• Operating Systems

    UNIT - V
    The Deadlock Problem

    A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set
    System has 2 disk drives
    P1 and P2 each hold one disk drive and each needs another one
    semaphores A and B, initialized to 1
      P0            P
    wait (A);   wait(B)
    wait (B);   wait(A)
    Bridge Crossing Example

    Traffic only in one direction
    Each section of a bridge can be viewed as a resource
    If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback)
    Several cars may have to be backed up if a deadlock occurs
    Starvation is possible
    Note – Most OSes do not prevent or deal with deadlocks

    System Model
    Resource types R1, R2, . . ., Rm
    CPU cycles, memory space, I/O devices
    Each resource type Ri has Wi instances.
    Each process utilizes a resource as follows:

    Deadlock Characterization
    Deadlock can arise if four conditions hold simultaneously
    Mutual exclusion: only one process at a time can use a resource
    Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes
    No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task
    Circular wait: there exists a set {P0, P1, …, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0.

    Resource-Allocation Graph
    A set of vertices V and a set of edges E
    V is partitioned into two types:
    P = {P1, P2, …, Pn}, the set consisting of all the processes in the system
    R = {R1, R2, …, Rm}, the set consisting of all resource types in the system
    request edge – directed edge P1 ® Rj
    assignment edge – directed edge Rj ® Pi

    Example of a Resource Allocation Graph

    Resource Allocation Graph With A Deadlock

    Graph With A Cycle But No Deadlock

    Basic Facts

    If graph contains no cycles Þ no deadlock
    If graph contains a cycle Þ
    if only one instance per resource type, then deadlock if several instances per resource type, possibility of deadlock

    Methods for Handling Deadlocks
    Ensure that the system will never enter a deadlock state
    Allow the system to enter a deadlock state and then recover
    Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including UNIX

    Deadlock Prevention
    Restrain the ways request can be made
    Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources
    Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources Require process to request and be allocated all its resources before it begins
    execution, or allow process to request resources only when the process has none Low resource utilization; starvation possible
    No Preemption– If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released Preempted resources are added to the list of resources for which the process is waiting Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting
    Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration
    Deadlock Avoidance
        Requires that the system has some additional a priori information available Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes

    Safe State
        When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state System is in safe state if there exists a sequence <P1, P2, …, Pn> of ALL the processes is the systems such that for each Pi, the resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj, with j < inThat is:
    If Pi resource needs are not immediately available, then Pi can wait until all Pj have finished When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate
    When Pi terminates, Pi +1 can obtain its needed resources and so on
    Basic Facts
    If a system is in safe state Þ no deadlocks
    If a system is in unsafe state Þ possibility of deadlock
    Avoidance Þ ensure that a system will never enter an unsafe state.

    Safe, Unsafe , Deadlock State


    Avoidance algorithms
    Single instance of a resource type
    Use a resource-allocation graph
    Multiple instances of a resource type
    Use the banker’s algorithm

    Resource-Allocation Graph Scheme
    Claim edge Pi ® Rj indicated that process Pj may request resource Rj; represented by a dashed line
    Claim edge converts to request edge when a process requests a resource
    Request edge converted to an assignment edge when the resource is allocated to the process
    When a resource is released by a process, assignment edge reconverts to a claim edge
    Resources must be claimed a priori in the system
    Resource-Allocation Graph
    Unsafe State In Resource-Allocation Graph
    Resource-Allocation Graph Algorithm
    Suppose that process Pi requests a resource Rj
    The request can be granted only if converting the request edge to an assignment edge does not result in the formation of a cycle in the resource allocation graph
    Banker’s Algorithm
    Multiple instances
    Each process must a priori claim maximum use
    When a process requests a resource it may have to wait
    When a process gets all its resources it must return them in a finite amount of time
    Data Structures for the Banker’s Algorithm
    Let n = number of processes, and m = number of resources types.
    Available: Vector of length m. If available [j] = k, there are k instances of resource type Rj available
    Max: n x m matrix. If Max [i,j] = k, then process Pi may request at most k instances of resource type Rj
    Allocation: n x m matrix. If Allocation[i,j] = k then Pi is currently allocated k instances of Rj
    Need: n x m matrix. If Need[i,j] = k, then Pi may need k more instances of Rj to complete its task Need [i,j] = Max[i,j] – Allocation [i,j]
    Safety Algorithm
    1. Let Work and Finish be vectors of length m and n, respectively. Initialize:
    Work = Available
    Finish [i] = false for i = 0, 1, …, n- 1
    2. Find and i such that both:
    (a) Finish [i] = false(b) Needi £ Work
    If no such i exists, go to step 4
    3. Work = Work + Allocationi
    Finish[i] = true
    go to step 2
    4. If Finish [i] == true for all i, then the system is in a safe state
    Resource-Request Algorithm for Process Pi
    Request = request vector for process Pi.
    If Requesti [j] = k then process Pi wants k instances of resource type Rj
    1. If Requesti £ Needi go to step 2.
    Otherwise, raise error condition, since process has exceeded its maximum claim
    2. If Requesti £ Available, go to step 3.
    Otherwise Pi must wait, since resources are not available
    3. Pretend to allocate requested resources to Pi by modifying the state as follows:
    Available = Available – Request;
    Allocationi = Allocationi + Requesti;
    Needi = Needi – Requesti;
    lIf safe Þ the resources are allocated to Pi
    lIf unsafe Þ Pi must wait, and the old resource-allocation state is restored
    Example of Banker’s Algorithm
    5 processes P0 through P4;
    3 resource types:
    A (10 instances), B (5instances), and C (7 instances)
    Snapshot at time T0:
      Available             Allocation              Max
        A B C                   A B C                  A B C
                         P0          0 1 0                   7 5 3
        3 3 2
                         P1         2 0 0                    3 2 2
                         P2         3 0 2                    9 0 2
                         P3         2 1 1                    2 2 2
                         P4         0 0 2                    4 3 3
    The content of the matrix Need is defined to be Max – Allocation
         Need                A B C
           P0                   7 4 3
           P1                   1 2 2
           P2                   6 0 0
           P3                   0 1 1
           P4                   4 3 1
    The system is in a safe state since the sequence < P1, P3, P4, P2, P0> satisfies safety criteria
    Example: P1 Request (1,0,2)
    Check that Request £ Available (that is, (1,0,2) £ (3,3,2) Þ true
    Allocation            Need             Available
    A B C                                           A B C                   A B C
                                  P0                   0 1 0                      7 4 3
    2 3 0
                                  P1       3 0 2                     0 2 0
                                  P2                   3 0 1                      6 0 0
                                  P3                   2 1 1                      0 1 1
                                  P4                   0 0 2                      4 3 1
    Executing safety algorithm shows that sequence < P1, P3, P4, P0, P2> satisfies safety requirement
    Can request for (3,3,0) by P4 be granted?
    Can request for (0,2,0) by P0 be granted?
    Deadlock Detection
    Allow system to enter deadlock state
    Detection algorithm
    Recovery scheme
    Single Instance of Each Resource Type
    Maintain wait-for graph Nodes are processes
    Pi ® Pj if Pi is waiting for Pj
    Periodically invoke an algorithm that searches for a cycle in the graph. If there is a cycle, there exists a deadlock
    An algorithm to detect a cycle in a graph requires an order of n2 operations, where n is the number of vertices in the graph
    Resource-Allocation Graph and Wait-for Graph

    Several Instances of a Resource Type
    Available: A vector of length m indicates the number of available resources of each type.
    Allocation: An n x m matrix defines the number of resources of each type currently allocated to each process.
    Request: An n x m matrix indicates the current request of each process.
    If Request [ij] = k, then process Pi is requesting k more instances of resource type. Rj.
    Detection Algorithm
    1. Let Work and Finish be vectors of length m and n, respectively Initialize:
    (a) Work = Available(b) For i = 1,2, …, n, if Allocation i¹ 0, then Finish[i] = false; otherwise, Finish[i] = true
    2. Find an index i such that both:
    (a) Finish[i] == false(b) Requesti £ Work If no such i exists, go to step 4
    3. Work = Work + Allocation i Finish[i] = true go to step 2
    4. If Finish[i] == false, for some i, 1 £ i £ n, then the system is in deadlock state. Moreover, if Finish[i] == false, then Pi is deadlocked
    Algorithm requires an order of O(m x n²) operations to detect whether the system is in deadlocked state                 
    Example of Detection Algorithm
    Five processes P0 through P4; three resource types
    A (7 instances), B (2 instances), and C (6 instances)
    nSnapshot at time T0:
         Allocation                 Request              Available
           A B C                        A B C                    A B C
                        P0    0 1 0                     0 0 0      0 0 0
                        P1                 2 0 0                      2 0 2
                        P2                 3 0 3        0 0 0
                        P3                 2 1 1                     1 0 0
                        P4                 0 0 2                     0 0 2
    Sequence <P0, P2, P3, P1, P4> will result in Finish[i] = true for all i
    P2 requests an additional instance of type C
                    A B C                Request
                       P0                    0 0 0
                       P1                    2 0 1
                       P2                    0 0 1
                       P3                    1 0 0
                       P4                    0 0 2
    State of system?
    Can reclaim resources held by process P0, but insufficient resources to fulfill other processes; requests
    lDeadlock exists, consisting of processes P1, P2, P3, and P4
    Detection-Algorithm Usage
    When, and how often, to invoke depends on:
    How often a deadlock is likely to occur?
    How many processes will need to be rolled back?
    * one for each disjoint cycle
    If detection algorithm is invoked arbitrarily, there may be many cycles in the resource graph and so we would not be able to tell which of the many deadlocked processes “caused” the deadlock
    Recovery from Deadlock: Process Termination
    Abort all deadlocked processes
    Abort one process at a time until the deadlock cycle is eliminated
    In which order should we choose to abort?
    Priority of the process
    How long process has computed, and how much longer to completion
    Resources the process has used
    Resources process needs to complete
    How many processes will need to be terminated
    Is process interactive or batch?
    Recovery from Deadlock: Resource Preemption
    Selecting a victim – minimize cost
    Rollback – return to some safe state, restart process for that state
    Starvation – same process may always be picked as victim, include number of rollback in cost factor
    I/O Systems
    Explore the structure of an operating system’s I/O sub system
    Discuss the principles of I/O hardware and its complexity
    Provide details of the performance aspects of I/O hardware and software
    I/O Hardware
    Incredible variety of I/O devices
    Common concepts
    Bus (daisy chain or shared direct access)
    Controller (host adapter)
    I/O instructions control devices
    Devices have addresses, used by
    Direct I/O instructions
    Memory-mapped I/O
    A Typical PC Bus Structure
    Device I/O Port Locations on PCs (partial)
    Determines state of device
    Busy-wait cycle to wait for I/O from device
    CPU Interrupt-request line triggered by I/O device
    Interrupt handler receives interrupts
    Maskable to ignore or delay some interrupts
    Interrupt vector to dispatch interrupt to correct handler
    Based on priority
    Some nonmaskable
    Interrupt mechanism also used for exceptions
    Interrupt-Driven I/O Cycle
    Intel Pentium Processor Event-Vector Table
    Direct Memory Access
    Used to avoid programmed I/O for large data movement
    Requires DMA controller
    Bypasses CPU to transfer data directly between I/O device and memory
    Six Step Process to Perform DMA Transfer
    Application I/O Interface
    I/O system calls encapsulate device behaviors in generic classes
    Device-driver layer hides differences among I/O controllers from kernel
    Devices vary in many dimensions
    Character-stream or block
    Sequential or random-access
    Sharable or dedicated
    Speed of operation
    read-write, read only, or write only

    A Kernel I/O Structure
    Characteristics of I/O Devices
    Block and Character Devices
    Block devices include disk drives
    Commands include read, write, seek
    Raw I/O or file-system access
    Memory-mapped file access possible
    Character devices include keyboards, mice, serial ports
    Commands include get(), put()
    Libraries layered on top allow line editing
    Network Devices
    Varying enough from block and character to have own interface
    Unix and Windows NT/9x/2000 include socket interface
    Separates network protocol from network operation
    Includes select() functionality
    Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)
    Clocks and Timers
    Provide current time, elapsed time, timer
    Programmable interval timer used for timings, periodic interrupts
    ioctl() (on UNIX) covers odd aspects of I/O such as clocks and timers
    Blocking and Nonblocking I/O
    Blocking - process suspended until I/O completed
    Easy to use and understand
    Insufficient for some needs
    Nonblocking - I/O call returns as much as available
    User interface, data copy (buffered I/O)
    Implemented via multi-threading
    Returns quickly with count of bytes read or written
    Asynchronous - process runs while I/O executes
    Difficult to use
    I/O subsystem signals process when I/O completed
    Two I/O Methods
    Kernel I/O Subsystem
    Some I/O request ordering via per-device queue
    Some OSs try fairness
    Buffering - store data in memory while transferring between devices
    To cope with device speed mismatch
    To cope with device transfer size mismatch
    To maintain “copy semantics”
    Device-status Table
    Sun Enterprise 6000 Device-Transfer Rates
    Kernel I/O Subsystem
    Caching - fast memory holding copy of data
    Always just a copy
    Key to performance
    Spooling - hold output for a device
    If device can serve only one request at a time i.e., Printing
    Device reservation - provides exclusive access to a device
    System calls for allocation and deallocation
    Watch out for deadlock
    Error Handling
    OS can recover from disk read, device unavailable, transient write failures
    Most return an error number or code when I/O request fails
    System error logs hold problem reports
    I/O Protection
    User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructions
    All I/O instructions defined to be privileged
    I/O must be performed via system calls
    * Memory-mapped and I/O port memory locations must be protected too
    Use of a System Call to Perform I/O
    Kernel Data Structures
    Kernel keeps state info for I/O components, including open file tables, network connections, character device state
    Many, many complex data structures to track buffers, memory allocation, “dirty” blocks
    Some use object-oriented methods and message passing to implement I/O
    UNIX I/O Kernel Structure
    I/O Requests to Hardware Operations
    Consider reading a file from disk for a process:
    Determine device holding file
    Translate name to device representation
    Physically read data from disk into buffer
    Make data available to requesting process
    Return control to process
    Life Cycle of An I/O Request
    STREAM – a full-duplex communication channel between a user-level process and a device in Unix System V and beyond
    A STREAM consists of:
    - STREAM head interfaces with the user process
    - driver end interfaces with the device
    - zero or more STREAM modules between them.
    Each module contains a read queue and a write queue
    Message passing is used to communicate between queues
    The STREAMS Structure
    I/O a major factor in system performance:
    Demands CPU to execute device driver, kernel I/O code
    Context switches due to interrupts
    Data copying
    Network traffic especially stressful
    Intercomputer Communications
    Improving Performance
    Reduce number of context switches
    Reduce data copying
    Reduce interrupts by using large transfers, smart controllers, polling
    Use DMA
    Balance CPU, memory, bus, and I/O performance for highest throughput
    Device-Functionality Progression

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