Chapter 3: Processes modified by your instructor

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Chapter 3 Processesmodified by your instructor
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Chapter 3 Processes

• Process Concept
• Process Scheduling
• Operations on Processes
• Cooperating Processes
• Interprocess Communication
• Communication in Client-Server Systems
• This is a very important chapter now that we have
  completed the introductory materials.
• This will require two to three lectures.

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Introductory Remarks Quick Review Concepts
Slide

• We speak of programs and processes.
• It is important to distinguish between the two.
• A process is a program in execution a unit of
  work.
• Programs are files stored somewhere static and
  passive
• A process is executing and needs resources from
  the computing system such as
• CPU cycles
• Memory
• Files
• I/O devices.
• But the operating system is responsible for
  executing processes and all management of overall
  resources and those related to processes
• Creation / deletion of user and system processes
  (user code, system code).
• Process scheduling
• Synchronization of processes, communication among
  processes, and the handling of deadlock
  conditions that can occur among processes.

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Process Concept

• An operating system executes a variety of
  programs
• Batch system jobs
• These usually run from beginning to end
• Time-shared systems user programs or tasks
• Especially interactive tasking
• Textbook uses terms job and process almost
  interchangeably (for some systems, this is okay,
  but not all!)
• Process a program in execution
• A process includes
• program counter (contained in a CPU register)
• Current register settings
• Stack (will see its use ahead)
• data section global variables

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Process in Memory
Text the object code instructions Program
Counter address of the next instruction to be
fetched from memory, brought into the CPU,
decoded, and executed Register settings
current values of working registers Stack
temporary area for program parameters, return
addresses, stack frames (used for recursion)
local variables (specific method) Data global
variables other data buffers Heap
(optional) hunk of memory that may be
made available for dynamic storage allocation
like for creation of objects and more We
usually refer to all this as a processes
address space.
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Process State

• As any process executes, it changes state.
• All processes have state.
• new The process is being created
• running Instructions are being executed
• waiting The process is waiting for some event
  to occur
• ready The process is waiting to be assigned to
  a process
• terminated The process has finished execution
• Even when a process is running, its internal
  state is changing! (values of its variables,
  etc.)

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Diagram of Process State
Discuss states and their transitions
Important to note that at any given instant in
time, a CPU can only be executing a single
instruction from a user or operating system
process. Other processes are in other states
like waiting or ready, or
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Process Control Block (PCB)

• Every process has a PCB an abstraction of the
  process itself.
• Some vendors call PCBs task control blocks (IBM,
  for example)
• The PCB is a data structure used to manage the
  process.
• It contains (shown on next slide) everything
  needed to represent the process.
• Process state ready, waiting, running, etc.
• Program counter address of next instruction
• CPU registers instruction registers, stack
  pointers, index registers, condition-code and/or
  flag registers.
• CPU scheduling information scheduling
  priorities specific queues
• Memory-management information
• Base / limit registers page tables segment
  tables
• these depend upon the memory management scheme
  used
• Accounting information amount of CPU time
  memory used, account numbers, time limits,
  other resource computations used
• I/O status information list of I/O devices
  allocated to the process list of open files,
  their status, etc.

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Process Control Block (PCB)
When a process changes state, say from Running to
Wait, the values / contents of the CPUs
registers, instruction counter, etc. are copied
into this abstraction of the process to provide
for resumption later. All processes have a PCB
to represent them and the resources,
constraints, temporary values, etc. associated
with the process they represent. Used to manage
the process resumption of processing,
protection of areas of memory, accounting info
and much more.
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CPU Switch From Process to Process
Originating outside or within the process.
Note
Discuss Context Switching!
Or some other Ready process!
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Process Scheduling Queues

• Remember goal of what we call multiprogramming
  is to have some process running all the time to
  maximize CPU utilization
• Goal of time sharing is to switch the context
  from one process to another so users can interact
  with each program while it is running.
• ? With a single processor system, only one
  program can be running at any instant in time.
• Thus we have a real need of very robust
  scheduling.
• This can be a very complex undertaking with
  several queues and algorithms for determining the
  best CPU utilization while providing required
  services.
• We call this the mix and refer to this as CPU
  loading.

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Process Scheduling Queues

• Job queue set of all processes in the system
• These may likely not be ready for execution,
  but they are in the system and desire to be
  executed. (have not been made ready)
• Ready queue set of all processes residing in
  main memory, ready and waiting to execute that
  is, ready for CPU dispatching.
• The Ready Queue is often implemented as a
  linked list
• Unfortunately, there may be a number of Ready
  queues (later!)
• The Ready-queue header contains a pointer to the
  first PCB in ready queue.
• Other PCBs are linked via a singly-linked forward
  pointer part of each PCB.
• Device queues set of processes waiting for an
  I/O device, as you might expect.
• Two processes requesting access to the same
  device will require queueing.
• Different data structure may be used to manage
  different queues Different searching techniques
  may be used within different queues to obtain the
  next process. Regular queues, priority queues,
  etc. may be used here and there. ?
• Important to note Processes migrate among the
  various queues

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Observe the Structure of the PCB in Linux

• Of course, Linux (written in C) uses structures.
  E.g. struct data.
• A processs parent is the process that created it
  (for multi threading).
• A few of the fields are
• pid_t pid // a unique process
  identifier (usually integer)
• long state // state of the process
• unsigned int time_slide //
  scheduling info
• struct files_struct files // list
  of open files (string data)
• struct mm_struct mm // address
  space of the process
• State of the process is in state (captured in
  this PCB)
• In Linux kernel, all active processes are
  presented using a doubly-linked list of
  task_struct and the kernel maintains a pointer
  called current to the process currently
  executing.
• Good figure in your textbook. Pretty
  straightforward.

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Ready Queue And Various I/O Device Queues
Remember from your data structures course that
regular queues have items (places in an array,
nodes, etc. (PCBs here) inserted into the
tail and items (PCBs here too) removed (or
accessed) from the head of the queue. A
regular queue, then, has at least these two
pointers one to the head, and one to the
tail.
These headers are permanent. But PCBs are
created and destroyed as needed.
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Representation of Process Scheduling

• Brief look at process scheduling
• There are two queues here
• a ready queue and
• a device queue.
• When a new process
• is made ready it is put
• into a ready queue
• until it is dispatched!
• Once dispatched, process
• may continue until
• - process requests an I/O
• - process creates a new
• sub-process and will now
• await sub-process termination, or
• process could be forcibly removed from CPU via
  an interrupt and be placed back in
• ready queue.

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Schedulers

• Clearly, a process will migrate among different
  queues as it continues along a path of execution
  until process terminates.
• But he actual selection of which process to
  execute is carried out by a scheduling algorithm.
• Long-term scheduler (or job scheduler) selects
  which processes should be brought into the ready
  queue from outside of what is being executed
• Particularly useful for batch jobs.
• Usually these are spooled onto disk ..
• Not executed too frequently maybe even minutes
  between creation of a new process
• The job scheduler controls the degree of
  multiprogramming, a.k.a. the number of processes
  in memory.
• Ideally, the rate of process termination
  influences the rate at which the job scheduler
  schedules a new job for execution assuming the
  job mix is in a stable operation.
• (All kinds of parameters in the selection process
  ahead)Again, we are after a good mix of
  processes depending on their characteristics
  to provide for excellent service to users and
  high CPU utilization and other system resources.

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Scheduler

• Short-term scheduler (or CPU scheduler)
  selects which process should be executed next and
  allocates CPU
• Processes provided to the CPU scheduler via
  queueing are frequently executed only for a very
  short time until they request an I/O, cause a
  trap, fork a process, etc.
• But then these are rapidly placed back into the
  ready queue (CPU queue).
• Great care must be taken by the job scheduler in
  selecting processes into the mix.
• The differences between I/O bound jobs and
  CPU-bound jobs is incredibly important!
• Much more directly ahead on this

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Schedulers (Cont.)

• Processes can be described as either
• I/O-bound process spends much more time doing
  I/O than computations, many short CPU bursts.
• This means that these jobs may spend a lot of
  time in a wait queue before they become ready
  again (once the I/O request has been
  accommodated.
• Too many of these in the mix and the CPU becomes
  idle. This is bad!
• Ready queue may become empty, and the short-term
  scheduler (CPU scheduler) may have little to do.
• These tend to be business-oriented, file
  processing, data base jobs.
• CPU-bound process spend more time doing
  computations few long CPU bursts
• If the mix contains too many computationally-orien
  ted jobs, the I/O waiting queue may be empty
  I/O devices unused, and system load unbalanced.
  This too is bad.
• These tend to be more scientific or engineering
  type jobs where there may be a lot of
  computations rather than lots of input/output.
• Systems providing the best overall performance
  must have a good mix of both I/O bound jobs and
  CPU-bound jobs in a general purpose processing
  environment.

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More on Schedulers

• Unix and Microsoft Windows do not have a job
  scheduler.
• Put all input processes into memory for the CPU
  scheduler.
• Stability depends on the physical limitations of
  the computing system (like number of terminals,
  devices, etc.).
• System can come to a grinding halt too.
• Other operating systems may use a medium-term
  scheduler that may be used to remove processes
  from memory (and thus for active contention for
  the CPU) and thus reduce the degree of
  multiprogramming, but lighten the burden on the
  operating system and improve throughput should
  throughput become an issue.
• This scheme is called swapping, and the swapped
  process will be later swapped back in.
• Discussed in later chapters.

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Context Switch

• When CPU switches to another process, the system
  must save the state of the old process and load
  the saved state for the new process
• A very common, typical occurrence on
  general-purpose computing systems that occurs
  thousands and thousands of time in a unit
  time..
• When getting an interrupt, the system saves the
  current context of the process being executed
  (that is, saves its state).
• Its PCB is used to store the interrupted
  processs state and all necessary parameters for
  resumption.
• The PCB will be later used resumption.
• Changing the CPU from one process to another is
  called context switching.
• Context-switch time is overhead the system does
  no useful work while switching
• ? The time it takes for context switching is
  very hardware dependent.(10 mics)
• Copying register contents to/from memory takes
  time not much, but when this is done many
  thousands of time in a minute, this overhead adds
  up.
• The details of preserving the address space and
  related issues are discussed in memory
  management, because the space used by the
  currently executing process may well need to be
  freed up for some other process(es) but maybe
  not.

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Shells and the Kernel

• Before we get into Operations on Processes, I
  want to present a short discussion on Shells and
  the Kernel.
• These may be sometimes confusing
• The Kernel and the Shell

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Kernel

• WE know that the kernel is that part of an
  operating system software where the real work is
  done. It performs all those tasks dealing with
  input and output devices such as managing disk
  drives, CPU scheduling, memory management, and so
  very much more..
• The Unix kernel is the part of the operating
  system that contains code for
• Sharing the CPU and RAM between competing
  processes
• Processing all system calls
• Handling peripherals
• The kernel is a program that is loaded from disk
  into RAM when the computer is first turned on
  (recall System Boot).
• It always stays in RAM and runs until the system
  is turned off or crashes (or at least most of
  it).
• Kernel design and coding must be terribly
  efficient.
• is written mostly in C, although some parts are
  written in assembly language for efficiency
  reasons.
• User programs make use of the kernel via the
  system call interface, which we have talked
  about.

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Kernel Subsystems

• Kernel facilities are typically divided into five
  subsystems with specific hunks of functionality
  for managing each
• Memory management
• Process management
• Inter-process communications (IPC)
• Input/output
• File management
• These are the facilities / management / control /
  services that the kernel provides for us as
  services for both user and system process.
• The kernel is the big cookie.

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Kernel Subsystems a view of files and I/O

• Hierarchically, a view that addresses file
  management, I/O, and peripherals may appear as

File management
Input/output
peripherals
And
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Kernel Subsystems

• And views at lower level are

Inter-process Communication
Process Management
Memory Management
CPU RAM
hardware
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Talking to the Kernel

• Processes access kernel facilities via the system
  call interface (SCI), and peripherals (special
  files) communicate with the kernel via hardware
  interrupts.
• ? System calls and hardware interrupts are the
  only ways in which the outside world can talk to
  the kernel.
• Since these are very important, we will devote a
  lot of time to these in the appropriate sections.
  
• But for now, you may get a visual on this by

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Visual on Talking to the Kernel
Process
Process
Process
This visual says a great deal!
System Calls
Kernel
Hardware Interrupts
peripheral
peripheral
peripheral
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Shells

• A shell is a program that sits between you and
  the raw Unix operating system.
• There are four shells commonly supported by Unix
  vendors, and weve mentioned them in the past.
  They are the
• 1. Bourne shell (sh)
• 2. Korn shell (ksh)
• 3. C shell (csh), and the
• 4. Bourne Again shell (bash).
• There is a common core set of operations that
  make life in the Unix system a little easier and
  somewhat consistent.
• Example all the shells allow the output of a
  process to be stored in a file or piped to
  another process. (e.g. ls more )
• They also allow the use of wildcards in filenames
  so its easy to say things like list all of the
  files whose names end with the suffix .c as in
  ls .c
• Pick your shell. These all work in the same
  way.

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Shells

• Shell selection is a matter of taste, power,
  compatibility, and availability.
• Interactive Work Some feel that the C shell is
  better than the Bourne shell for interactive
  work, but slightly worse in some respects for
  script programming.
• Upwards Compatibility The Korn shell was
  designed to be upwards compatible with the Bourne
  shell, and it incorporates the best features of
  the Bourne and C shell, plus some more features
  of its own.
• Optimal Bash also takes a best of all worlds
  approach including features from all the other
  major shells.

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Bourne Shell

• The Bourne shell comes with absolutely every
  version of UNIX. The others come with most
  versions these days, but you might not always
  find your favorite.
• Bash probably ships with the fewest versions of
  UNIX, but is available for the most.
• (Steve Bourne is a personal friend of mine. We
  work each year together at the ICPC (Inter
  Collegiate Programming Contest sponsored by IBM).
  He works with Upsilon Pi Epsilon (UPE) to assist
  us in annual registration of programming teams.)

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Shells last for now

• It is very simple to change your default login
  shell, if you wish. You only need to know the
  full pathnames of the four shells normally,
  /bin/ksh or /bin/sh, etc.
• Each shells start up is a wee bit different.
• Most execute some special start-up file usually
  in the users home directory that contains
• some initialization information (some info is
  usually gotten from your profile),
• displays a prompt, and
• waits for a user command.
• Entering a control-D on a line of its own is
  interpreted by the shell as end of input and
  causes shell to terminate otherwise, the shell
  executes the users command.
• Bottom line for those interested in the shell
  game is that the shell is essentially an
  interface to the kernel, where all the work is
  done.

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End of Chapter 3.1