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8-IO-Management.md

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I/O Management

Operating system

  • Has protocols
    • Interfaces for device I/O
  • Has dedicated handlers
    • Device drivers, interrupt handlers
  • Decouple I/O details from core processing
    • abstract I/O device detail from applications

I/O Device Features

  • Control registers (accessed by CPU)
    • Command
    • Data Transfers
    • Status
  • Microcontroller : device's CPU
  • On device memory
  • Other logic
    • e.g. analog to digital

Device drivers

  • per each device type
  • responsible for device access management and control
  • provided by device manufacturers per OS /version
  • each OS standardizes interfaces
    • device independence
    • device diversity

Types of devices

  • Block
    • e.g. disk
    • read/write blocks of data
    • direct access to arbitrary block
  • Character
    • e.g. keyboard
    • get/put character
  • Network devices

OS representation of a device : special device file

UNIX like systems:

  • /dev
  • tmpfs
  • devfs

Linux supports a number of pseudo "virtual" devices that provide special functionality to a system.

CPU device interactions

iointeractions.png

access device registers : memory load/store

  1. Memory mapped I/0
    • part of 'host' physical memory dedicated for device interactions
    • Base Address Registers (BAR)
  2. I/O Port
    • dedicated in low instructions for device access
    • target device (I/0 port) and value in register

Path from Device to CPU

  1. Interrupt
    • Overhead: Interrupt handling steps
    • +: Can be generated as soon as possible
  2. Polling
    • Overhead: Delay or CPU overhead
    • when convenient for OS

Device access : Programmed I/O (PIO)

  • No additional hardware support
  • CPU "programs" the device
    • via command registers
    • data movement
  • E.g. NIC(Network Interface Card)
    • data = network packet
  • Write command to request packet information
  • Copy packet to data registers
  • Repeat until packet sent

E.g. 1500B packet; 8 byte registers or bus => 1(for bus command) + 188(for data) = 189 CPU store instructions

Direct Memory Access (DMA)

  • Relies on DMA controller
  • CPU "programs" the device
    • via command registers
    • via DMA controls
  • E.g. NIC (data = network packet)
  • Write command to request packet information
  • Configure DMA controller with in memory address and size of packet buffer

E.g. 1500B packet; 8 byte registers or bus => 1(for bus command) + 1(for DMA configuration) = total 2 CPU store instructions. Less steps, but DMA configuration is more complex.

For DMAs

  • data buffer must be in physical memory until transfer completes
  • pinning regions (non-swappable)

Typical Device Access

typicaldeviceaccess.png

  • System call
  • In-kernel stack
  • Driver Invocation
  • Device request configuration
  • Device performs request

OS bypass

osbypass.png

  • device registers/data
    • directly available
  • OS configures
    • then gets out of the way
  • "user level driver"
    • in library
  • OS retains coarse-grain control
  • relies on device features
    • sufficient registers
    • demux capability

What happens to a calling thread?

access.png

  • Synchronous I/O operations
    • process blocks
  • Asynchronous I/O operations
    • process continues
    • Later, process checks and retrieves result
    • OR
    • process is notified that operation is completed and results are ready

Block Device Stack

Block device typical storage for files:

blockdevicestack.png

  • processes use files => logical storage unit
  • kernel file system (KFS)
    • where how to find and access file
    • OS specifies interface
  • generic block layer
    • OS standardized block interface
  • Device driver

Virtual File System

vfs.png

Virtual File System Abstractions

  • File : Elements on which the VFS operates
  • File Descriptor : OS representation of file
    • open, read, write, send file , lock, close
  • inode : Persistent representation of file "index"
    • list of all data blocks
    • device, permissions, size
  • dentry : Directory entry, corresponding to the single path component,
    • dentry cache
  • super block : file system specific information regarding the File System layout

VFS on disk

  • File : data blocks on disk
  • inode : track file blocks
    • also resides on disk in some block
  • super block : overall map of disk blocks
    • inode blocks
    • data blocks
    • free blocks

Inodes

Index of all disk blocks corresponding to a file

  • File : identified by inode
  • inode : list of all blocks + other metadata

+: Easy to perform sequential or random access
-: Limit on file size

Inodes with indirect pointers

  • Index of all disk blocks corresponding to a file
  • Index contain:
    • metadata
    • pointers to blocks
  • Direct pointer : Points to data block
    • 1 KB per entry
  • Indirect pointer : Points to block of pointers
    • 256 KB per entry
  • Double Indirect pointer : Points to block of block of pointers
    • 64 MB per entry

+: Small inode => large file size
-: File access slowdown

Disk access optimizations

Reducing file access overheads

  1. Caching/buffering : reducenumber of disk accesses
    • buffer cache in main menu
    • read/write from cache
    • periodically flush to disk - fsync()
  2. I/O scheduling : reduce disk head movement
    • maximize sequential vs random access
  3. Prefetching : increases cache hits
    • leverages locality
  4. Journaling/logging: reduce random access (ext3, ext4)
    • "describe" write in log : block, offset, value..
    • periodically apply updates to proper disk locations