Interrupts

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Interrupts
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An Interrupt

• A signal sent to a process
• An action taken by the interrupting process
• It is an event because it occurs at a specific
  instant and has no duration
• An interrupt is associated with and interrupt
  level

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An Interrupt (Cont.)

• A process interrupts the normal activity of
  another process
• The first process is called the interrupting
  process, the second one is called the interrupted
  process
• The interrupted process will later continue its
  activity from the point of interruption

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The Interrupted Process

• Decides what to do when it detects an interrupt.
• Usually releases any resources held, saves the
  remaining service time, joins a queue, and
  suspends itself.
• Can detect one or more levels of interrupt, each
  one can trigger a different response activity.

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Interrupt Handling

• When interrupted, a process executes an interrupt
  routine before suspending
• In the interrupt routine, the process
• Computes the remaining time
• Releases resources
• Other immediate activity
• There is usually an interrupt routine for every
  interrupt level

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Flow of Activity for Interrupt Routine
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Interrupting and Interrupted Processes
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Multi-Server Multi-Queue Model with Preemption

• Every customer has a priority
• An arriving customer can interrupt the service of
  another customer with a lower priority
• The interrupted customer is returned to the head
  of his queue, this customer is the one with the
  lowest priority and that started most recently.

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The Interrupting Process

• Only sends an interrupt to a lower priority
  process
• Waits for the availability of a server, after the
  interrupt
• Waits for the availability of resources released
  by the interrupted process
• Can determine the remaining time of the
  interrupted process.

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After the Interrupt

• The interrupting process gets access to server(s)
  and resources available
• Then it can reactivate the interrupted process
• Any other process can reactivate the interrupted
  process
• The interrupted process resumes service for its
  remaining period, or can be interrupted again

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Selection of the Process to Interrupt

• The lowest priority process, lower than the
  priority of the interrupting process
• If there are several processes with the same
  priority
• the process that started most recently
• the process that has the shortest remaining time
• the process that started first

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Interrupts with PsimJ
In the interrupting process int intr_level
1 // intrrupt level pp_object.p_interrupt(
intr_level) where pp_object is a reference to
the interrupted process and intr_level is the
interrupt level
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Interrupted Process

• The interrupted process executes the following
  code
• if (int_level() 1) // interrupt level 1?
• // ... execute interrupt service routine
• deactivate(this) // suspend itself

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Parts Replacement System With Interrupts

• When a part fails, the machine object
• Removes the damaged part
• Place the damaged part in the container for
  damaged parts
• Interrupt the repairman object if it is carrying
  out background tasks
• If available, take a replacement part from the
  replacement container, or wait (suspend)
• Install the replacement part

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Repairman Object

• Check if there are parts in the damaged parts
  container
• If available, take a damaged part and repair it
• Place the repaired part in the proper container
• If there are no damaged parts available, carry
  out some background task
• Check for pending background job to complete
• If there is an interruption, carry out interrupt
  service routine

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Java Code for Repairman Process
delay(job_per) // execute
background job pending_job false
if (int_level() REPAIR) // job
interrupted? rem_time
get_remain_t() Intrep.out.println(get_
name() " interrupted at "
get_clock()) clear_int()
// clear interrupts pending_job
true // end if
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CarWash System With Interrupts

• Multi-server system
• Priority queue for customer objects
• Simple queue for available servers
• Priority queue for bust servers
• A server can be interrupted by a customer

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Interrupts in Carwash Model

• A customer can interrupt a server that is
  servicing a lower priority customer
• When there are no available servers, an arriving
  customer will interrupt the server with the
  lowest priority customer that started most
  recently
• There are two queues for Server objects
  available and busy_queue

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Car Process

• An arriving car joins the car priority queue
• If there are no servers available
• Examine the busy server queue, try to get the
  lowest priority server that started service most
  recently
• If a server is found with a lower priority than
  current car, interrupt the server
• Suspend itself to wait for service

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Java Code in Car Process
// there are no available servers // interrupt a
busy server with lowest priority machine
(Server) Carwash.busy_queue.pllast(mtype) if
(machine ! null) Carwash.out.println(get_
name() " interrupting "
machine.get_name()) machine.p_interrupt(1)
// interrupt server else
Carwash.out.println(get_name() "
no server with lower priority available ")
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Server Process

• If car (priority) queue is empty, suspend
• If queue not empty, get the next car with the
  highest priority from the priority queue
• Service the car
• If interrupted
• store the remaining time of current service
• get the next car with higher priority from the
  car queue

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Java Code in Server Process
// Was this service interrupted? if
(int_level() 1) // interrupt level 1
// return customer object to customer queue
Carwash.car_queue.pback (currentCustomer)
rem_time get_remain_t() // remaining
service time currentCustomer.change_servp(
rem_time) currentCustomer.set_pend()
// pending flag clear_int()
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Controller Interrupts Gate

• // interrupt gate opening and get remaining time
  left
• cout ltlt "Controller interrupting gate at "
• ltlt get_clock() ltlt endl
• gt_rem gate_obj-gtp_interrupt(INT_CONT) //
  interrupt Gate

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Gate Checking for Interrupt

• // Check if interrupted by the controller
• // because of excessive waiting to communicate
• if (int_level() INT_CONT)
• cout ltlt "Gate interrupted, resetting
  at "
• ltlt get_clock() ltlt endl
• clear_int()