CS252/KubiatowiczLec 22.1
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CS252Graduate Computer Architecture
Lecture 22
I/O Introduction
November 16th, 2003
Prof. John Kubiatowicz
http://www.cs.berkeley.edu/~kubitron/courses/cs252-F03
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• Motivation:– DRAM is dense Signals are easily disturbed– High Capacity higher probability of failure
• Approach: Redundancy– Add extra information so that we can recover from errors– Can we do better than just create complete copies?
• Block Codes: Data Coded in blocks– k data bits coded into n encoded bits– Measure of overhead: Rate of Code: K/N – Often called an (n,k) code– Consider data as vectors in GF(2) [ i.e. vectors of bits ]
• Code Space is set of all 2n vectors, Data space set of 2k vectors– Encoding function: C=f(d)– Decoding function: d=f(C’)– Not all possible code vectors, C, are valid!
• Systematic Codes: original data appears within coded data
Review: Error Correction
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• Not every vector in the code space is valid• Hamming Distance (d):
– Minimum number of bit flips to turn one code word into another
• Number of errors that we can detect: (d-1)• Number of errors that we can fix: ½(d-1)
Code Space
d0
C0=f(d0)
Code Distance(Hamming Distance)
General Idea:Code Vector Space
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Review: Code Types• Linear Codes:
Code is generated by G and in null-space of H• Hamming Codes: Design the H matrix
– d = 3 Columns nonzero, Distinct– d = 4 Columns nonzero, Distinct, Odd-weight
• Reed-solomon codes:– Based on polynomials in GF(2k) (I.e. k-bit symbols)– Data as coefficients, code space as values of polynomial:
– P(x)=a0+a1x1+… ak-1xk-1
– Coded: P(0),P(1),P(2)….,P(n-1)– Can recover polynomial as long as get any k of n– Alternatively: as long as no more than n-k coded symbols
erased, can recover data.
• Side note: Multiplication by constant in GF(2k) can be represented by kk matrix: ax– Decompose unknown vector into k bits: x=x0+2x1+…+2k-
1xk-1
– Each column is result of multiplying a by 2i
CHS dGC
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Motivation: Who Cares About I/O?
• CPU Performance: 60% per year• I/O system performance limited by mechanical
delays (disk I/O)< 10% per year (IO per sec or MB per sec)
• Amdahl's Law: system speed-up limited by the slowest part!
10% IO & 10x CPU => 5x Performance (lose 50%)10% IO & 100x CPU => 10x Performance (lose 90%)
• I/O bottleneck: Diminishing fraction of time in CPUDiminishing value of faster CPUs
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I/O Systems
Processor
Cache
Memory - I/O Bus
MainMemory
I/OController
Disk Disk
I/OController
I/OController
Graphics Network
interruptsinterrupts
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Technology Trends
Disk Capacity now doubles every 18 months; before1990 every 36 motnhs
• Today: Processing Power Doubles Every 18 months
• Today: Memory Size Doubles Every 18 months(4X/3yr)
• Today: Disk Capacity Doubles Every 18 months
• Disk Positioning Rate (Seek + Rotate) Doubles Every Ten Years!
The I/OGAP
The I/OGAP
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Storage Technology Drivers
• Driven by the prevailing computing paradigm– 1950s: migration from batch to on-line processing– 1990s: migration to ubiquitous computing
» computers in phones, books, cars, video cameras, …
» nationwide fiber optical network with wireless tails
• Effects on storage industry:– Embedded storage
» smaller, cheaper, more reliable, lower power– Data utilities
» high capacity, hierarchically managed storage
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Historical Perspective• 1956 IBM Ramac — early 1970s Winchester
– Developed for mainframe computers, proprietary interfaces– Steady shrink in form factor: 27 in. to 14 in.
• 1970s developments– 5.25 inch floppy disk formfactor (microcode into mainframe)– early emergence of industry standard disk interfaces
» ST506, SASI, SMD, ESDI
• Early 1980s– PCs and first generation workstations
• Mid 1980s– Client/server computing – Centralized storage on file server
» accelerates disk downsizing: 8 inch to 5.25 inch– Mass market disk drives become a reality
» industry standards: SCSI, IPI, IDE» 5.25 inch drives for standalone PCs, End of proprietary interfaces
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Disk History
Data densityMbit/sq. in.
Capacity ofUnit ShownMegabytes
1973:1. 7 Mbit/sq. in140 MBytes
1979:7. 7 Mbit/sq. in2,300 MBytes
source: New York Times, 2/23/98, page C3, “Makers of disk drives crowd even mroe data into even smaller spaces”
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Historical Perspective
• Late 1980s/Early 1990s:– Laptops, notebooks, (palmtops)– 3.5 inch, 2.5 inch, (1.8 inch formfactors)– Formfactor plus capacity drives market, not so
much performance» Recently Bandwidth improving at 40%/ year
– Challenged by DRAM, flash RAM in PCMCIA cards» still expensive, Intel promises but doesn’t
deliver» unattractive MBytes per cubic inch
– Optical disk fails on performace (e.g., NEXT) but finds niche (CD ROM)
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Disk History
1989:63 Mbit/sq. in60,000 MBytes
1997:1450 Mbit/sq. in2300 MBytes
source: New York Times, 2/23/98, page C3, “Makers of disk drives crowd even mroe data into even smaller spaces”
1997:3090 Mbit/sq. in8100 MBytes
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MBits per square inch: DRAM as % of Disk over
time
0%
10%
20%
30%
40%
50%
1974 1980 1986 1992 1998
source: New York Times, 2/23/98, page C3, “Makers of disk drives crowd even mroe data into even smaller spaces”
470 v. 3000 Mb/si
9 v. 22 Mb/si
0.2 v. 1.7 Mb/si
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Disk Performance Model /Trends
• Capacity+ 100%/year (2X / 1.0 yrs)
• Transfer rate (BW)+ 40%/year (2X / 2.0 yrs)
• Rotation + Seek time– 8%/ year (1/2 in 10 yrs)
• MB/$> 100%/year (2X / <1.5 yrs)Fewer chips + areal density
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Photo of Disk Head, Arm, Actuator
Actuator
ArmHead
Platters (12)
{Spindle
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Nano-layered Disk Heads• Special sensitivity of Disk head comes from “Giant
Magneto-Resistive effect” or (GMR) • IBM is leader in this technology
– Same technology as TMJ-RAM breakthrough we described in earlier class.
Coil for writing
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Disk Device Terminology
• Several platters, with information recorded magnetically on both surfaces (usually)
• Actuator moves head (end of arm,1/surface) over track (“seek”), select surface, wait for sector rotate under head, then read or write– “Cylinder”: all tracks under heads
• Bits recorded in tracks, which in turn divided into sectors (e.g., 512 Bytes)
Platter
OuterTrack
InnerTrackSector
Actuator
HeadArm
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Disk Performance Example
Disk Latency = Queuing Time + Seek Time + Rotation Time + Xfer Time + Ctrl Time
Order of magnitude times for 4K byte transfers:Seek: 12 ms or less
Rotate: 4.2 ms @ 7200 rpm = 0.5 rev/(7200 rpm/60m/s) (8.3 ms @ 3600 rpm )Xfer: 1 ms @ 7200 rpm (2 ms @ 3600 rpm)
Ctrl: 2 ms (big variation)
Disk Latency = Queuing Time + (12 + 4.2 + 1 + 2)ms = QT + 19.2msAverage Service Time = 19.2 ms
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Disk Time Example
• Disk Parameters:– Transfer size is 8K bytes– Advertised average seek is 12 ms– Disk spins at 7200 RPM– Transfer rate is 4 MB/sec
• Controller overhead is 2 ms• Assume that disk is idle so no queuing delay• What is Average Disk Access Time for a Sector?
– Ave seek + ave rot delay + transfer time + controller overhead
– 12 ms + 0.5/(7200 RPM/60) + 8 KB/4 MB/s + 2 ms– 12 + 4.15 + 2 + 2 = 20 ms
• Advertised seek time assumes no locality: typically 1/4 to 1/3 advertised seek time: 20 ms => 12 ms
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State of the Art: Ultrastar 72ZX– 73.4 GB, 3.5 inch disk– 2¢/MB– 10,000 RPM;
3 ms = 1/2 rotation– 11 platters, 22 surfaces– 15,110 cylinders– 7 Gbit/sq. in. areal den– 17 watts (idle)– 0.1 ms controller time– 5.3 ms avg. seek– 50 to 29 MB/s(internal)
source: www.ibm.com; www.pricewatch.com; 2/14/00
Latency = Queuing Time + Controller time +Seek Time + Rotation Time + Size / Bandwidth
per access
per byte{+
Sector
Track
Cylinder
Head PlatterArmTrack Buffer
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CS 252 Administrivia• Upcoming schedule of project events in CS
252– People who have not talked to me about their projects
should do so soon! – Oral Presentations: Tue/Wed 12/9-10– Final projects due: Friday 12/12?
• Final topics:– Some queuing theory– Multiprocessing
• Final Midterm: 12/1 (Monday after thanksgiving)
• Alternative: 12/3?
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Alternative Data Storage Technologies: Early 1990s
Cap BPI TPI BPI*TPI Data Xfer Access
Technology (MB) (Million) (KByte/s) Time
Conventional Tape:
Cartridge (.25") 150 12000 104 1.2 92 minutes
IBM 3490 (.5") 800 22860 38 0.9 3000 seconds
Helical Scan Tape:
Video (8mm) 4600 43200 1638 71 492 45 secs
DAT (4mm) 1300 61000 1870 114 183 20 secs
Magnetic & Optical Disk:
Hard Disk (5.25") 1200 33528 1880 63 3000 18 ms
IBM 3390 (10.5") 3800 27940 2235 62 4250 20 ms
Sony MO (5.25") 640 24130 18796 454 88 100 ms
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Tape vs. Disk
• Longitudinal tape uses same technology as hard disk; tracks its density improvements
• Disk head flies above surface, tape head lies on surface
• Disk fixed, tape removable
• Inherent cost-performance based on geometries: fixed rotating platters with gaps (random access, limited area, 1 media / reader)vs. removable long strips wound on spool (sequential access, "unlimited" length, multiple / reader)
• New technology trend: Helical Scan (VCR, Camcoder, DAT) Spins head at angle to tape to improve density
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Current Drawbacks to Tape• Tape wear out:
– Helical 100s of passes to 1000s for longitudinal
• Head wear out: – 2000 hours for helical
• Both must be accounted for in economic / reliability model
• Long rewind, eject, load, spin-up times; not inherent, just no need in marketplace (so far)
• Designed for archival
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Automated Cartridge System
STC 4400
6000 x 0.8 GB 3490 tapes = 5 TBytes in 1992 $500,000 O.E.M. Price
6000 x 10 GB D3 tapes = 60 TBytes in 1998
Library of Congress: all information in the world; in 1992, ASCII of all books = 30 TB
8 feet
10 feet
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Relative Cost of Storage Technology—Late 1995/Early
1996Magnetic Disks
5.25” 9.1 GB $2129 $0.23/MB$1985 $0.22/MB
3.5” 4.3 GB $1199 $0.27/MB$999 $0.23/MB
2.5” 514 MB $299 $0.58/MB1.1 GB $345 $0.33/MB
Optical Disks5.25” 4.6 GB $1695+199 $0.41/MB
$1499+189 $0.39/MB
PCMCIA CardsStatic RAM 4.0 MB $700 $175/MBFlash RAM 40.0 MB $1300 $32/MB
175 MB $3600 $20.50/MB
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Manufacturing Advantages
of Disk Arrays
14”10”5.25”3.5”
3.5”
Disk Array: 1 disk design
Conventional: 4 disk designs
Low End High End
Disk Product Families
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Replace Small # of Large Disks with Large # of Small
Disks! (1988 Disks)
Data Capacity
Volume
Power
Data Rate
I/O Rate
MTTF
Cost
IBM 3390 (K)
20 GBytes
97 cu. ft.
3 KW
15 MB/s
600 I/Os/s
250 KHrs
$250K
IBM 3.5" 0061
320 MBytes
0.1 cu. ft.
11 W
1.5 MB/s
55 I/Os/s
50 KHrs
$2K
x70
23 GBytes
11 cu. ft.
1 KW
120 MB/s
3900 IOs/s
??? Hrs
$150K
Disk Arrays have potential for
large data and I/O rates
high MB per cu. ft., high MB per KW
reliability?
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Array Reliability
• Reliability of N disks = Reliability of 1 Disk ÷ N
50,000 Hours ÷ 70 disks = 700 hours
Disk system MTTF: Drops from 6 years to 1 month!
• Arrays (without redundancy) too unreliable to be useful!
Hot spares support reconstruction in parallel with access: very high media availability can be achievedHot spares support reconstruction in parallel with access: very high media availability can be achieved
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Redundant Arrays of Disks
• Files are "striped" across multiple spindles• Redundancy yields high data availability
Disks will fail
Contents reconstructed from data redundantly stored in the array
Capacity penalty to store it
Bandwidth penalty to update
Mirroring/Shadowing (high capacity cost)
Horizontal Hamming Codes (overkill)
Parity & Reed-Solomon Codes
Failure Prediction (no capacity overhead!)VaxSimPlus — Technique is controversial
Techniques:
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Redundant Arrays of DisksRAID 1: Disk Mirroring/Shadowing
• Each disk is fully duplicated onto its "shadow" Very high availability can be achieved
• Bandwidth sacrifice on write: Logical write = two physical writes
• Reads may be optimized
• Most expensive solution: 100% capacity overhead
Targeted for high I/O rate , high availability environments
recoverygroup
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Redundant Arrays of Disks RAID 3: Parity Disk
P100100111100110110010011
. . .
logical record 10010011
11001101
10010011
00110000
Striped physicalrecords
• Parity computed across recovery group to protect against hard disk failures 33% capacity cost for parity in this configuration wider arrays reduce capacity costs, decrease expected availability, increase reconstruction time• Arms logically synchronized, spindles rotationally synchronized logically a single high capacity, high transfer rate disk
Targeted for high bandwidth applications: Scientific, Image Processing
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Redundant Arrays of Disks RAID 5+: High I/O Rate Parity
A logical writebecomes fourphysical I/Os
Independent writespossible because ofinterleaved parity
Reed-SolomonCodes ("Q") forprotection duringreconstruction
A logical writebecomes fourphysical I/Os
Independent writespossible because ofinterleaved parity
Reed-SolomonCodes ("Q") forprotection duringreconstruction
D0 D1 D2 D3 P
D4 D5 D6 P D7
D8 D9 P D10 D11
D12 P D13 D14 D15
P D16 D17 D18 D19
D20 D21 D22 D23 P
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.Disk Columns
IncreasingLogical
Disk Addresses
Stripe
StripeUnit
Targeted for mixedapplications
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Problems of Disk Arrays: Small Writes
D0 D1 D2 D3 PD0'
+
+
D0' D1 D2 D3 P'
newdata
olddata
old parity
XOR
XOR
(1. Read) (2. Read)
(3. Write) (4. Write)
RAID-5: Small Write Algorithm
1 Logical Write = 2 Physical Reads + 2 Physical Writes
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Subsystem Organization
hostarray
controller
single boarddisk
controller
single boarddisk
controller
single boarddisk
controller
single boarddisk
controller
hostadapter
manages interfaceto host, DMA
control, buffering,parity logic
physical devicecontrol
often piggy-backedin small format devices
striping software off-loaded from host to array controller
no applications modifications
no reduction of host performance
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System Availability: Orthogonal RAIDs
ArrayController
StringController
StringController
StringController
StringController
StringController
StringController
. . .
. . .
. . .
. . .
. . .
. . .
Data Recovery Group: unit of data redundancy
Redundant Support Components: fans, power supplies, controller, cables
End to End Data Integrity: internal parity protected data paths
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System-Level Availability
Fully dual redundantI/O Controller I/O Controller
Array Controller Array Controller
. . .
. . .
. . .
. . . . . .
.
.
.RecoveryGroup
Goal: No SinglePoints ofFailure
Goal: No SinglePoints ofFailure
host host
with duplicated paths, higher performance can beobtained when there are no failures
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OceanStore:Global Scale Persistent
Storage
Global-Scale Persistent Storage
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Pac Bell
Sprint
IBMAT&T
CanadianOceanStore
• Service provided by confederation of companies– Monthly fee paid to one service provider– Companies buy and sell capacity from each other
IBM
Utility-based Infrastructure
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Important P2P Technology(Decentralized Object Location and Routing)
GUID1
DOLR
GUID1GUID2
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The Path of an OceanStore Update
Second-TierCaches
Multicasttrees
Inner-RingServers
Clients
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Archival Disseminationof Fragments
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Aside: Why erasure coding?High Durability/overhead ratio!
• Exploit law of large numbers for durability!• 6 month repair, FBLPY:
– Replication: 0.03– Fragmentation: 10-35
Fraction Blocks Lost Per Year (FBLPY)
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The Dissemination Process:Achieving Failure Independence
Model Builder
Set Creator
IntrospectionHuman Input
Network
Monitoringmodel
Inner Ring
Inner Ring
set
set
probe
type
fragments
fragments
fragments
Anti-correlationAnalysis/Models• Mutual Information• Human Input• Data Mining
Independent SetGeneration
Effective Dissemination
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• OceanStore Concepts Applied to Tape-less backup– Self-Replicating, Self-Repairing, Self-Managing– No need for actual Tape in system
» (Although could be there to keep with tradition)
The Berkeley PetaByte Archival Service
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But: What about queue time?
Or: why nonlinear response
Response time = Queue + Device Service time
100%
ResponseTime (ms)
Throughput (Utilization)(% total BW)
0
100
200
300
0%
Proc
Queue
IOC Device
Metrics: Response Time Throughput
latency goes as Tser×u/(1-u) u = utilization
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•Queueing Theory applies to long term, steady state behavior Arrival rate = Departure rate•Little’s Law: Mean number tasks in system = arrival rate x mean reponse time
– Observed by many, Little was first to prove– Simple interpretation: you should see the same number of tasks
in queue when entering as when leaving.
•Applies to any system in equilibrium, as long as nothing in black box is creating or destroying tasks
“Black Box”Queueing
System
Arrivals Departures
Introduction to Queueing Theory
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•Server spends a variable amount of time with customers– Weighted mean m1 = (f1 x T1 + f2 x T2 +...+ fn x Tn)/F
= p(T)xT 2 = (f1 x T12 + f2 x T22 +...+ fn x Tn2)/F – m12
= p(T)xT2 - m12
– Squared coefficient of variance: C = 2/m12
» Unitless measure (100 ms2 vs. 0.1 s2)• Exponential distribution C = 1 : most short relative to
average, few others long; 90% < 2.3 x average, 63% < averageHypoexponential distribution C < 1 : most close to average, C=0.5 => 90% < 2.0 x average, only 57% < averageHyperexponential distribution C > 1 : further from average C=2.0 => 90% < 2.8 x average, 69% < average
Avg.
A Little Queuing Theory: Use of random distributions
Avg.
0
Proc IOC Device
Queue server
System
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•Disk response times C 1.5 (majority seeks < average)•Yet usually pick C = 1.0 for simplicity
– Memoryless, exponential dist– Many complex systems well described
by memoryless distribution!
•Another useful value is average time must wait for server to complete current task: m1(z)– Called “Average Residual Wait Time”– Not just 1/2 x m1 because doesn’t capture variance– Can derive m1(z) = 1/2 x m1 x (1 + C)– No variance C= 0 => m1(z) = 1/2 x m1– Exponential C= 1 => m1(z) = m1
A Little Queuing Theory: Variable Service Time
Proc IOC Device
Queue server
System
Avg.
0 Time
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•Calculating average wait time in queue Tq:– All customers in line must complete; avg time: m1Tser= 1/– If something at server, it takes to complete on average m1(z)
» Chance server is busy = u=/; average delay is u x m1(z)
Tq = u x m1(z) + Lq x Tser
Tq = u x m1(z) + x Tq x Tser
Tq = u x m1(z) + u x Tq
Tq x (1 – u) = m1(z) x uTq = m1(z) x u/(1-u) = Tser x {1/2 x (1+C)} x u/(1 – u))
Notation: average number of arriving customers/second
Tser average time to service a customeru server utilization (0..1): u = x Tser
Tq average time/customer in queueLq average length of queue:Lq= x Tq
m1(z) average residual wait time = Tser x {1/2 x (1+C)}
A Little Queuing Theory: Average Wait
Time
Little’s Law
Defn of utilization (u)
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A Little Queuing Theory: M/G/1 and M/M/1
• Assumptions so far:– System in equilibrium– Time between two successive arrivals in line are random– Server can start on next customer immediately after prior
finishes– No limit to the queue: works First-In-First-Out
– Afterward, all customers in line must complete; each avg Tser
• Described “memoryless” or Markovian request arrival (M for C=1 exponentially random), General service distribution (no restrictions), 1 server: M/G/1 queue
• When Service times have C = 1, M/M/1 queueTq = Tser x u x(1 + C) /(2 x (1 – u)) = Tser x u /(1 – u)
Tser average time to service a customeru server utilization (0..1): u = r x Tser
Tq average time/customer in queue
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A Little Queuing Theory: An Example
• processor sends 10 x 8KB disk I/Os per second, requests & service exponentially distrib., avg. disk service = 20 ms
• On average, how utilized is the disk?– What is the number of requests in the queue?– What is the average time spent in the queue?– What is the average response time for a disk request?
• Notation: average number of arriving customers/second = 10
Tser average time to service a customer = 20 ms (0.02s)u server utilization (0..1): u = x Tser= 10/s x .02s = 0.2Tq average time/customer in queue = Tser x u / (1 – u)
= 20 x 0.2/(1-0.2) = 20 x 0.25 = 5 ms (0 .005s)Tsys average time/customer in system: Tsys =Tq +Tser= 25 msLq average length of queue:Lq= x Tq
= 10/s x .005s = 0.05 requests in queueLsys average # tasks in system: Lsys = x Tsys = 10/s x .025s = 0.25
CS252/KubiatowiczLec 22.53
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A Little Queuing Theory:Yet Another Example
• Processor access memory over “network”• DRAM service properties
– speed = 9cycles+ 2cycles/word– With 8 word cachelines: Tser= 25cycles, =1/25=.04 ops/cycle– Deterministic Servicetime! (C=0)
• Processor behavior– CPI=1, 40% memory, 7.5% cache misses – Rate: = 1 inst/cycle*.4*.075 = .03 ops/cycle
• Notation: average number of arriving customers/cycle=.03
Tser average time to service a customer= 25 cyclesu server utilization (0..1): u = x Tser= .0325=.75Tq average time/customer in queue = Tser x u x (1 + C) /(2 x (1 – u))
= (25 x 0.75 x ½)/(1 – 0.75) = 37.5 cyclesTsys average time/customer in system: Tsys = Tq +Tser= 62.5 cyclesLq average length of queue:Lq= x Tq
= .03 x 37.5 = 1.125 requests in queueLsys average # tasks in system :Lsys = x Tsys = .03 x 62.5 = 1.875
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Summary• Disk industry growing rapidly, improves:
– bandwidth 40%/yr , – areal density 60%/year, $/MB faster?
• queue + controller + seek + rotate + transfer• Advertised average seek time benchmark much
greater than average seek time in practice• Response time vs. Bandwidth tradeoffs• Queueing theory: or (c=1):
• Value of faster response time:– 0.7sec off response saves 4.9 sec and 2.0 sec (70%) total
time per transaction => greater productivity– everyone gets more done with faster response,
but novice with fast response = expert with slow
u
uxCW
1
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