Chapter 3 Process Scheduling

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Chapter 3 Process Scheduling

• Bernard Chen
• Spring 2007

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Schedulers

• Long-term scheduler
• (or job scheduler) selects which processes
  should be brought into the ready queue
• Short-term scheduler
• (or CPU scheduler) selects which process
  should be executed next and allocates CPU

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Schedulers

• On some systems, the long-term scheduler maybe
  absent or minimal
• Just simply put every new process in memory for
  short-term scheduler
• The stability depends on physical limitation or
  self-adjustment nature of human users

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Schedulers

• Sometimes it can be advantage to remove process
  from memory and thus decrease the degree of
  multiprogrammimg
• This scheme is called swapping

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Addition of Medium Term Scheduling
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Share Memory Parallelization System Example

• m_set_procs(number) prepare number of child for
  execution
• m_fork(function) childes execute function
• m_kill_procs() terminate childs

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Real Example

• main(argc , argv)
• int nprocs9
• m_set_procs(nprocs) / prepare to launch this
  many processes /
• m_fork(slaveproc) / fork out processes /
• m_kill_procs() / kill activated
  processes /
• void slaveproc()
• int id
• id m_get_myid()
• m_lock()
• printf(" Hello world from process d\n",id)
• printf(" 2nd line Hello world from process
  d\n",id)
• m_unlock()

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• include "stdio.h"
• include "sys/types.h"
• include "sys/times.h"
• include "ulocks.h"
• include "sys/param.h"
• include "math.h"
• define MAXTHRDS 6

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Real Example

• int array_size1000
• int global_arrayarray_size
• main(argc , argv)
• int nprocs4
• m_set_procs(nprocs) / prepare to launch this
  many processes /
• m_fork(sum) / fork out processes /
• m_kill_procs() / kill activated
  processes /
• void sum()
• int id
• id m_get_myid()
• for (iid(array_size/nprocs)
  ilt(id1)(array_size/nprocs) i)
• global_arrayidarray_size/nprocsglobal_array
  i

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Cooperating Processes

• Independentprocess cannot affect or be affected
  by the execution of another process
• Cooperatingprocess can affect or be affected by
  the execution of another process
• Advantages of process cooperation
• Information sharing
• Computation speed-up
• Modularity
• Convenience

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Interprocess Cpmmunication (IPC)

• Two fundamental models
• Share Memory
• Message Passing

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Communication Models

• (a) MPI (b) Share memory

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Shared-Memory Systems

• Consider the producer-consumer problem
• A producer process produce information that is
  consumed by customer process, for example, a web
  server produces HTML files and images which are
  consumed by client web browser

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Shared-Memory Systems

• The producer and consumer must be synchronized,
  so that consumer does not try to consume an item
  that has not yet been produced.
• Two types of buffer can be used
• Unbounded buffer
• Bounded buffer

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Shared-Memory Systems

• Unbounded Buffer the consumer may have to wait
  for new items, but producer can always produce
  new items.
• Bounded Buffer the consumer have to wait if
  buffer is empty, the producer have to wait if
  buffer is full

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Bounded Buffer

• define BUFFER_SIZE 6
• Typedefstruct
• . . .
• item
• item bufferBUFFER_SIZE
• intin 0
• intout 0

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Bounded Buffer (producer iew)

• while (true)
• / Produce an item /
• while (((in (in 1) BUFFER SIZE count)
  out)
• / do nothing --no free buffers /
• bufferin item
• in (in 1) BUFFER SIZE

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Bounded Buffer (Consumer view)

• while (true)
• while (in out)
• // do nothing --nothing to consume
• // until remove an item from the buffer
• item bufferout
• out (out 1) BUFFER SIZE
• return item

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Message-Passing Systems

• A message passing facility provides at least two
  operations send(message), receive(message)

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Message-Passing Systems

• If 2 processes want to communicate, a
  communication link must exist
• It has the following variations
• Direct or indirect communication
• Synchronize or asynchronize communication
• Automatic or explicit buffering

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Message-Passing Systems

• Direct communication
• send(P, message)
• receive(Q, message)
• Properties
• A link is established automatically
• A link is associated with exactly 2 processes
• Between each pair, there exists exactly one link

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Message-Passing Systems

• Indirect communication the messages are sent to
  and received from mailbox
• send(A, message)
• receive(A, message)

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Message-Passing Systems

• Properties
• A link is established only if both members of the
  pair have a shared mailbox
• A link is associated with more than 2 processes
• Between each pair, there exists a number of links

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Message-Passing Systems

• Mailbox sharing
• P1, P2, andP3 share mailbox A
• P1, sends P2 andP3 receive
• Who gets the message?
• Solutions
• Allow a link to be associated with at most two
  processes
• Allow only one process at a time to execute a
  receive operation
• Allow the system to select arbitrarily the
  receiver. Sender is notified who the receiver was.

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Message-Passing Systems

• If the mailbox owned by process, it is easy to
  tell who is the owner and user.
• And there is no confuse we send the message and
  who receives it.
• When process terminates, the mailbox disappear

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Message-Passing Systems

• If the mailbox owned by OS, it requires the
  following functions
• Create a new mailbox
• Send and Receive message through the mailbox
• Delete a mailbox

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Message-Passing Systems

• Synchronization synchronous and asynchronous
• Blocking is considered synchronous
• Blocking send has the sender block until the
  message is received
• Blocking receive has the receiver block until a
  message is available

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Message-Passing Systems

• Non-blockingis considered asynchronous
• Non-blocking send has the sender send the message
  and continue
• Non-blocking receive has the receiver receive a
  valid message or null

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Message-Passing Systems

• Buffering Queue of messages attached to the
  link, there are 3 variations
• Zero capacity 0 messages
• Sender must wait for receiver
• 2.Bounded capacity finite length of n
• messages, sender must wait if link full
• 3.Unbounded capacity infinite length
• Sender never waits

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MPI Program example

• include "mpi.h"
• include ltmath.hgt
• include ltstdio.hgt
• include ltstdlib.hgt
• int main (int argc, char argv)
• int id / Process rank /
• int p / Number of processes
  /
• int i,j
• int array_size100
• int arrayarray_size / or array
  and then use malloc or vector to increase the
  size /
• int local_arrayarray_size/p
• int sum0
• MPI_Status stat
• MPI_Comm_rank (MPI_COMM_WORLD, id)
• MPI_Comm_size (MPI_COMM_WORLD, p)

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MPI Program example

• if (id0)
• for(i0 iltarray_size i)
• arrayii / initialize array/
• for(i0 iltp i)
• MPI_Send(arrayiarray_size/p,
  / Start from/
• array_size/p,
  / Message size/
• MPI_INT,
  / Data type/
• i,
  / Send to which process/
• MPI_COMM_WORLD)
• else
• MPI_Recv(local_array0,array_size/p,MPI_IN
  T,0,0,MPI_COMM_WORLD,stat)

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MPI Program example

• for(i0iltarray_size/pi)
• sumarrayi
• MPI_Reduce (sum, sum, 1, MPI_INT, MPI_SUM,
  0, MPI_COMM_WORLD)
• if (id0)
• printf("d ",sum)