Operating-Systems-Notes

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

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

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

OS bypass

• 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?

• 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:

• 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

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

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