CSE380 - Operating Systems. Communicating with Devices

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1 CSE380 - Operating Systems Notes for Lecture 15-11/4/04 Matt Blaze (some examples by Insup Lee) Communicating with Devices Modern architectures support convenient communication with devices memory mapped I/O ports via special I/O instructions Hardware can hide details and support efficient use of devices interrupts device controllers DMA Main problems for OS: protocol for communicating with device controllers linking processes with the devices they are using

2 Kinds of devices Some devices are naturally associated with the OS itself clock interrupt, etc, OS problem: protocol for communicating w/ device this is what we ve looked at so far Others naturally associated with some user process serial ports, terminal lab instruments, etc. additional OS problems: associating process with device; relaying data between device and process Still others shared with many processes simultaneously disks, network interfaces, etc. additional OS problems: demultiplexing (e.g., file system abstraction); relaying data between device and appropriate process Devices and user processes Naming problem: how does a user process reference a given device? Communication problem: how does a user process send & received data to & from a given device? Also, blocking and unblocking process when I/O not ready Control problem: how can user process issue privileged commands to a device Contention and multiplexing problems: how does OS manage multiple processes access to a given device

3 Naming Devices Even with memory mapped I/O, it isn t usually reasonable to require processes to hard-code the addresses used for communicating with the various devices installed on a system Instead, many operating systems have a notion of a device namespace that processes can use to refer to devices System calls can operate on devices in this namespace, similar to system calls operating on files e.g., open them, etc. On Unix systems, this is integrated with the file system e.g., the first printer might be named /dev/printer1 User Process Communication with Devices Once a process tells the OS the name of a device it wants to use, how does it communicate with it? If the device uses memory-mapped I/O, the OS could map the device s addresses to a page in the process virtual address space e.g., a system call that takes the name of the device and returns a pointer to the mapped memory advantage: no direct O/S involvement on each I/O disadvantage: no chance for O/S involvement in I/O Or the process could treat the device like a file syscall returns file descriptor, process does read, write, etc assumes device can use file-style I/O Or the OS could have special system calls for communicating with a particular device

4 User Process Control of Devices Not all operations on devices can/should be available to user processes e.g., installing interrupt handlers But some functions need to be made available e.g., setting the bit rate on a serial interface device interface to such operations is often esoteric Usual solution is to perform these functions through standardized system calls device driver provides interface between control system call and device Unix systems use the ioctl system call for this Multiplexing, contention, scheduling devices Some devices might be used by more than one process at a time e.g., disks (via file accesses) Some sequences of operations must be done atomically, others must be translated first e.g., move disk head and write sector Solutions: Have all access go through a subsystem that translates requests, schedules I/O and enforces order might be in OS kernel or in a user process e.g., file system manages access to disks

5 Device drivers Device drivers are the interface between the OS and a device OS usually provides a standard set of hooks for installing new device drivers driver includes device-specific interrupt handlers and device-specific I/O routines and device-specific control routines (ioctl) Device drivers may be hard-coded part of OS kernel, dynamically loaded, or even part of a user process depends on OS and kind of device Example device: serial interface ( tty ) Asynchronous serial port (RS-232) is a standard communication interface to many kinds of peripheral equipment modems, terminals, printers, lab equipment, etc Can send and receive bits at low-moderate speed (e.g., 28Kbps) typical transfer unit is one byte (8 bits) devices usually issue an interrupt when input comes in or output queue ready Many options and configurations speed, parity bit, start/stop bits, flow control, etc.

6 Serial interface device drivers Read and write syscalls would be a natural interface for sending and receiving data Need input and output buffers: interface is much slower than CPU might still be sending last byte when process tries to send next one data might arrive at any time even though process hasn t done read yet buffering avoids need for busy waiting and helps prevent lost data Also need some way to set options e.g., speed, parity, etc Serial interface in Unix Namespace in file system e.g., /dev/tty04 open of a tty returns a file descriptor can do read & write with it, just like a file Device driver includes interrupt handler device sends interrupt when new data arrives and when output queue available Options set by an ioctl arguments include file descriptor and options ioctl options determined by driver man 4 tty, man 2 ioctl for details

7 Pseudo-terminals in Unix The serial interface is so useful that it s even used when there s no physical serial device two process can be hooked together via a pseudo terminal; a write to one is available as a read to the other as far as the processes are concerned they re talking to a serial port A virtual device driver relays bytes between pseudo terminals No actual hardware involved, but still uses device drivers Next example: Disk Drives We ve already talked about how a disk works Soon we ll talk about how file systems use disks file system acts as the interface between most user processes and the disk devices processes don t communicate directly with the disk driver file system does disk scheduling and implements the system s file structure But right now, we ll talk about how the OS communicates with disks

8 Disk Access Disk interface: move head to location, read/write sector addresses are complex (cylinder number, head number, sector number) sectors can be given logical numbers that get decoded by controller Driver provides simpler interface to processes open, seek to byte #, read, write provides view of disk as one big linear array of bytes standard open, seek, read and write system calls work Design considerations for driver what is the interface provided by the controller? how to optimize performance Disk Scheduling Time to read or write a sector is influenced by: seek time (usually longest) rotational delay (usually in the middle) transfer time (usually smaller) Controller and device driver can each exploit knowledge of disk structure to improve performance in general, there will be a queue of pending requests for reads and write in what order should they be sent to the disk?

9 Disk-Arm Scheduling: FCFS Basic strategy for moving disk arm across cylinders: First-come-first-served queue disk queue: 98, 183, 37, 122, 14, 124, 65, 67 initial r/w head position: cylinder ! ! " total head movement = = we move across 640 tracks In practice: OK for small load, low variance of waiting time Disk-Arm Scheduling:SSF Shortest Seek-Time First (SSF): Pick the closest one next Consider example from last slide: ordering will be 65, 67, 37, 14, 98, 122, 124, 183 total head movement (starting at 53) = = 236 tracks In practice: Better performance, but high variance (starvation possible)

10 Disk-Arm Scheduling: Scan Maintain a direction bit Pick the request with shortest seek time in current direction first, and change direction only at edges. Previous example: Assume head was moving toward 0. Ordering: 37, 14, 0, 65, 67, 98, 122, 124, 183 total head movement = = 236 In practice: Almost as good as SSF in mean wait time, but lower variance. Disk-Arm Scheduling Algorithms Elevator (Look): Variation of Scan algorithm Change direction if no more requests farther down in current direction Preferable to Scan (gives total head movement 208 in previous example) Another variant: circular elevator Always scan in the same direction servicing cylinders in a circular manner (i.e. go to lowest numbered pending request if no requests are higher than current position)

11 Fault Tolerance for Disks Underlying disk mechanical disk technology is unreliable and failure prone Corrupted bits and sectors Manufacturing defects Disk crashes Many countermeasures implemented directly by disk and controller hardware Error correction within sectors Extra sectors within a disk usually not visible to OS Other measures can be implemented by OS Backups at intervals Automatic mirroring RAID (provides redundancy and parallelism) Bad Sectors Sectors become defective for a various reasons disk hardware often can detect this via ECC Every track has extra (spare) sectors that are not part of logical address space Disk controller keeps track of bad sectors and does remapping of sector numbers Two choices for renumbering Substitute a spare sector for defective Renumber all by shifting Obvious tradeoff Efficiency of reading consecutive sectors Initial effort of rewriting preambles in each sector

12 Read Errors After reading a sector, what if reduantcy code indicates an error? ECC may allow error correction Read again, and error may go away If read error seems to happen repeatedly, then the sector can be marked as bad, and replaced by a spare sector OS may need to keep track of bad sectors, or it may be transparently handled by controller hardware Backup process may have to manage bad sectors Reliability by Redundancy Mirroring (or Shadowing): duplicate disks Write writes every block to both of the disks Read can be performed from either disk this can be exploited to allow parallelism and speeding up of reads mainly for error correction: if first read fails, try the other Crash recovery: If one disk crashes, then the other one can be used to restore data If failures are independent, this dramatically increases meantime to failure Mirroring can be managed by device controller hardware (if it can handle multiple disks) or by the OS software Disadvantage: duplicate disks are expensive (twice the cost) and can be slow if not mirrored in hardware

13 RAID: Redundant Array of Independent Disks Basic idea: spread work over several physical disks RAID controller talks to OS just like a device driver and manages a suite of disks to improve reliability and performance Striping: Organize logically consecutive chunks of data across different disks to improve performance Striping can be at bit-level, byte-level, or sector-level Incoming request is split into all relevant disks by RAID controller, and their outputs are put together Redundancy through duplication: Multiple requests can be load-balanced Error recovery on faults RAID Level 0 Array of disks with block-level striping Stripe = k sectors (for suitably chosen k) First k sectors are on first stripe of first disk, next k sectors are on first strip of second disk, and so on Advantages: A large request spanning multiple stripes can be parallelized easily Not much overhead Used in high-performance applications Disadvantages No redundancy Mean-time to failure is, in fact, less than standard disks (why?) No parallelism for small requests

14 RAID Level RAID Level 1 Like mirroring, but with striping of level 0 Use same number of backup disks with same striping Same benefits/issues as in mirroring

15 RAID Level 2 Called memory-style error correcting code organization Uses bit-level striping Some of the bits used for parity or other error correction Sample: 4-bits of data encoded by additional 3-bits of ECC, and resulting 7 bits stored on 7 different disks CM-2 machine: 32-bits of data with 7-bits of ECC spanning over array of 39 disks Advantages Great parallelism for read as well as write Even if one disk crashes, all data can be recovered (due to ECC) Disadvantages Drives have to synchronized Overhead in putting together reads from different disks Crash recovery much slower (compared to RAID-1) RAID Levels 3-5 Raid levels 3 through 5 Backup and parity drives are shaded

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