-byte units and has a data rate of 50 KB/second. No data transfer can be missed. • Hard disk: transfers data in 16-byte chunks and can transfer at 8 MB/second. Again, no transfer can be missed.

  15. % Processor time to poll mouse, floppy • Mouse Polling Clocks/sec = 30 * 400 = 12000 clocks/sec % Processor for polling = 12*103/500*106 = 0.002%  Polling mouse little impact on processor • Times Polling Floppy/sec = 50 KB/s /2B = 25K polls/sec Floppy Polling Clocks/sec = 25K * 400 = 10,000,000 clocks/sec % Processor for polling = 10*106/500*106 = 2%  OK if not too many I/O devices

  16. % Processor time to hard disk • Times Polling Disk/sec = 8 MB/s /16B = 500K polls/sec Disk Polling Clocks/sec = 500K * 400 = 200,000,000 clocks/sec % Processor for polling: 200*106/500*106 = 40%  Unacceptable

  17. What is the alternative to polling? Interrupt • Wasteful to have processor spend most of its time “spin-waiting” for I/O to be ready • Wish we could have an unplanned procedure call that would be invoked only when I/O device is ready • Solution: use exception mechanism to help I/O. Interrupt program when I/O ready, return when done with data transfer

  18. (1) I/O interrupt Memory add sub and or (2) save PC user program (3) interrupt service addr (4) read store ... jr interrupt service routine (5) Interrupt Driven Data Transfer

  19. Benefit of Interrupt-Driven I/O • 500 clock cycle overhead for each transfer, including interrupt. Find the % of processor consumed if the hard disk is only active 5% of the time. • Interrupt rate = polling rate • Disk Interrupts/sec = 8 MB/s /16B = 500K interrupts/sec • Disk Polling Clocks/sec = 500K * 500 = 250,000,000 clocks/sec • % Processor for during transfer: 250*106/500*106= 50% • Disk active 5%  5% * 50%  2.5% busy

  20. 4 Responsibilities leading to OS • The I/O system is shared by multiple programs using the processor • Low-level control of I/O device is complex because requires managing a set of concurrent events and because requirements for correct device control are often very detailed • I/O systems often use interrupts to communicate information about I/O operations • Would like I/O services for all user programs under safe control

  21. Direct Memory Access (DMA) • How to transfer data between a Device and Memory? Wastage of CPU cycles if done through CPU. • Let the device controller transfer data directly to and from memory => DMA • The CPU sets up the DMA transfer by supplying the type of operation, memory address and number of bytes to be transferred. • The DMA controller contacts the bus directly, provides memory address and transfers the data • Once the DMA transfer is complete, the controller interrupts the CPU to inform completion. • Cycle Stealing – Bus gives priority to DMA controller thus stealing cycles from the CPU

  22. Why Networks? • Originally sharing I/O devices between computers (e.g., printers) • Then Communicating between computers (e.g, file transfer protocol) • Then Communicating between people (e.g., email) • Then Communicating between networks of computers  Internet, WWW

  23. Computer switch switch switch network interface What makes networks work? • links connecting switches to each other and to computers or devices • ability to name the components and to route packets of information - messages - from a source to a destination • Layering, protocols, and encapsulation as means of abstraction

  24. Typical Types of Networks • Local Area Network (Ethernet) • Inside a building: Up to 1 km • (peak) Data Rate: 10 Mbits/sec, 100 Mbits /sec,1000 Mbits/sec (1.25, 12.5, 125 MBytes/s) • Run, installed by network administrators • Wide Area Network • Across a continent (10km to 10000 km) • (peak) Data Rate: 1.5 Mbits/sec to 2500 Mbits/sec • Run, installed by telephone companies • Wireless Networks, ...

  25. appln appln OS OS network interface device ABCs: many computers • switches and routers interpret the header in order to deliver the packet • source encodes and destination decodes content of the payload

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