Concurrent Systems II - Bad Request - Trinity College Dublin

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Disk scheduling algorithms (1) • FCFS scheduling ◾ The I/O request queue is follows the FIFO queuing discipline ◾ Requests are serviced in the order in which they were submitted to the operating system’s I/O manager ◾ Although the algorithm is fair, the distribution of requests across the disk means a significant proportion of the disk access time is spent seeking • SSTF scheduling ◾ Shortest seek time first ◾ Of the queued requests, select the one that is closest to the current disk head position (the position of the last disk access) ◾ Reduces time spent seeking ◾ Problem: Some requests may be starved ◾ Problem: The algorithm is still not optimal 218 Trinity College Dublin © Mike Brady 2007–2009
Disk scheduling algorithms (2) • SCAN scheduling ◾ Starting from either the hub or the outer edge of the disk and moving to the other edge, service all the queued requests along the way ◾ When the other edge is reached, reverse the direction of head movement and service the queued requests in the opposite direction ◾ If a requests arrives in the queue just before the head reaches the requested track, the request will be serviced quickly ◾ However, if a requests in the queue just after the head has serviced the requested position, the request may take a long time to be serviced (depending on the current head position) • Consider a request for a track at the hub that arrives just after the hub has been serviced • That request will not be serviced until the head has serviced the requests in both directions 219 Trinity College Dublin © Mike Brady 2007–2009
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Concurrent Systems II (CS3D4/CS3BA2
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POSIX Threads • POSIX Threads aka
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Six Separate Threads Master Thread
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Where… • ‘thread’ is the ID
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Hello World -- Creating Threads int
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Hello World -- Thread Function void
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Wilde/Stoker/Ubuntu h-3.1$ cc -lpth
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Interaction • No thread interacti
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Interactions & Dependencies • Int
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…to do Parallel Programming • H
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Hardware Main Memory Level 2 Cache
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Shared Memory Machine • Each proc
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Concurrent Program • A Concurrent
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Communication • A parallel progra
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1 - Flow Dependence S1: write to lo
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3 - Output Dependence S1: write to
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Dependence in Loops j k k i 1 2 j 5
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How can we transform it? • We can
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Issues: • The variable sum, as wr
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Summarising: • We could paralleli
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Example • Sample function: y = 0.
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Synchronisation • We’ve looked
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Shared Variable S: Thread 1 after T
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Mutual Exclusion in pthreads. • M
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Shared Variable S Protected by Mute
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Lock and Unlock Mutex • pthread_m
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} /********** Critical Section ****
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Problem Tutorial • The transpose
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Producers and Consumers • Essenti
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Condition Variables (2) • Dramati
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From Thread U’s POV (2) • If C
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From Thread U’s POV (4) • Acqui
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Thread S (2) • Acquire Mutex M
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Semaphores • Computer Semaphores
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Semaphore Consumer ◾Acquire Mutex
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Semaphores (3) • Useful for count
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Sample Solution for ∏ • Calcula
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unsigned long long clock_cycles() {
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The Challenge of Concurrency • Co
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Concurrent Execution • A concurre
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Notation Trivial Concurrent Program
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Trivial Concurrent Program p1: n
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Example — Jumping Frogs • A fro
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If the frogs can only move “forwa
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State Diagrams for Processes • A
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Book Reference Principles of concur
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Correctness • (Correctness in Seq
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Using State machines • We can des
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The Critical Section Problem (2)
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Observations & Implications • Rem
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Simplify Critical Section Problem i
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State Transitions r1,s1,1 r1: await
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Deadlock Free? • Deadlock Free: I
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Scorecard Mutual Exclusion Deadlock
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Attempt #2 Critical Section Problem
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Attempt #2 Fails… • Attempt 2 f
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Attempt #3 Critical Section Problem
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Attempt #4 loop forever Critical Se
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Attempt #5, derived from #4 Critica
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Verification of concurrent programs
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A quick overview of some mathematic
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A quick overview of some mathematic
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Proving correctness by induction
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A quick overview of some temporal l
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A quick overview of some temporal l
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Temporal logic at work • Consider
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Promela • Promela [Protocol/Proce
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Process (2) • A process type (pro
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SPIN Hello World Example SPIN = Sim
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Promela Variables (2) • Arrays
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Statements (2) • The skip stateme
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If Statement if :: (n % 2 != 0) ->
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Atomic Statement atomic { stat1; st
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Process Execution / Evaluation Sema
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The Closed World Assumption • The
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Process memory management • In a
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Virtual memory • Give each proces
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Virtual memory 0 0 process 1 2 32 0
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Memory management requirements and
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E.g. Virtual memory implementation
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Multi-level page tables Virtual add
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Page 169 and 170: Translation look aside buffer • E
Page 171 and 172: Page fault handling Start Access pa
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Page 175 and 176: Page replacement policy • A page
Page 177 and 178: Page replacement policy • First-I
Page 179 and 180: Clock Algorithm 179 Trinity College
Page 181 and 182: Resident set management • Variabl
Page 183 and 184: Resident set management Local Repla
Page 185 and 186: Resident set management Δ Working
Page 187 and 188: Resident set management 187 Trinity
Page 189 and 190: Cleaning policy • Page buffering
Page 191 and 192: Load control • We can use a few d
Page 193 and 194: Process state transitions • Proce
Page 195 and 196: Preemptive and nonpreemptive schedu
Page 197 and 198: Scheduling algorithms • First-Com
Page 199 and 200: Scheduling algorithms • Shortest-
Page 201 and 202: Scheduling algorithms • Priority
Page 203 and 204: Scheduling algorithms • Round-Rob
Page 205 and 206: Example Process Arrival Time Next B
Page 207 and 208: Scheduling algorithms • Multi-lev
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Page 211 and 212: Multi-processor scheduling • Flyn
Page 213 and 214: Layered file I/O (1) Application Di
Page 215 and 216: Disk access • Disks are slow ◾
Page 217: Disk scheduling • So, the order i
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Concurrent Systems II - Bad Request - Trinity College Dublin

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