Darwin

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Darwin

• Kevin Centofanti
• Jason Snyder
• Kevin Turner

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What is it?

• Unix core to the Mac OS X Operating System
• Based on BSD 4.3 and Mach 3.0
• File System and Networking part of BSD
• Mach handles Scheduling and Memory Management

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Mac OS X Architecture
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Mac OS X Architecture
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Mac OS X Kernel Architecture
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Scheduling

• Tasks Basic unit of resource ownership
• Contains address space, threads, and ports
• Threads Unit of CPU execution within a task
• Ports - Communication channel with associated
  rights

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Scheduling Policy

• Originally developed as part of Mach at CMU
• Combination of Real Time and traditional
  scheduling
• Allows for multimedia applications to run
  simultaneously with interactive applications

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Priority Bands

• Normal - Normal application thread priorities
• System High - Above normal
• Kernel Mode - Threads created in kernel that have
  higher priority than user threads
• Real Time - Priority is based on getting a
  well-defined fraction of total clock cycles

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• Task priority can move up or down but only within
  a priority band
• Computation heavy tasks given low priority to
  improve response time of interactive tasks
• Still run frequently as most high priority I/O
  tasks do a small amount of processing then block,
  allowing low priority tasks to run again

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Real Time Threads

• Real time threads are treated differently by
  scheduler.
• Must tell scheduler how many cycles it needs a
  of next b cycles
• Soft real time so kernel tries to honor request
• If thread uses too many cycles it may be demoted
  to normal priority

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Memory Management

• handled by Mach 3.0

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Kernel Abstractions

• Address space -  populated by memory objects
• Memory objects -  units of memory management

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Virtual Addresses

• Demand Paging

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Pagers

• pager - owner of a memory object
• default pager - handles non-persistent memory,
  known as anonymous memory
• vnode pager - maps files into memory objects

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Default Pager

• anonymous memory - zero-initialized, exists only
  during the life of a task

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vnode Pager

• maps files into memory objects
• EMMI - External Memory Management Interface

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Memory Management

• named memory entries - virtual memory handles,
  denoted by ports

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Direct Allocation

• vm_allocate

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Shared Ranges

• shared memory window - named entry mapped in two
  tasks
• inheritance - new tasks cloned from parent
• copy-on-write - delayed copy

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Another Form of Sharing

• named region - a form of a named entry, backed by
  a virtual map fragment instead of a virtual
  memory object
• virtual map fragment - hold mappings to numerous
  virtual memory objects

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Darwin Virtual File System

• Layer between operations and storage devices
• Uses Access Control Lists
• Simplifies User Experience

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HFS plus

• Volume Header
• Allocation File
• Catalog File
• Extent Overflow File
• Startup File
• Forks
• data fork
• resource fork

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Journaling

• log operations before taking action
• data recovery
• built in support in HFS
• supported by VFS other FS
• transaction
• commit
• replay

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Symbolic Links Aliases

• symbolic links rely on pathname
• Apples aliases store pathname and unique
  identifier
• Aliases update as changes are made in file system

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Device Drivers

• I/O Kit Completely redesigned to support OO
• Implemented as a subset of C to support code
  reuse
• Provides an abstract view of hardware to the
  upper layers of OS X

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Families

• Collection of abstractions for a specific type of
  device
• Example SCSI, USB, Firewire, Disk, HCI
• Devices of the same type share many operations
• Developers only have to code for quirks of
  individual devices
• Resultant drivers are up to 75 smaller than Mac
  OS 9 counterparts

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Drivers

• I/O Kit object that manages a specific device or
  bus
• Upon loading, its dependent families are loaded
  to provide common functionality
• Drivers maintain list of all dependent families
  in a property list

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Nubs

• I/O Kit object that represents a connection to a
  device
• Loaded as a part of the family that instantiates
  it
• Provides access to the device and services such
  as arbitration and power management

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References

• Golub, David Operating System Support for
  Coexistence of Real-Time and Conventional
  Scheduling School of Computer Science Carnegie
  Mellon University
• Apple Kernel Programming Guide http//developer.ap
  ple.com/documentation/Darwin/Conceptual/KernelProg
  ramming/index.
• HFS Plus Volume Formathttp//developer.apple.com/
  technotes/tn/tn1150.html
• The home of Hexley the Platypus The Darwin-OS
  Mascot http//www.hexley.com/