2Functions of Device Management Device management involves four basic functions:Track status of each device (such as tape drives, disk drives, printers, plotters, and terminals).Use preset policies to determine which process will get a device and for how long.Allocate the devices.Deallocate the devices at 2 levels:At process level when I/O command has been executed & device is temporarily releasedAt job level when job is finished & device is permanently released.
3Types of devicesThe system’s peripheral devices generally fall into one of three categories:DedicatedSharedVirtualThe differences are a function of the characteristics of the devices, as well as how they’re managed by the Device Manager.
4Dedicated DevicesAre assigned to only one job at a time.They serve that job for the entire time the job is active or until it releases them.Some devices demand this kind of allocation scheme, because it would be awkward to let several users share them.Example: tape drives, printers, and plottersDisadvantagesThey must be allocated to a single user for the duration of a job’s execution, which can be quite inefficient, even though the device is not used 100% of the time.
5Shared DevicesCan be assigned to several processes.For example – a disk (DASD) can be shared by several processes at the same time by interleaving their requests;This interleaving must be carefully controlled by the Device ManagerAll conflicts must be resolved based on predetermined policies.
6Virtual DevicesA combination of the first two typesThey’re dedicated devices that have been transformed into shared devices.Example: printerConverted into a shareable device through a spooling program that reroutes all print requests to a disk.Only when all of a job’s output is complete, and the printer is ready to print out the entire document, is the output sent to the printer for printing.Because disks are shareable devices, this technique can convert one printer into several virtual printers, thus improving both its performance and use.
7Example: universal serial bus (USB) Acts as an interface between the OS, device drivers, and applications and the devices that are attached via the USB host.One USB host (assisted by USB hubs) can accommodate up to 127 different devices.Each device is identified by the USB host controller with a unique identification number, which allows many devices to exchange data with the computer using the same USB connection.The USB controller assigns bandwidth to each device depending on its priority:Highest priority is assigned to real-time exchanges where no interruption in the data flow is allowed such as video or sound data.Medium priority is assigned to devices that can allow occasional interrupts without jeopardizing the use of the device, such as a keyboard or joystick.Lowest priority is assigned to bulk transfers or exchanges that can accommodate slower data flow, such as printers or scanners.
8Every device is different Every device is different. Regardless of the specific attributes of the device, the most important differences among them are speed and degree of sharability.
9Components of the I/O Subsystem The pieces of the I.O subsystem all have to work harmoniously and it works in a manner similar to the mythical “McHoes and Flynn Taxicab Company” shown in Fig.1.Many requests come in from all over the city to the taxi company dispatcher.It’s the dispatcher’s job to handle the incoming calls as fast as they arrive and to find out who needs transportation, where they are, where they are going and when.The dispatcher then organizes calls into an order that will use the company’s resources as efficiently as possible.It’s not easy as the company has several drivers and a variety of vehicles at its disposal: ordinary taxicabs, station wagons, vans and a minibus.Once the order is set, the dispatcher calls the drivers who ideally jump into the appropriate vehicles, pick up the waiting passengers and deliver them quickly to their respective destinations.
10Components of the I/O Subsystem CabDriverStation wagonVanDriverCabPhone callsDriverVanDispatcherCabDriverMinibusStation wagonFig. 1. The mythical “McHoes and Flynn Taxicab Company”DriverCab
11Components of the I/O Subsystem Disk 1Disk 2Disk 3Control Unit 1Channel 1Control Unit 2Tape 1Tape 2Tape 3Tape 4CPUControl Unit 3Channel 2Control Unit 4Disk 4Disk 5
12I/O Subsystem : I/O Channel I/O Channel -- keeps up with I/O requests from CPU and pass them down the line to appropriate control unit.Programmable units placed between CPU and control unit.Synchronize fast speed of CPU with slow speed of the I/O device.Make it possible to overlap I/O operations with processor operations so the CPU and I/O can process concurrently.Use channel programs that specifies action to be performed by devices & controls transmission of data between main memory & control units.Entire path must be available when an I/O command is initiated.
13I/O Control UnitI/O control unit interprets signal sent by channel.One signal for each function.At start of I/O command, info passed from CPU to channel:I/O command (READ, WRITE, REWIND, etc.)Channel numberAddress of physical record to be transferred (from or to secondary storage)Starting address of a memory buffer from which or into which record is to be transferred
14Device Manager Must Know which components are busy and which are free. Be able to accommodate requests that come in during heavy I/O traffic.Accommodate disparity of speeds between CPU and I/O devices.Solved by structuring interaction between unitsHandled by “buffering” records & queueing requests
15Communication Among Devices Each unit in I/O subsystem can finish its operation independently from others.CPU is free to process data while I/O is being performed, which allows for concurrent processing and I/O.Success of operation depends on system’s ability to know when device has completed operation.Uses a hardware flag that must be tested by CPU.
16Hardware Flag Used To Communicate When A Device Has Completed An Operation Composed made up of three bits.Each bit represents a component of I/O subsystem.One each for channel, control unit, and device.Resides in the Channel Status Word (CSW)In a predefined location in main memory and contains info indicating status of channel.Each bit is changed from zero to one to indicate that unit has changed from free to busy.
17Testing the Flag : Polling or Interrupts Polling uses a special machine instruction to test flag.CPU periodically tests the channel status bit (in CSW).Major disadvantage with this scheme is determining how often the flag should be polled.If polling is done too frequently, CPU wastes time testing flag just to find out that channel is still busy.If polling is done too seldom, channel could sit idle for long periods of time.
18InterruptsUse of interrupts is a more efficient way to test flag.Hardware mechanism does test as part of every machine instruction executed by CPU.If channel is busy flag is set so that execution of current sequence of instructions is automatically interrupted.Control is transferred to interrupt handler, which resides in a predefined location in memory.Some sophisticated systems are equipped with hardware that can distinguish between several types of interrupts.
19Management of I/O Requests Device Manager divides task into 3 parts, with each handled by specific software component of I/O subsystem.I/O traffic controller watches status of all devices, control units, and channels.I/O scheduler implements policies that determine allocation of, and access to, devices, control units, and channels.I/O device handler performs actual transfer of data and processes the device interrupts.
20I/O Traffic Controller Monitors status of every device, control unit, and channel.Becomes more complex as number of units in I/O subsystem increases and as number of paths between these units increases.Three main tasks: (1) it must determine if there’s at least 1 path available; (2) if there’s more than 1 path available, it must determine which to select; and (3) if paths are all busy, it must determine when one will become available.Maintains a database containing status and connections for each unit in I/O subsystem, grouped into Channel Control Blocks, Control Unit Control Blocks, and Device Control Blocks.
21Traffic Controller Maintains Database For Each Unit In I/O Subsystem
22I/O SchedulerI/O scheduler performs same job as Process Scheduler-- it allocates the devices, control units, and channels.Under heavy loads, when # requests > # available paths, I/O scheduler must decide which request satisfied first.I/O requests are not preempted: once channel program has started, it’s allowed to continue to completion even though I/O requests with higher priorities may have entered queue.Some systems allow I/O scheduler to give preferential treatment to I/O requests from “high-priority” programs.If a process has high priority then its I/O requests also has high priority and is satisfied before other I/O requests with lower priorities.I/O scheduler must synchronize its work with traffic controller to make sure that a path is available to satisfy selected I/O requests.
23I/O Device HandlerI/O device handler processes the I/O interrupts, handles error conditions, and provides detailed scheduling algorithms, which are extremely device dependent.Each type of I/O device has own device handler algorithm.first come first served (FCFS)shortest seek time first (SSTF)SCAN (including LOOK, N-Step SCAN, C-SCAN, and C-LOOK)Every scheduling algorithm should :Minimize arm movementMinimize mean response timeMinimize variance in response time