Processes and Threads

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Processes and Threads
Creation and Termination States Usage Implementati
ons
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Processes

• Program in execution
• (cf. recipe vs. cooking)
• Multiprogramming - pseudo-parallelism
• (vs. true hardware parallelism of
  multiprocessor systems)

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The Process Model

• Multiprogramming of four programs
• Conceptual model of 4 independent, sequential
  processes
• Only one program active at any instant

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Process Creation

• Principal events that cause process creation
• System initialization (foreground a daemon
  processes)
• Execution of a process creation system call (data
  from network)
• User request to create a new process
• Initiation of a batch job
• UNIX fork system call ( execve)
• Windows CreateProcess function call

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Process Termination

• Conditions which terminate processes
• Normal exit (voluntary) - (exit, ExitProcess)
• Error exit (voluntary)
• Fatal error (involuntary), e.g. program bug
• Killed by another process (involuntary) - kill,
  TerminateProcess

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Process Hierarchies

• Parent creates a child process, child processes
  can create its own process
• Forms a hierarchy
• UNIX calls this a "process group
• init
• Windows has no concept of process hierarchy
• all processes are created equal

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Process States (1)

• Possible process states
• running
• blocked
• ready
• Transitions between states shown

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Process States (2)

• Lowest layer of process-structured OS
• handles interrupts, scheduling
• Above that layer are sequential processes

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Implementation of Processes (1)

• Fields of a process table entry

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Implementation of Processes (2)

• Skeleton of what lowest level of OS does when an
  interrupt occurs

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Threads

• Process
• resource grouping (code, data, open files, etc.)
• execution (program counter, registers, stack)
• Multithreading
• multiple execution takes place in the same
  process environment
• co-operation by sharing resources (address space,
  open files, etc.)

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The Thread Model (1)

• (a) Three processes each with one thread
• (b) One process with three threads

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The Thread Model (2)

• Items shared by all threads in a process
• Items private to each thread

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The Thread Model (3)

• Each thread has its own stack to keep track
  execution history (called procedures)

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Advantages

• Pseudo-parallelism with shared address space and
  data
• Easier to create and destroy than processes
• Better performance for I/O bound applications

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Thread Usage (1)

• A word processor with three threads
• Writing a book interactive and background
  threads sharing the same file

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Thread Usage (2)

• A multithreaded Web server

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Thread Usage (3)

• Rough outline of code for previous slide
• (a) Dispatcher thread
• (b) Worker thread

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Thread Usage (4)

• Three ways to construct a server

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Implementing Threads in User Space

• A user-level threads package

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(Dis)advantages

• no specific OS support needed
• faster than kernel instructions
• process-specific scheduling algorithms
• - blocking system calls (select)
• page faults

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Implementing Threads in the Kernel

• A threads package managed by the kernel

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(Dis)advantages

• handling blocking and page faults
• - more costly (but recycling threads)

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Hybrid Implementations

• Multiplexing user-level threads onto kernel-
  level threads

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Scheduler Activations

• Goal
• mimic functionality of kernel threads
• gain performance of user space threads
• Avoids unnecessary user/kernel transitions
• Kernel assigns virtual processors to each process
• lets runtime system allocate threads to
  processors, upcall
• Problem Fundamental reliance on kernel
  (lower layer)
• calling procedures in user space (higher
  layer)

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Pop-Up Threads

• Creation of a new thread when message arrives
• (a) before message arrives
• (b) after message arrives (quick)

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Making Single-Threaded Code Multithreaded (1)

• Conflicts between threads over the use of a
  global variable

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Making Single-Threaded Code Multithreaded (2)

• Threads can have private global variables.
• But non-reentrant library procedures.