nus.soc.cs5248 ooi wei tsang 1 course matters. nus.soc.cs5248 ooi wei tsang 2 make-up lecture this...

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Course Matters

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Make-Up LectureThis Saturday, 23 OctoberTR7, 1-3pm

Topic: “CPU scheduling”

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Multimedia Storage Issues

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Media vs. Documents

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Media vs. Documents large file sizewrite once, read many sequential accessperiodic accessdeadlines

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Average Service Time?

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OS Review: Disk

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Disk

head, spindle, track, sector, cylinder

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Latency components

seek time, rotational time and transfer time

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Disk Scheduler

disksched

I/O Request

read/write command

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OS Review: Disk Scheduling Algorithm

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FCFS

98 183 37 122 14 124 65 67

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SSTF

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SCAN (Elevator)

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C-SCAN

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EDF

98 183 37 122 14 124 65 6730 10 13 5 24 33 21 39

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EDF-SCAN

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Other SchemesFD-SCAN

SSEDO

SSEDV

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Data Placement

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Contiguous

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Fragmented

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Striping (RAID-0)

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Mirroring (RAID-1)

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Parity (RAID-5)

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Parity

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Summary of ConcernsHigh ThroughputFault TolerantLoad balancing

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This LectureConsider striping only (RAID-0)

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Efficient Striping Techniques for Multimedia File Servers P. Shenoy, H. VinNOSSDAV 97

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Design Parametersunit of striping (block size)degree of striping (num of disks)

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Stripe Unit

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ModelDisk serves clients in rounds

Time to read media from disk should be smaller than round time

Concern: minimize service time of most heavily loaded disk

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Effects of Block Size

Block Size

servicetime

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ProblemFind block size such that the

service time for the most heavily loaded disk is minimize

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Analysis Model: A Planservice time of the busiest disk

as a function of block sizeexpected num of blocks

accessed on the busiest diskexpected num of blocks

accessed on any disk

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AssumptionsNon-redundant arrayVBRAnalyze read operation only

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Variables DeclarationNd : Number of Disks

Nc : Number of Clients

B : Block size

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Variables DeclarationNb(i,j): Number of blocks client i

access from disk j

Pa(i,m): Probability of i access m blocks in a round

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AnalysisSuppose i request m blocks

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Reading 1 blockProb(Nb(i,j)=1)

Nd : Number of DisksNc : Number of ClientsB : Block sizeNb(i,j): Number of blocks client

i access from disk jPa(i,m): Probability of i access

m blocks in a round

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Reading k blockProb(Nb(i,j)=k)

Nd : Number of DisksNc : Number of ClientsB : Block sizeNb(i,j): Number of blocks client

i access from disk jPa(i,m): Probability of i access

m blocks in a round

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Blocks Read From a Disk Nb(*,j) =

Nd : Number of DisksNc : Number of ClientsB : Block sizeNb(i,j): Number of blocks client

i access from disk jPa(i,m): Probability of i access

m blocks in a round

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Busiest DiskNmax =

Nd : Number of DisksNc : Number of ClientsB : Block sizeNb(i,j): Number of blocks client

i access from disk jPa(i,m): Probability of i access

m blocks in a round

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Access TimeT = Nmax(ts + tr + Btt)

Depends on:block size Bdisk characteristic ts,tr,tt

server design Nd

workload characteristic Nc, Nb(i,j)_

Nd : Number of DisksNc : Number of ClientsB : Block sizeNb(i,j): Number of blocks client

i access from disk jPa(i,m): Probability of i access

m blocks in a round

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Model Verification

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Imbalance and Overhead

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Effects of Block Size

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Effects of Clients

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Effects of Num of Disks

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Choosing Block Size Given Nc, assume rest is fixed find B that minimize T

if T < duration of a round incr Nc and try again

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There is more.. redundant arrayfinding optimum degree of striping

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Cello: A Disk Scheduling Framework for Next Generation Operating Systems P. Shenoy, H. VinSIGMETRICS 98

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ProblemHow to co-exist with other

applications?

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Classes of ApplicationsReal-time

hard/soft periodic/aperiodic

Best-effort interactive troughput-intensive

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Attempt 1Priority-based schedulerAlways schedule real-time tasks

ahead of best-effort tasks

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Attempt 2Partition time slots into real-time

and best-effort

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Attempt 3Assign weight to application class,

based on their priorityService based on weight

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Attempt 3: CelloTwo-level scheduling

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Class Independent Scheduler

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Allocating Disk BandwidthAllocate in proportion to timeAllocate in proportion to bytes

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Proportionate Time-Allocation

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VariablesP : Interval of a period I : Idle time so farUi : Allocated time for class iwi : Weightage of class i

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Proportional Time Allocation

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Example

Weights 1:1:2P = 100I = 30

10 20 16 14

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r

next

prev

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Need to Make Sure..Does not exceed share for class i

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Need to Make Sure..Total service time does not exceed

available time

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Update and Repeat

next

prev

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Being Work ConservingWork conserving: “don’t work only

if no work to do”

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What if..a class have no pending request?

put in no_work group

a class violates one of the constraints? put in too_much_work group

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Improving UtilizationWhen disk is idle, distribute service

time among classes in too_much_work group

Pick a request from this class and put into scheduled queue

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Free Time Utilization

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Class Specific SchedulerHow to pick (r, prev, next)?

Must not affect tasks in queue Idea: Compute slack time

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Slack Time li Latest time must begin

servicing request iei Earliest time may begin

servicing request idi Deadline for request isi Slack time for i = li – ei

ti Time to service request i

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Slack Time

i

i

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Class 1: Interactive Best Effort

next

prev

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Class 2: High Throughput Best Effort

next

prev

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Class 3: Real Time Application

next

prev

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Evaluationsresponsetime

# video

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Evaluations%time

time

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Cello SummaryOS disk schedulerSupport multiple classesProtect classes from each otherWork conservingUse two-level scheduling