Processes

1
Processes

• Operating Systems
• Spring 2004

2
What is a process?

• An instance of an application execution
• Process is the most basic abstractions provided
  by OS
• An isolated computation context for each
  application
• Computation context
• CPU state address space environment

3
CPU stateRegister contents

• Process Status Word (PSW)
• exec. mode, last op. outcome, interrupt level
• Instruction Register (IR)
• Current instruction being executed
• Program counter (PC)
• Stack pointer (SP)
• General purpose registers

4
Address space

• Text
• Program code
• Data
• Predefined data (known in compile time)
• Heap
• Dynamically allocated data
• Stack
• Supporting function calls

5
Environment

• External entities
• Terminal
• Open files
• Communication channels
• Local
• With other machines

6
Process control block (PCB)
PCB
CPU
kernel
user
state
PSW
text
memory
IR
files
data
PC
accounting
heap
priority
SP
user
general purpose registers
CPU registers storage
stack
7
Process States
terminated
running
schedule
wait for event
preempt
created
ready
blocked
event done
8
UNIX Process States
running user
schedule
sys. call interrupt
return
ready user
zombie
interrupt
running kernel
terminated
preempt
wait for event
schedule
created
ready kernel
blocked
event done
9
Multiprocesses mechanisms

• Context switch
• Create process and dispatch (in Unix,
  fork()/exec())
• End process (in Unix, exit() and wait())

10
Threads

• Thread an execution within a process
• A multithreaded process consists of many
  co-existing executions
• Separate
• CPU state, stack
• Shared
• Everything else
• Text, data, heap, environment

11
Thread Support

• Operating system
• Advantage thread scheduling done by OS
• Better CPU utilization
• Disadvantage overhead if many threads
• User-level
• Advantage low overhead
• Disadvantage not known to OS
• E.g., a thread blocked on I/O blocks all the
  other threads within the same process

12
Multiprogramming

• Multiprogramming having multiple jobs
  (processes) in the system
• Interleaved (time sliced) on a single CPU
• Concurrently executed on multiple CPUs
• Both of the above
• Why multiprogramming?
• Responsiveness, utilization, concurrency
• Why not?
• Overhead, complexity

13
Responsiveness
Job 1 arrives
Job 2 arrives
Job 3 arrives
Job1
Job2
Job3
14
Workload matters!

• Would CPU sharing improve responsiveness if all
  jobs were taking the same time?
• No. It makes it worse!
• For a given workload, the answer depends on the
  value of coefficient of variation (CV) of the
  distribution of job runtimes
• CVstand. dev. / mean
• CV lt 1 gt CPU sharing does not help
• CV gt 1 gt CPU sharing does help

15
Real workloads

• Exp. Dist CV1Heavy Tailed Dist CVgt1
• Dist. of job runtimes in real systems is heavy
  tailed
• CV ranges from 3 to 70
• Conclusion
• CPU sharing does improve responsiveness
• CPU sharing is approximated by
• Time slicing interleaved execution

16
Utilization
1st I/O operation
I/O ends
2nd I/O operation
I/O ends
3rd I/O operation
idle
CPU
idle
idle
Disk
idle
idle
idle
CPU
idle
Job1
Job2
Job1
Job2
Disk
idle
Job1
Job2
idle
idle
Job1
17
Workload matters!

• Does it really matter?
• Yes, of course
• If all jobs are CPU bound (I/O bound),
  multiprogramming does not help to improve
  utilization
• A suitable job mix is created by a long-term
  scheduling
• Jobs are classified on-line to be CPU (I/O) bound
  according to the jobs history

18
Concurrency

• Concurrent programming
• Several process interact to work on the same
  problem
• ls l more
• Simultaneous execution of related applications
• Word Excel PowerPoint
• Background execution
• Polling/receiving Email while working on smth else

19
The cost of multiprogramming

• Switching overhead
• Saving/restoring context wastes CPU cycles
• Degrades performance
• Resource contention
• Cache misses
• Complexity
• Synchronization, concurrency control, deadlock
  avoidance/prevention

20
Short-Term Scheduling
terminated
running
schedule
wait for event
preempt
created
ready
blocked
event done
21
Short-Term scheduling

• Process execution pattern consists of alternating
  CPU cycle and I/O wait
• CPU burst I/O burst CPU burst I/O burst...
• Processes ready for execution are hold in a ready
  (run) queue
• STS schedules process from the ready queue once
  CPU becomes idle

22
Metrics Response time

• Response time (turnaround time) is the average
  over the jobsTresp

Job terminates/ blocks waiting for I/O
Job arrives/ becomes ready to run
Starts running
Trun
Twait
Tresp
Tresp Twait Trun
23
Other Metrics

• Wait time average of Twait
• This parameter is under the system control
• Response ratio or slowdown
• slowdownTresp / Trun
• Throughput, utilization depend on user imposed
  workloadgt
• Less useful

24
Note about running time (Trun)

• Length of the CPU burst
• When a process requests I/O it is still running
  in the system
• But it is not a part of the STS workload
• STS view I/O bound processes are short processes
• Although text editor session may last hours!

25
Off-line vs. On-line scheduling

• Off-line algorithms
• Get all the information about all the jobs to
  schedule as their input
• Outputs the scheduling sequence
• Preemption is never needed
• On-line algorithms
• Jobs arrive at unpredictable times
• Very little info is available in advance
• Preemption compensates for lack of knowledge

26
First-Come-First-Serve (FCFS)

• Schedules the jobs in the order in which they
  arrive
• Off-line FCFS schedules in the order the jobs
  appear in the input
• Runs each job to completion
• Both on-line and off-line
• Simple, a base case for analysis
• Poor response time

27
Shortest Job First (SJF)

• Best response time

Long job
Short
Short
Long job

• Inherently off-line
• All the jobs and their run-times must be
  available in advance

28
Preemption

• Preemption is the action of stopping a running
  job and scheduling another in its place
• Context switch Switching from one job to another

29
Using preemption

• On-line short-term scheduling algorithms
• Adapting to changing conditions
• e.g., new jobs arrive
• Compensating for lack of knowledge
• e.g., job run-time
• Periodic preemption keeps system in control
• Improves fairness
• Gives I/O bound processes chance to run

30
Shortest Remaining Time first (SRT)

• Job run-times are known
• Job arrival times are not known
• When a new job arrives
• if its run-time is shorter than the remaining
  time of the currently executing job
• preempt the currently executing job and schedule
  the newly arrived job
• else, continue the current job and insert the new
  job into a sorted queue
• When a job terminates, select the job at the
  queue head for execution

31
Round Robin (RR)

• Both job arrival times and job run-times are not
  known
• Run each job cyclically for a short time quantum
• Approximates CPU sharing

Job 1 arrives
Job 2 arrives
Job 3 arrives
Job 1 terminates
Job 2 terminates
Job 3 terminates
Job1
3
2
1
2
3
1
2
3
1
2
1
Job2
32
Responsiveness
Job 1 arrives
Job 2 arrives
Job 3 arrives
Job1
Job2
Job3
33
Priority Scheduling

• RR is oblivious to the process past
• I/O bound processes are treated equally with the
  CPU bound processes
• Solution prioritize processes according to their
  past CPU usage

• Tn is the duration of the n-th CPU burst
• En1 is the estimate of the next CPU burst

34
Multilevel feedback queues
terminated
new jobs
quantum10
quantum20
quantum40
FCFS
35
Multilevel feedback queues

• Priorities are implicit in this scheme
• Very flexible
• Starvation is possible
• Short jobs keep arriving gt long jobs get starved
• Solutions
• Let it be
• Aging

36
Priority scheduling in UNIX

• Multilevel feedback queues
• The same quantum at each queue
• A queue per priority
• Priority is based on past CPU usage
• pricpu_usebasenice
• cpu_use is dynamically adjusted
• Incremented each clock interrupt 100 sec-1
• Halved for all processes 1 sec-1

37
Fair Share scheduling

• Given a set of processes with associated weights,
  a fair share scheduler should allocate CPU to
  each process in proportion to its respective
  weight
• Achieving pre-defined goals
• Administrative considerations
• Paying for machine usage, importance of the
  project, personal importance, etc.
• Quality-of-service, soft real-time
• Video, audio

38
Perfect Fairness
A fair share scheduling algorithm achieves
perfect fairness in time interval (t1,t2) if
39
Wellness criterion for FSS

• An ideal fair share scheduling algorithm achieves
  perfect fairness for all time intervals
• The goal of a FSS algorithm is to receive CPU
  allocations as close as possible to a perfect FSS
  algorithm

40
Fair Share scheduling algorithms

• Weighted Round Robin
• Shares are not uniformly spread in time
• Lottery scheduling
• Each process gets a number of lottery tickets
  proportional to its CPU allocation
• The scheduler picks a ticket at random and gives
  it to the winning client
• Only statistically fair, high complexity

41
Fair Share scheduling VTRR

• Virtual Time Round Robin (VTRR)
• Order ready queue in the order of decreasing
  shares (the highest share at the head)
• Run Round Robin as usual
• Once a process that exhausted its share is
  encountered
• Go back to the head of the queue

42
Multiprocessor Scheduling

• Homogeneous vs. heterogeneous
• Homogeneity allows for load sharing
• Separate ready queue for each processor or common
  ready queue?
• Scheduling
• Symmetric
• Master/slave

43
A Bank or a Supermarket?
departing jobs
CPU1
CPU1
shared queue
departing jobs
CPU2
CPU2
arriving jobs
arriving jobs
departing jobs
CPU3
CPU3
departing jobs
CPU4
CPU4
M/M/4
4 x M/M/1
44
It is a Bank!