Processes and threads

1
Processes and threads

• A process is like a person
• It has consciousness and possessions
• Once started it is like a large group of people,
  coming and going and doing their thing
• Some resources are system-wide
• Others are part of the process

2
More on processes

• What objects are global, local, and shared
• Global
• The file system
• I/O devices and their handlers
• Physical memory
• Network interfaces
• Local
• Transactions on a file
• The memory space of a process
• The currently running program
• Shared
• Shared memory regions
• Anything commonly owned by a process group

3
Processes and threads

• Thread
• The stream of consciousness includes
• Program counter and instruction register
• Other register contents
• Memory regions (stack) reserved to this thread
• It exists on a one processor
• Process
• The possessions include
• Memory regions
• Open files
• Network connections
• Possessions can be available to several threads
  and several processors
• This possession set is often called user context.
• Switching user context is slow, a thread switch
  is fast

4
User and kernel-level threads

• Relationship between processes and their threads
• Originally processes were all single-threaded
• User-level threads are controlled by the
  underlying processKernel is unaware of them it
  schedules processesprocess starts and schedules
  its own threads
• Kernel-level threads are known to and scheduled
  by the kernelIn Linux, they, not processes are
  the unit of schedulingThreads inside a process
  are just those that share parts or most of their
  environment
• Threads are to processes as individual family
  members are to processes
• A third variety is the kernel thread
• This is known only inside the kernel
• It has no user context, so starting it does not
  cause a context switch
• In Linux, most kernel work is done by kernel
  threads

5
Concurrency

• The theoretical core of OS
• An OS creates a set of processes that run
  concurrently
• A process is like a person - it has
• a stream of consciousness
• possessions - memory, files, data
• is created, reproduces, dies
• interacts with other processes
• In actuality, it is a program in execution

6
Concurrency and mutual exclusion

• Concurrent process misbehaviors
• Race conditions - unpredictable results because
  processes simultaneously modify data or devices
• Deadlocks - Each has what the other wants
• Starvation - some processes go hungry - others
  eat well
• Mutual exclusion (ME)
• Motivation is practical some resources dont
  function if modified simultaneously by more than
  one process
• A satisfactory ME mechanism must satisfy
• Mutual exclusion
• No preemption
• Bounded waiting
• Progress

7
Structure of a process

• In any OS
• Consciousness
• Program counter and IP
• Register contents
• Link to possessions
• Possessions
• Memory regions
• Allocated resources
• Open files (shared with other processes)
• Individual file (exclusive during operations)
• Interprocess communication instances (shared with
  other processes)
• note these are used to control use of other
  shared resources

8
Context and context switching

• Process context
• The consciousness (PC, IP, registers) of a
  process
• The possessions / especially memory regions and
  resources
• Context switch
• The act of suspending one process and activating
  another
• Requires saving old register context, restoring
  new
• Requires remapping user memory
• Relatively slow operation (typically 1 ms)
• Kernel context
• Remains the same (not remapped at context switch
  time)
• Only usable context during interrupts

9
Thread and kernel switching

• Thread
• Stream of consciousness only
• Uses (and shares) possessions of some process
  environment
• May be of several types
• kernel-level threads (KLTs) known to and
  scheduled by OS
• user-level threads (ULTs) not known to OS,
  scheduled inside their user process
• Kernel threads - known to and scheduled by OS,
  only kernel context is used, so no context switch
  is used to handle them.
• Thread advantages
• Thread switching within process or kernel is much
  faster than full context switch
• Multiple processes using same resource set are a
  common happening

10
Memory space of a process

• Who allocates these
• Compiler, linker and loader
• User code
• Static data
• reserved space for kernel memory
• Run-time support
• Heap and stack
• Heap is allocated by new
• stack growth area by default after initial
  allocation

Kernel memory
Stack
Unallocated Growth area For stack and heap
Dynamic data (the heap)
Static data
User code
11
Some operating system ideas - also review

• Everything is a file - even devices
• This allows a program to work with human input,
  or a device, or a temporary or permanent file
• Interrupt-driven behavior
• Example - windows - process is simultaneously
  sensitive to events from mouse, keyboard, process
  subsystem, and window manager
• Caching - Foreground and background copies
• Used in
• Cache memory
• Swapping
• The disk block cache - recent transactions are in
  memory
• Networking utilities

12
Processes and their states

• The process state includes the following
• Running/ready/blocked (waiting on an
  event/events)
• Priority
• Kernel/User mode
• In-memory/swapped
• Process ID
• Priority

13
The process owns

• U-area (but user cant examine/modify it)
• Entries in the system open-file table
• Its own file control blocks (linked to the system
  table)
• Its own memory space

14
The system process table

• Used for scheduling and control of processes
• Scheduler acts on timer interrupt or a process
  state change
• It schedules the highest-priority process that is
  ready
• It causes a context switch (gives the process
  control)
• This mechanism
• Gives system and I/O processes high priority
• Keeps the CPU puercos from monopolizing
• Interrupts are outside process country
• This avoids make a context switch for each
  interrupt
• It means the interrupt service routines use only
  the kernels memory

15
Mutual Exclusion - software solutions

• Unsuccessful solutions (on separate slides)
• Strict alternation
• Algorithms where mutual exclusion fails
• Petersons algorithm
• Description of Bakery Algorithm
• Just like Baskin-Robbins - incoming process takes
  a number
• Since two users can grab same number,
  lowest-number process gets priority

16
Avoiding busy waiting - Semaphores and messages

• Classical semaphores
• The semaphore is an ADT with a counter, a process
  queue, and two operations
• The down operation sleeps enqueued if the counter
  is zero, and then decrements the counter
• The up operation increments the counter and
  awakens a sleeper if the counter was 0.
• The down operation enters a critical section, the
  up leaves it.
• Messages
• Reliable messages to an administrator work like a
  down or up
• Messages and semaphores are equivalent
• Thats why

17
Semaphores and messages - continued

• Mistakes are easy
• Deadlock
• Gateway-controlling and resource-controlling
  semaphores invoked in wrong order - deadlocks and
  slow performance

18
Language Mechanisms - Monitors

• Monitor is a class with mutual exclusion it has
• Member functions
• Data members
• Condition variables (user processes wait on
  these)
• Two ways to enter
• Normal
• Returning from wait on a condition variable
• Two ways to leave
• Normal (return from a member function)
• Waiting on a condition variable
• Monitors can be implemented by
• Preprocessor that generates semaphore code
• Compiler modification

19
Classical IPC problems

• Producer-consumer
• Scenario
• Producer produces information
• Consumer consumes it
• Both must wait on buffer availability
• Problem resembler disk I/O and print servers
• Code is in book - both C and monitor
• Readers-Writers
• Scenario
• Readers can write whenever no writer is active
• Writers must have solitude
• Problem resembles database
• Code in book - both C and monitor

20
Classical IPC problems (continued)

• Dining Philosphers
• Scenario
• N philosophers think, wait for chopsticks, and
  eat
• Problem is useless but illustrative
• Code - both C and monitor