Process Management                     1
Introduction   In conventional (centralized) operating system, process    management deals with mechanisms and policies f...
Cont…   Process allocation deals with the process of    deciding which process should be assigned to which    processor....
Process Migration   Relocation of a process from its current location to    another node.   Process may be migrated    ...
Cont… : Flow of execution of amigration process                              Source node                                  ...
Cont…   Preemptive process migration is costlier.   Process Migration Policy       Selection of a process that should b...
Desirable features of a good processmigration mechanism   Transparency   Minimal Interference   Minimal Residual Depend...
TransparencyLevel of transparency:     Object Access Level     System Call & Interprocess Communication level           ...
Cont… : Object Access level Transparency   Minimum requirement for a system to support non-    preemptive process migrati...
Cont… : System Call & IPC level   For migrated process, system calls should be    location independent.   For transparen...
Minimal Interference   Migration of a process should cause minimal    interference of progress of the process involved. ...
Minimal Residual Dependencies   No residual dependency should be left on    previous node.                               ...
Efficiency   Minimum time required for migrating a process.   Minimum cost of locating an object.   Minimum cost of sup...
Robustness   The failure of a node other than the one on which a    process is currently running should not in any way   ...
Communication between coprocessors of a job   To reduce communication cost, it is necessary that    coprocesses are able ...
Process Migration MechanismsFour major activities Freezing the process on its source node and  restarting it on its desti...
Cont… : Mechanisms for Freezing andRestarting a Process   Freezing the process:       The execution of the process is su...
Cont… :Immediate and delayedblocking of the process   If the process is not executing a system call, it can    be immedia...
Cont… : Fast and Slow I/O Operations   Process is frozen after the completion of all fast I/O    operations like disk acc...
Cont… : Information about open files    Includes name and identifier of the file, their access     modes and the current ...
Cont…   Approaches :     Link is created to the file and the pathname of the      link is used as an access point to the...
Cont… : Reinstating the process on itsDestination Node   On the destination node, an empty process state is    created. ...
Cont…   Once all the state of the migrating process has been    transferred from the source to destination node and    co...
Address Space Transfer Mechanisms   The migration of a process involves the    transfer of       Process’s state       ...
Cont…   Process State consists of       Execution Status – Register Contents       Memory Tables       I/O State : I/O...
Cont…   Process address space       code,       data and       program stack   The size of the process’s address spac...
Cont…   Mechanisms for address space transfer:       Total freezing       Pretransferring       Transfer on reference ...
Cont… : Total Freezing   A process’s execution is stopped while its address    space is being transferred.   Disadvantag...
Cont…                                 Source       Destination node                                  node                 ...
Cont … : Pretransferring   Also known as precopying.   The address space is transferred while the process is    still ru...
Cont …   The pretransfer operation is executed at a higher    priority than all other programs on the source node.   Red...
Cont…                                Source       Destination node                                 node                Tim...
Cont… : Transfer on Reference   The process address space is left behind on its    source node, and as the relocated proc...
Cont…   Very short switching time of the process from its    source node to its destination node.   Imposes a continued ...
Cont…                            Source              Destination                             node                 node    ...
Message-forwarding Mechanisms   In moving a process, it must be ensured that all    pending, re-route, and future message...
Cont…   Mechanisms:       Mechanism of resending the message       Origin site mechanism       Link traversal mechanis...
Cont… : Mechanisms of resending themessage   Messages of the types 1 and 2 are returned to the    sender as not deliverab...
Cont…   Disadvantage:       The message forwarding mechanism of process        migration operation is nontransparent to ...
Cont… : Origin Site Mechanism   Each site is responsible for keeping information    about the current locations of all th...
Cont…   Disadvantage:       Failure of the origin site will disrupt the message        forwarding mechanism.       Cont...
Cont… : Link Traversal Mechanism   To redirect message type 1, a message queue for the    migrant process is created on i...
Cont…   Disadvantage:       Several link may have to be traversed to locate a process        from a node and if any node...
Cont… : Link Update Mechanisms   During the transfer phase of the migrant process,    the source node sends link-update m...
Mechanisms for HandlingCoprocesses   To provide efficient communication between a    process and its sub processes which ...
Cont… : Disallowing Separation ofcoprocesses   Easiest method of handling communication    between coprocesses is to disa...
Cont…   Disadvantage:       It does not allow the use of parallelism within jobs.                                       ...
Cont… : Home node or Origin SitesConcept   Used for communication between a process and its    sub process when the two a...
Cont…   Disadvantage:       All communication between a parent process and its        children processes take place via ...
Process Migration in Heterogeneous Systems   All the concerned data must be translated from the    source CPU format to t...
Cont…                                   1          Processor of                            Processor of             type 1...
Cont…                               Processor of                                  type 1                               1  ...
Cont… : Handling Exponent   The number of bits for the exponent varies from    processor to processor.   The EDR have at...
Cont…   Some solutions:       Ensuring that numbers used by the programs that migrate        have a smaller exponent val...
Cont… : Handling the Mantissa   Suppose processor A uses 32 bits and processor B    uses 64 bits.       No problem from ...
Cont…   Problem:       Loss of precision due to multiple migrations.   Solution:       Remote computations can be view...
Advantages of Process Migration   Reducing average response time of processes       To reduce the average response time ...
Cont…   Gaining higher throughput       Process migration facility may also be used properly to        mix I/O and CPU-b...
Cont…   Reducing network traffic       Migrating a process closer to the resources it is using        most heavily.   I...
Process   A basic unit of CPU utilization in traditional OS.   A program in execution.   Defines a data space.   Has i...
Threads   It is a basic unit of CPU utilization used for    improving a system performance through    parallelism.   A p...
Cont…   On a uniprocessor, threads run in quasi-parallel    (time sharing), whereas on a shared-memory    multiprocessor,...
Threads      Address space              Address space         Thread               Thread   Thread   Thread   (a) Single-t...
Motivations for using Threads   Overheads involved in creating a new process is more    than creating a new thread.   Co...
Different models to construct a server    process: As a single-thread process    Use blocking system calls but without an...
Cont… : As a finite state machine   Model support parallelism but with nonblocking    system calls.   Implemented as a s...
Cont… : As a group of threads   Supports parallelism with blocking system calls.   Server process is comprised of a sing...
Models for organizing threads   Dispatcher-workers model   Team model   Pipeline model                                68
Cont… : Dispatcher-worker model   Single dispatcher thread and multiple worker    threads.   Dispatcher thread accepts r...
Cont…             Requests                                 A Server process for                 Port            processing...
Cont… : Team Model   There is no dispatcher-worker relationship for    processing clients requests.   Each thread gets a...
Cont…                 Requests                       Port            A Server process for processing                      ...
Cont… : Pipeline Model   Useful for application based on the producer-    consumer model.   The threads of a process are...
Cont…                 Requests              A Server process for processing                                       incoming...
Issues in designing a Thread Package1.   Threads creation2.   Thread termination3.   Threads synchronization4.   Threads s...
Threads Creation   Created either statically or dynamically.   Static approach:       Threads remain fixed for its enti...
Threads Termination   A thread may either destroy itself when it finishes    its job by making an exit call              ...
Threads Synchronization   Threads share a common address space, so it is    necessary to prevent multiple threads from tr...
Cont…   Segment of code in which a thread may be accessing    some shared variable is called a critical region.   Two co...
Cont…   Mutex have only 2 states, hence simple to    implement.   Condition variable associated with mutex variable &   ...
Cont…                     Thread1    Thread2   Lock (mutex_A)                      Lock (mutex_A) fails      succeeds     ...
Threads Scheduling    Special features for threads scheduling :    1.   Priority assignment facility    2.   Flexibility ...
Cont… : Priority assignment facility   Threads are scheduled on the first-in, first-out basis    or the round-robin polic...
Cont… : Flexibility to vary quantum sizedynamically   Use of round-robin scheduling scheme.   Varies the size of fixed-l...
Cont… : Handoff Scheduling   Allows a thread to name its successor if it wants to.   Provides flexibility to bypass the ...
Cont… : Affinity scheduling   A thread is scheduled on the CPU it last ran on in    hopes that part of its address space ...
Signal handling   Signals provide software-generated interrupts and    exceptions.   Issues :       A signal must be ha...
Cont…   Approach:       Create a separate exception handler thread in each        process.       Assign each thread its...
Thread Implementation   Often provided in form of a thread package.   Two important approaches:       First is to const...
User level threads   It is cheap to create and destroy threads.   Cost of creating or destroying thread is determined by...
Cont…   Drawback: Invocation of a blocking system call will    immediately block the entire process to which the    threa...
User and Kernel Level   Threads are useful to structure large application into    parts that could be logically executed ...
Cont…   Price: Every thread operation will have to be carried    out by kernel requiring system calls. Hence benefits    ...
Implementing a Threads PackageUser-level approach   Consists of a runtime system that is a collection of threads    manag...
Cont… : User level                     Processes and their                           threads     User space               ...
Cont…Kernel-level approach   No runtime system is used and threads are managed by the    kernel.   Threads status info. ...
Cont… : Kernel level     User space     Processes and their                          threads     Kernel space    Kernel (m...
Cont…Advantages of the approaches   User-level approach can be implemented on top of an    existing OS that does not supp...
Cont…Disadvantages of the approaches:   Since there is no clock interrupt within a single process, so    once CPU is give...
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process management

  1. 1. Process Management 1
  2. 2. Introduction In conventional (centralized) operating system, process management deals with mechanisms and policies for sharing the processor of the system among all processes. In a distributed operating system, the main goal of process management is to make the best possible use of the processing resources of the entire system by sharing them among all processes. Three important concepts are used to achieve this goal: -Processor allocation -Process migration -Threads 2
  3. 3. Cont… Process allocation deals with the process of deciding which process should be assigned to which processor. Process migration deals with the movement of a process from its current location to the processor to which it has been assigned. Threads deals with fine-grained parallelism for better utilization of the processing capability of the system. 3
  4. 4. Process Migration Relocation of a process from its current location to another node. Process may be migrated  either before it start executing on its source node  known as non-preemptive process migration.  or during the course of its execution  Known as preemptive process migration. 4
  5. 5. Cont… : Flow of execution of amigration process Source node Destination node Time Process P1 in execution Execution suspended Transfer of control Freezing time Execution resumed Process p1 in execution 5
  6. 6. Cont… Preemptive process migration is costlier. Process Migration Policy  Selection of a process that should be migrated.  Selection of the destination node to which the selected process should be migrated. Process Migration Mechanism  Actual transfer of the selected process to the destination node. 6
  7. 7. Desirable features of a good processmigration mechanism Transparency Minimal Interference Minimal Residual Dependencies Efficiency Robustness Communication between coprocesses of a job 7
  8. 8. TransparencyLevel of transparency:  Object Access Level  System Call & Interprocess Communication level 8
  9. 9. Cont… : Object Access level Transparency Minimum requirement for a system to support non- preemptive process migration facility. Access to objects such as files and devices can be location independent. Allows free initiation of program at arbitrary node. Requires transparent object naming and locating. 9
  10. 10. Cont… : System Call & IPC level For migrated process, system calls should be location independent. For transparent redirection of messages during the transient state of a process. Transparency must be provided to support preemptive process migration facility. 10
  11. 11. Minimal Interference Migration of a process should cause minimal interference of progress of the process involved. Achieve by minimizing the freezing time of the process being migrated.  Freezing time is the time period for which the execution of the process is stopped for transferring its information to the destination node. 11
  12. 12. Minimal Residual Dependencies No residual dependency should be left on previous node. 12
  13. 13. Efficiency Minimum time required for migrating a process. Minimum cost of locating an object. Minimum cost of supporting remote execution once the process is migrated. 13
  14. 14. Robustness The failure of a node other than the one on which a process is currently running should not in any way affect the accessibility or execution of that process. 14
  15. 15. Communication between coprocessors of a job To reduce communication cost, it is necessary that coprocesses are able to directly communicate with each other irrespective of their locations. 15
  16. 16. Process Migration MechanismsFour major activities Freezing the process on its source node and restarting it on its destination node. Transferring the process’s address space from its source node to its destination node. Forwarding massages meant for the migrant process. Handling communication between cooperating processes that have been separated as a result of process migration. 16
  17. 17. Cont… : Mechanisms for Freezing andRestarting a Process Freezing the process:  The execution of the process is suspended and all external interactions with the process are deferred. Issues:  Immediate and delayed blocking of the process  Fast and slow I/O operations  Information about open files  Reinstating the process on its Destination node 17
  18. 18. Cont… :Immediate and delayedblocking of the process If the process is not executing a system call, it can be immediately blocked from further execution. If the process is executing a system call but is sleeping at an interruptible priority waiting for a kernel event to occur, it can be immediately blocked from further execution. If the process is executing a system call but is sleeping at an non-interruptible priority waiting for a kernel event to occur, it cannot be blocked immediately. 18
  19. 19. Cont… : Fast and Slow I/O Operations Process is frozen after the completion of all fast I/O operations like disk access. Slow I/O operation (pipe or terminal) is done after process migration and when process is executed on destination node. 19
  20. 20. Cont… : Information about open files  Includes name and identifier of the file, their access modes and the current positions of their file pointers.  OS returns a file descriptor to process that is used for all I/O.  It is necessary to somehow preserve a pointer to the file so that migrated process could continue to access it. 20
  21. 21. Cont… Approaches :  Link is created to the file and the pathname of the link is used as an access point to the file after the process migrates.  An open file’s complete pathname is reconstructed when required by modifying the kernel. Keeping Track of file replicas. 21
  22. 22. Cont… : Reinstating the process on itsDestination Node On the destination node, an empty process state is created. Newly allocated process may or may not have the same process identifier as the migrating process. 22
  23. 23. Cont… Once all the state of the migrating process has been transferred from the source to destination node and copied into the empty state, new copy of the process is unfrozen and old copy is deleted. The process is restarted on its destination node in whatever state it was in before being migrated. 23
  24. 24. Address Space Transfer Mechanisms The migration of a process involves the transfer of  Process’s state  Process’s address space from the source node to the destination node. 24
  25. 25. Cont… Process State consists of  Execution Status – Register Contents  Memory Tables  I/O State : I/O Queue, I/O buffers, Interrupts  Capability list  Process’s Identifier  Process’s user and group identifier  Information about Open Files 25
  26. 26. Cont… Process address space  code,  data and  program stack The size of the process’s address space (several megabytes) overshadows the size of the process’s state information (few kilobytes). 26
  27. 27. Cont… Mechanisms for address space transfer:  Total freezing  Pretransferring  Transfer on reference 27
  28. 28. Cont… : Total Freezing A process’s execution is stopped while its address space is being transferred. Disadvantage:  Process is suspended for long time during migration, timeouts may occur, and if process is interactive, the delay will be noticed by the user. 28
  29. 29. Cont… Source Destination node node Time Migration Execution decision made suspended Transfer of Freezing time address space Execution resumed 29
  30. 30. Cont … : Pretransferring Also known as precopying. The address space is transferred while the process is still running on the source node. It is done as an initial transfer of the complete address space followed by repeated transfers of the page modified during the previous transfer. 30
  31. 31. Cont … The pretransfer operation is executed at a higher priority than all other programs on the source node. Reduces the freezing time of the process but it may increase the total time for migrating due to the possibility of redundant page transfers. 31
  32. 32. Cont… Source Destination node node Time Migration decision made Execution Transfer of suspended address spaceFreezing time Execution resumed 32
  33. 33. Cont… : Transfer on Reference The process address space is left behind on its source node, and as the relocated process executes on its destination node. Attempts to reference memory page results in the generation of requests to copy in the desired blocks from their remote location. A page is transferred from its source node to its destination node only when referenced. 33
  34. 34. Cont… Very short switching time of the process from its source node to its destination node. Imposes a continued load on the process’s source node and results in the process if source node fails or is rebooted. 34
  35. 35. Cont… Source Destination node node Time Execution Migration suspended decision made Freezing time Execution resumed On-demand transfer of address space 35
  36. 36. Message-forwarding Mechanisms In moving a process, it must be ensured that all pending, re-route, and future messages arrive at the process’s new location. Types of messages: 1. Messages received at the source node after the process’s execution has been stopped on its source node and the process’s execution has not yet been started on its destination node. 2. Messages received at the source node after the process’s execution has started on its destination node. 3. Messages that are to be sent to the migrant process from any other node after it has started executing on the destination node. 36
  37. 37. Cont… Mechanisms:  Mechanism of resending the message  Origin site mechanism  Link traversal mechanism  Link update mechanism 37
  38. 38. Cont… : Mechanisms of resending themessage Messages of the types 1 and 2 are returned to the sender as not deliverable or are simply dropped, with the assurance that the sender of the message is storing a copy of the data and is prepared to retransmit it. Does not require any process state to be left behind on the process’s source node. 38
  39. 39. Cont… Disadvantage:  The message forwarding mechanism of process migration operation is nontransparent to the processes interacting with the migrant process. 39
  40. 40. Cont… : Origin Site Mechanism Each site is responsible for keeping information about the current locations of all the processes created on it. Messages for a particular process are always first sent to its origin site. The origin site then forwards the message to the process’s current location. 40
  41. 41. Cont… Disadvantage:  Failure of the origin site will disrupt the message forwarding mechanism.  Continuous load on the migrant process’s origin site even after the process has migrated from that node. 41
  42. 42. Cont… : Link Traversal Mechanism To redirect message type 1, a message queue for the migrant process is created on its source node. For 2 and 3 type message, a forwarding address known as link is left at the source node pointing to the destination of the migrant process. Two component of link, one is unique process identifier and second is last known location. 42
  43. 43. Cont… Disadvantage:  Several link may have to be traversed to locate a process from a node and if any node in chain of link fails, the process cannot be located. 43
  44. 44. Cont… : Link Update Mechanisms During the transfer phase of the migrant process, the source node sends link-update messages to the kernels controlling all of the migrant process’s communication partners. Link update message  Tells the new address of each link held by the migrant process.  Acknowledged for synchronization purposes. 44
  45. 45. Mechanisms for HandlingCoprocesses To provide efficient communication between a process and its sub processes which might have been migrated on different nodes. Mechanisms :  Disallowing separation of coprocesses.  Home node or origin site concept. 45
  46. 46. Cont… : Disallowing Separation ofcoprocesses Easiest method of handling communication between coprocesses is to disallow their separation. Methods :  By disallowing the migration of processes that wait for one or more of their children to complete.  By ensuring that when a parent process migrates, its children process will be migrated along with it. 46
  47. 47. Cont… Disadvantage:  It does not allow the use of parallelism within jobs. 47
  48. 48. Cont… : Home node or Origin SitesConcept Used for communication between a process and its sub process when the two are running on different nodes. Allows the complete freedom of migrating a process or its sub process independently and executing them on different nodes of the system. 48
  49. 49. Cont… Disadvantage:  All communication between a parent process and its children processes take place via the home node.  The message traffic and the communication cost increase considerably. 49
  50. 50. Process Migration in Heterogeneous Systems All the concerned data must be translated from the source CPU format to the destination CPU format before it can be executed on the destination node. A heterogeneous system having n CPU types must have n(n-1) pieces of translation software. Handles problem of different data representations such as characters, integers and floating-point numbers. 50
  51. 51. Cont… 1 Processor of Processor of type 1 4 type 2 3 10 5 8 11 7 6 2 Processor of 9 Processor of type 3 type 4 12 Example: The need for 12 pieces of translation software required in a heterogeneous system having 4 types of processors 51
  52. 52. Cont… Processor of type 1 1 2 8 3 Processor of External data Processor of type 4 representation type 2 4 7 5 6 Processor of type 3Example: The need for only 8 pieces of translation software in aheterogeneous system having 4 types of processors when the EDRmechanism is used 52
  53. 53. Cont… : Handling Exponent The number of bits for the exponent varies from processor to processor. The EDR have at least as many bits in the exponent as the largest exponent. Overflow or underflow can be managed upon conversion from the external data representation. 53
  54. 54. Cont… Some solutions:  Ensuring that numbers used by the programs that migrate have a smaller exponent value than the smallest processor’s exponent value in the system.  Emulating the larger processor’s value.  Restricting the migration of the process to only those nodes whose processor’s exponent representation is at least as large as that of the source node’s processor. 54
  55. 55. Cont… : Handling the Mantissa Suppose processor A uses 32 bits and processor B uses 64 bits.  No problem from A to B migration.  Half-Precision problem from B to A migration. Solution:  The external data representation must have sufficient precision to handle the largest mantissa. 55
  56. 56. Cont… Problem:  Loss of precision due to multiple migrations. Solution:  Remote computations can be viewed as ‘extra precision’ calculation. 56
  57. 57. Advantages of Process Migration Reducing average response time of processes  To reduce the average response time of the processes, processes of a heavily loaded node are migrated to idle or underutilized nodes. Speeding up individual jobs  A migration of job to different node is done and execute them concurrently.  Migrate a job to a node having a faster CPU or to a node at which it has minimum turnaround time.  More speed up more migration cost involved. 57
  58. 58. Cont… Gaining higher throughput  Process migration facility may also be used properly to mix I/O and CPU-bound processes on a global basis for increasing the throughput of the system. Utilizing resources effectively  Depending upon the nature of a process, it can be migrated to suitable node to utilize the system resource in the most efficient manner. 58
  59. 59. Cont… Reducing network traffic  Migrating a process closer to the resources it is using most heavily. Improving system reliability  Migrate a copy of a critical process to some other node and to execute both the original and copied processes concurrently on different nodes. Improving system security  A sensitive process may be migrated and run on a secure node. 59
  60. 60. Process A basic unit of CPU utilization in traditional OS. A program in execution. Defines a data space. Has its own program counter, its own register states, its own stack and its own address space. Has at least one associated thread. Unit of distribution. 60
  61. 61. Threads It is a basic unit of CPU utilization used for improving a system performance through parallelism. A process consist of an address space and one or more threads of control. Threads share same address space but having its own program counter, register states and its own stack. Less protection due to the sharing of address space. 61
  62. 62. Cont… On a uniprocessor, threads run in quasi-parallel (time sharing), whereas on a shared-memory multiprocessor, as many threads can run simultaneously as there are processors. States of Threads:  Running, blocked, ready, or terminated. Threads are viewed as miniprocesses. 62
  63. 63. Threads Address space Address space Thread Thread Thread Thread (a) Single-threaded (b) Multithreaded processes 63
  64. 64. Motivations for using Threads Overheads involved in creating a new process is more than creating a new thread. Context switching between threads is cheaper than processes due to their same address space. Threads allow parallelism to be combined with sequential execution and blocking system calls. Resource sharing can be achieved more efficiently and naturally between threads due to same address space. 64
  65. 65. Different models to construct a server process: As a single-thread process Use blocking system calls but without any parallelism. If a dedicated machine is used for the file server, the CPU remains idle while the file server is waiting for a reply from the disk space. No parallelism is achieved in this method and fewer client requests are processed per unit of time. 65
  66. 66. Cont… : As a finite state machine Model support parallelism but with nonblocking system calls. Implemented as a single threaded process and is operated like a finite state machine. An event queue is maintained for request & reply. During time of a disk access, it records current state in a table & fetches next request from queue. When a disk operation completes, the appropriate piece of client state must be retrieved to find out how to continue carrying out the request. 66
  67. 67. Cont… : As a group of threads Supports parallelism with blocking system calls. Server process is comprised of a single dispatcher thread and multiple worker threads. Dispatcher thread keeps waiting in a loop for request from the clients. A server process designed in this way has good performance and is also easy to program. 67
  68. 68. Models for organizing threads Dispatcher-workers model Team model Pipeline model 68
  69. 69. Cont… : Dispatcher-worker model Single dispatcher thread and multiple worker threads. Dispatcher thread accepts requests from clients and after examining the request, dispatches the request to one of the free worker threads for further processing of the request. Each worker thread works on a different client request. 69
  70. 70. Cont… Requests A Server process for Port processing incoming requests Dispatcher Thread Worker Worker Worker Thread Thread Thread 70
  71. 71. Cont… : Team Model There is no dispatcher-worker relationship for processing clients requests. Each thread gets and processes clients’ requests on its own. Each thread of the process is specialized in servicing a specific type of request. Multiple types of requests can be simultaneously handled by the process. 71
  72. 72. Cont… Requests Port A Server process for processing incoming requests that may be of three different types, each type1 types of request being handled type3 type2 by a different thread. Thread Thread Thread 72
  73. 73. Cont… : Pipeline Model Useful for application based on the producer- consumer model. The threads of a process are organized as a pipeline so that the output data generated by the first thread is used for processing by the second thread and so on. 73
  74. 74. Cont… Requests A Server process for processing incoming requests, each request processed in three steps, each Port step handled by a different thread and output of one step as input to the next step Thread Thread Thread 74
  75. 75. Issues in designing a Thread Package1. Threads creation2. Thread termination3. Threads synchronization4. Threads scheduling5. Signal handling 75
  76. 76. Threads Creation Created either statically or dynamically. Static approach:  Threads remain fixed for its entire lifetime.  Fixed stack is allocated to each thread.  No. of threads of a process is decided at the time of writing a program or during compilation. Dynamic approach:  Number of threads changes dynamically. 76
  77. 77. Threads Termination A thread may either destroy itself when it finishes its job by making an exit call or be killed from outside by using the kill command and specifying the thread identifier as its parameter. 77
  78. 78. Threads Synchronization Threads share a common address space, so it is necessary to prevent multiple threads from trying to access the same data simultaneously. Mechanism for threads synchronization:  Use of global variable within the process.  mutex variable and condition variable. 78
  79. 79. Cont… Segment of code in which a thread may be accessing some shared variable is called a critical region. Two commonly used mutual exclusion techniques in a threads package are mutex variables and condition variables. 79
  80. 80. Cont… Mutex have only 2 states, hence simple to implement. Condition variable associated with mutex variable & reflects Boolean state of that variable. Condition – wait and signal operation. 80
  81. 81. Cont… Thread1 Thread2 Lock (mutex_A) Lock (mutex_A) fails succeeds Wait (A_free) Critical region (uses shared resource A) Blocked state Unlock (mutex_A) Signal (A_free) Lock (mutex_A) succeeds Mutex_A is a mutex variable for exclusive use of shared resource A. A_free is a condition variable for resource A to become free. 81
  82. 82. Threads Scheduling Special features for threads scheduling : 1. Priority assignment facility 2. Flexibility to vary quantum size dynamically 3. Handoff scheduling 4. Affinity scheduling 82
  83. 83. Cont… : Priority assignment facility Threads are scheduled on the first-in, first-out basis or the round-robin policy is used. Used to timeshares the CPU cycles. To provide more flexibility priority is assigned to the various threads of the applications. Priority thread maybe non-preemptive or preemptive. 83
  84. 84. Cont… : Flexibility to vary quantum sizedynamically Use of round-robin scheduling scheme. Varies the size of fixed-length time quantum to timeshare the CPU cycle among the threads. Not suitable for multiprocessor system. Gives good response time to short requests, even on heavily loaded systems. Provides high efficiency on lightly loaded systems. 84
  85. 85. Cont… : Handoff Scheduling Allows a thread to name its successor if it wants to. Provides flexibility to bypass the queue of runnable threads and directly switch the CPU to the thread specified by the currently running thread. 85
  86. 86. Cont… : Affinity scheduling A thread is scheduled on the CPU it last ran on in hopes that part of its address space is still in that CPU’s cache. Gives better performance on a multiprocessor system. 86
  87. 87. Signal handling Signals provide software-generated interrupts and exceptions. Issues :  A signal must be handled properly no matter which thread of the process receives it.  Signals must be prevented from getting lost. 87
  88. 88. Cont… Approach:  Create a separate exception handler thread in each process.  Assign each thread its own private global variables for signaling conditions. 88
  89. 89. Thread Implementation Often provided in form of a thread package. Two important approaches:  First is to construct thread library that is entirely executed in user mode.  Second is to have the kernel be aware of threads and schedule them. 89
  90. 90. User level threads It is cheap to create and destroy threads. Cost of creating or destroying thread is determined by the cost for allocating memory to set up a thread stack. Switching thread context is done in few instructions. Only CPU registers need to be stored & subsequently reloaded with the previously stored values of the thread to which it is being switched. There is no need to change memory maps, flush the TLB, do CPU accounting etc. 90
  91. 91. Cont… Drawback: Invocation of a blocking system call will immediately block the entire process to which the thread belongs. 91
  92. 92. User and Kernel Level Threads are useful to structure large application into parts that could be logically executed at the same time. In user level thread, blocking on I/O does prevent other parts to be executed in the meantime. This can be circumvented by implementing threads in the OS Kernel. 92
  93. 93. Cont… Price: Every thread operation will have to be carried out by kernel requiring system calls. Hence benefits of using threads disappear. 93
  94. 94. Implementing a Threads PackageUser-level approach Consists of a runtime system that is a collection of threads management routines. Threads run in the user space on the top of the runtime system and are managed by it. System maintains a status information table having one entry per thread. Two-level scheduling is preformed in this approach. 94
  95. 95. Cont… : User level Processes and their threads User space Runtime system (maintain threads status info.) Kernel Kernel space (maintains processes status info.) 95
  96. 96. Cont…Kernel-level approach No runtime system is used and threads are managed by the kernel. Threads status info. table maintained within kernel. Single-level scheduling is used in this approach. All calls that might block a thread are implemented as system calls that trap to the kernel. 96
  97. 97. Cont… : Kernel level User space Processes and their threads Kernel space Kernel (maintains threads status info.) 97
  98. 98. Cont…Advantages of the approaches User-level approach can be implemented on top of an existing OS that does not support threads. In user-level approach due to use of two-level scheduling, users have the flexibility to use their own customized algorithm to schedule the threads of a process. Switching the context from one thread to another is faster in the user-level approach than in the kernel approach. No status information cause poor scalability in the kernel as compared to the user-level approach. 98
  99. 99. Cont…Disadvantages of the approaches: Since there is no clock interrupt within a single process, so once CPU is given to a thread to run, there is no way to interrupt it. To solve this runtime system can request a clock interrupt after every fixed unit of time 99
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