operating systems: scheduling 1 scheduling processes can be in one of several states –5 state...

Operating Systems: Scheduling

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Scheduling

•Processes can be in one of several states

–5 state model :

–‘short-term’ scheduling

»organising transitions between states on page-fault, waiting for or getting semaphores, I/O transfer

completions etc.

»deciding order in which ready processes should be run priorities etc. and queue handling


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–7 state model with medium and long-term scheduling :


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•Medium-term scheduling

–when main memory is full, processes need to be swapped out to disc

–medium-term sched. decides which and when to swap out and back in

»level of multiprogramming to achieve desired performance

»to have processes ready and waiting to run when running process blocks

–separate swap area on disc commonly used

»local disc to be effective

»networked discs too slow

–possible to use file storage sites instead of swap area

»for files mapped into virtual space from disc storage site accessing VM equivalent to accessing file storage site on disc

stacks and heaps etc. can also be mapped files

all VM space can be mapped files

»often simpler than separate swap area each page only has one corresponding disc site instead of possibly two

paging out a dirty page updates the file

clean pages need not be written out (if disc file sites initially cleared to zero)


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»drawbacks : cannot page out across a network to a file serve

potential file inconsistency on system crash

»EMAS system used mapped files first version used a swap area on a dedicated drum (fixed head disc)

later versions just paged to and from disc file sites

worked because it was for a stand-alone mainframe with local discs

–swap areas are usually fixed partitions on disc

»drawbacks : may not be large enough

may waste disc space if large enough for any eventuality

–swap areas probably more efficient overall

»all page transfers can be initiated together probably to a contiguous disc area

»disc driver will be more effective in optimising transfers minimises head movement


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•Long-term scheduling

–whether to allow new processes to enter a system

–for a compute server or background job stream :

»when to start next job depends on job priority, CPU-time needs, memory needs etc.

also depends on existing load

–for multiple user interactive system

»how many users to allow on each should get acceptable performance

or is it better to let all requestors on and let performance degrade?

CPUUtulisation

No. of Users

knee


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Short-term scheduling

•Define the objectives and criteria to be met; then invent a scheme

•Precise scheme will depend on type of system :

–Compute server or background job stream processor

»overall throughput most important

–Single-user workstation

»foreground interactive response most important

–Multiple-user system

»interactive time-sharing

»transaction processing travel agent enquiry and booking systems, banking terminals etc.

»interactive response with fairness between users

–Real-time systems

»meeting hard deadlines keeping up with processing data streams e.g. comms, audio and video

etc.

industrial process control


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•System Manager objectives :

–throughput - to maximise number of jobs completed per unit time

–turn-around time for jobs

–utilisation - to make best use of expensive resources

»CPU - proportion of time spent executing user programs

»memory usage

»usage of peripherals

»overall cost-effectiveness a mix of CPU-bound and I/O bound tasks might be desirable for balance

–to be fair

»no favouring or starvation of some processes

»ensuring priorities met

–performance to degrade gracefully under load

–to be reasonably predictable

»wide variations in performance can be distracting

–to be adaptable to varying circumstances without need for intervention


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•User objectives :

–turn-around time for submitted jobs

–adequate response time for interactive working

»< 0.1 sec for immediate feedback e.g. key depressions, menu highlighting etc.

may need special fast path through kernel to achieve

or peripheral processor - keyboard interface or video processor

»< 1 sec needed to maintain user attention and interest for long periods

»> 1 sec : response can be very distracting - concentration will falter

»> 10 secs : intolerable for interaction even ‘talk’ conversations impossible

time to go for a coffee!

–observed phenomenon :

»thinking time drops as response time drops

»an effect of short-term memory and attention span


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Scheduling Criteria

•Priority of process

–basic priority usually decided outwith the scheduler - may be dynamic later

–e.g. interactive v. background

•CPU boundedness

–does process always use its CPU quantum allocation without blocking?

•I/O boundedness

–does process frequently block for I/O ?

•Page-fault frequency

–a small PFF usually means the process has all the memory it needs and

can make good progress

–a large PFF means the process will not use much CPU - always waiting for

the page to be brought in from disc


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•Urgency of required response

–important for user interaction

–may be vitally urgent for a real-time system

»nearness to a deadline

•CPU time already received

–may decide to give CPU to a process that has not had much yet

–or ‘to him that hath shall be given’ ?

•CPU time to completion

–average waiting time minimised if process with least time to completion run

–need to know how much time still needed - usually unknown

•Regularity of requirements

–CPU required at fixed time intervals - real-time systems

–fairly straightforward to schedule given adequate system performance


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•Schedulers aim to optimise future performance

–need to know future characteristics of processes

–easiest to assume processes will continue to behave as previously

–must be able to adapt when processes change characteristics

•Pre-emptive scheduling

–currently running process can be interrupted before finishing

»put back onto the Run queue to get another go on the CPU later

–typically the scheduler is re-entered on regular clock or timer interrupts

»may also get re-entered whenever kernel entered peripheral interrupts, semaphore operations etc.

•Non Pre-emptive scheduling

–running process continues until it has finished or blocks

»low kernel overheads

»scheduler usually needs more control than this

»other processes may be seriously adversely affected


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Priority Queues

•Pre-emptive priority

–processes in highest priority queue always get run first

–those in lower priority queues always wait

–starvation likely


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•Non-Pre-Emptive Priority–higher priority processes still favoured but not exclusively

–every so often, take a process from a lower priority queue

» a priority ratio table (used in EMAS medium-term scheduling)

1 2 1 3 1 2 1 4 1 2 1 3 1 2 1

–make priorities dynamic

»lower a processes priority after each time it has been run for a quantum

»used in Windows NT process given an initial boost in priority, then gradual decay

favours short interactions

»boost a processes priority if it has not had a go on the CPU lately

–priority purchase ?

»a user may wish to pay more to get better service whether funny money i.e. computing time allocations, or real money

•Higher and lower priority bands–kernel processes v. background job stream

–real-time processes (NT)


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•First-Come-First-Served

–processes queued on run queue in order of arrival

–oldest process on queue always run next to completion - no pre-emption

–simple to implement

–primarily used for background and batch streams

–performs much better for long jobs than for short ones

»example: measure turnaround time normalised by service time : Tq/Ts

–favours CPU- bound processes over I/O bound processes

–unacceptable for interactive systems - might be OK if combined with priority queue system for known long run-time processes having lower priority


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•Round-Robin

–each process gets a quantum of time (a time-slice) in turn

–good for processes of equal priority

–widely used for multi-user interactive systems

»and single-user systems with multiple activities in progress

–a pre-emptive policy

»processes pre-empted by regular clock interrupts to kernel scheduler

–main issue is length of time-slice

»to optimise interactive response, make quantum just slightly longer than a typical interaction i.e. should complete within first time-slice :


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»if quantum not large enough :

»more than one quantum means additional round-robin delay

–short time-slices :

»all processes in round-robin queue get a go on the CPU quickly

»short processes complete in one go

»overheads will increase due to more frequent context changing


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–large time-slices :

»round-robin time longer

»some processes will always use their full time-slice

»overheads lower

–CPU-bound processes favoured over I/O bound or page-faulting processes

»CPU-bound processes take full time-slice

»I/O bound processes blocked before completing a time-slice

–put blocked processes in a special queue

»more equitable to give them higher priority when they unblock

»or put them on front of round-robin queue to get CPU next


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•Shortest Process Next

–process with shortest expected processing time run next - non-pre-emptive

–intended to reduce bias towards long processes

–achieves much better turnaround time than FCFS on average

–some risk of starvation for longer processes (if new processes admitted on fly)

–variability of turnaround time greater than FCFS

–need a good estimate of expected processing time (extra overhead)

»possible for batch and background streams, if user knows

»can be estimated from previous behaviour for interactive processes :

Keep a running average of what was used in previous bursts of CPU use:

Sn+1 = ( Ti ) / n

or Sn+1 = Tn/n + Sn*(n-1)/n

Better to use an exponential average function:

Sn+1 = Tn + (1-)*Sn


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For 0 < < 1, all previous observations carry decreasing weight:

Sn+1 = Tn + (1-)Tn-1 + ... + (1-)..(1-)Tn-i + (1-)..(1-)S0

For = 0.8, virtually all weight given to previous four observations :

Sn+1 = 0.8Tn + 0.16Tn-1 + 0.032Tn-2 + 0.0064Tn-3 + . . .

For = 0.5, all weight given to previous eight observations.

Higher values of reflect changes more quickly - but jerkily


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•Shortest Time Remaining

–process with least expected run-time to completion dispatched next

–in effect a pre-emptive version of SPN

»a new process entering the queue may pre-empt the running

process

–need estimate of remaining run-time for each process

»record previous elapsed times and use weighted average as in

SPN

–can use regular clock interrupts to re-evaluate best next process

–better turn-around time than SPN

–no bias in favour of long processes

–risk of starvation for longer processes

–in the example, three shortest processes all receive immediate service


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•Highest Response Ratio Next

–aim to minimise Tq/Ts for each process

–can approximate an a priori measure :

»Response ratio = (w + s)/s

»where w = waiting times = expected service time

»expected service time must be estimated again

»waiting time measured as time progresses

–process with highest RR dispatched next

»longer the wait, the higher the priority

»short processes favoured but long processes not starved

»good balance on the whole

–non-pre-emptive as defined but could be made pre-emptive as STR

–overheads in recomputing RR


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Threads and Scheduling

•Threads are each scheduled separately

•Threaded applications may wish to assign relative priorities to threads

–a background screen update thread - low priority

–foreground interactive thread - high priority

–check-point thread - low priority, but must not be starved

•In a multi-user environment, CPU must be allocated equitably

–spawning lots of threads must not gain the process unfair advantage

–need to add up CPU used in all threads belonging to a process when re-evaluating priorities or otherwise control allocation fairly

•In a single-user environment

–equitable CPU allocation less important

•Windows NT introduced fibers to give users more precise control of scheduling than threads

–important for database and transaction processing systems


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Scheduling Scheme Evaluation

•Analytic methods v. Simulation

•Queuing network models :

–arrival rates and service times

–multiple servers and queues

–expected performance can be analysed mathematically for simple systems

•Simulation :

–model of scheduling scheme programmed

–process characteristics, rate of interaction, service time needs, can be modelled to match experience

–traces of real sessions can be kept and used in simulations

»On EMAS multi-access system, sessions were recorded on a PDP-11

»re-run in simulation later

»ERTE - Edinburgh Remote Terminal Emulator

»produced useful data for tailoring scheduling schemes


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Real-time System Scheduling

•Soft and Hard deadlines for tasks

–soft deadlines are desirable aims but not obligatory

–hard deadlines must be met

»may need to reserve resources ahead of time

–schedule hard deadlines first, then fit soft deadlines in later

•Continuous data streams or regular data packets a feature

–much easier if timing characteristics and processing needs known in advance

•Need to avoid Priority Inversion :

–where a low priority task has a resource that is blocking a high priority task

–possible solution : priority inheritance

»low priority task inherits high priority of task needing the resource it has


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•Cyclic Static scheduling :

–for processing data appearing continuously or at regular times

–pre-allocate a fixed amount of CPU at regular intervals

»use timer interrupts to regain scheduling control from other tasks

–can interlace multiple processing needs :

•Dynamic scheduling : Most Urgent First

–dispatch process with earliest deadline

•Maximum Lateness First

–lateness = processing time left for task - time left until deadline

–task with largest lateness selected next

•Rate Monotonic

–assigns a priority to each task a priori proportional to the frequency of occurrence of its triggering event.

A A A AB B A B


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–task A : period 3ms, processing time 1.5ms

–task B : period 2ms, processing time 0,2ms

–task C : period 1ms, processing time 0.2ms

•A Priori Analysis

–mathematical and simulation techniques for predicting whether scheduling objectives can be met and a schedule to meet them


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•Round Robin and other schemes also used in real-time systems

• Hierarchical Scheduling :

–form groups of tasks and apply different scheduling schemes to each group

•Important that user able to select an appropriate scheduling scheme

Priority

Round RobinCyclic Static

T1 T2 T3 T4 T5 T6

operating systems: scheduling 1 scheduling processes can be in one of several states –5 state...

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disc file sites

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zerooperating systems

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file storage sites

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separate swap areaeach

dirty page