Concurrent Systems II - Bad Request - Trinity College Dublin

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Attempt #3 • Attempt #2 fails because of the ‘gap’ between finding out the other process is not attempting to get into its critical region and entering the critical region. ◾(The other process could enter its critical region in the gap.) • So, attempt #3 works by claiming to enter the critical region before checking it’s okay by everyone else. ◾i.e. Attempt #3 does assert wait before awaiting wait ◾c.f. Attempt #2 does await wait before assert wait 112 Trinity College Dublin © Mike Brady 2007–2009
Attempt #3 Critical Section Problem Attempt #3 boolean wantp ← false, wantq ← false loop forever p loop forever q p1: non-critical section q1: non-critical section p2: wantp ← true q2: wantq ← true p3: await wantq = false q3: await wantp = false p4: critical section q4: critical section p5: wantp ← false q5: wantq ← false Do you have an intuition about what might happen? 113 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
Page 61 and 62: From Thread U’s POV (2) • If C
Page 63 and 64: From Thread U’s POV (4) • Acqui
Page 65 and 66: Thread S (2) • Acquire Mutex M
Page 67 and 68: Semaphores • Computer Semaphores
Page 69 and 70: Semaphore Consumer ◾Acquire Mutex
Page 71 and 72: Semaphores (3) • Useful for count
Page 73 and 74: Sample Solution for ∏ • Calcula
Page 75 and 76: unsigned long long clock_cycles() {
Page 77 and 78: The Challenge of Concurrency • Co
Page 79 and 80: Concurrent Execution • A concurre
Page 81 and 82: Notation Trivial Concurrent Program
Page 83 and 84: Trivial Concurrent Program p1: n
Page 85 and 86: Example — Jumping Frogs • A fro
Page 87 and 88: If the frogs can only move “forwa
Page 89 and 90: State Diagrams for Processes • A
Page 91 and 92: Book Reference Principles of concur
Page 93 and 94: Correctness • (Correctness in Seq
Page 95 and 96: Using State machines • We can des
Page 97 and 98: The Critical Section Problem (2)
Page 99 and 100: Observations & Implications • Rem
Page 101 and 102: Simplify Critical Section Problem i
Page 103 and 104: State Transitions r1,s1,1 r1: await
Page 105 and 106: Deadlock Free? • Deadlock Free: I
Page 107 and 108: Scorecard Mutual Exclusion Deadlock
Page 109 and 110: Attempt #2 Critical Section Problem
Page 111: Attempt #2 Fails… • Attempt 2 f
Page 115 and 116: Attempt #3 Fails… • Attempt 3 f
Page 117 and 118: Attempt #4 loop forever Critical Se
Page 119 and 120: Attempt #4 Fails… • Attempt 3 f
Page 121 and 122: Attempt #5, derived from #4 Critica
Page 123 and 124: Verification of concurrent programs
Page 125 and 126: A quick overview of some mathematic
Page 127 and 128: A quick overview of some mathematic
Page 129 and 130: Proving correctness by induction
Page 131 and 132: A quick overview of some temporal l
Page 133 and 134: A quick overview of some temporal l
Page 135 and 136: Temporal logic at work • Consider
Page 137 and 138: Promela • Promela [Protocol/Proce
Page 139 and 140: Process (2) • A process type (pro
Page 141 and 142: SPIN Hello World Example SPIN = Sim
Page 143 and 144: Promela Variables (2) • Arrays
Page 145 and 146: Statements (2) • The skip stateme
Page 147 and 148: If Statement if :: (n % 2 != 0) ->
Page 149 and 150: Atomic Statement atomic { stat1; st
Page 151 and 152: Process Execution / Evaluation Sema
Page 153 and 154: The Closed World Assumption • The
Page 155 and 156: Process memory management • In a
Page 157 and 158: Virtual memory • Give each proces
Page 159 and 160: Virtual memory 0 0 process 1 2 32 0
Page 161 and 162: 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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Typical process memory layout memor
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Translation look aside buffer • E
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Page fault handling Start Access pa
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Fetch policy • When should a page
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Page replacement policy • A page
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Page replacement policy • First-I
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Clock Algorithm 179 Trinity College
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Resident set management • Variabl
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Resident set management Local Repla
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Resident set management Δ Working
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Resident set management 187 Trinity
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Cleaning policy • Page buffering
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Load control • We can use a few d
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Process state transitions • Proce
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Preemptive and nonpreemptive schedu
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Scheduling algorithms • First-Com
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Scheduling algorithms • Shortest-
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Scheduling algorithms • Priority
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Scheduling algorithms • Round-Rob
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Example Process Arrival Time Next B
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Scheduling algorithms • Multi-lev
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Scheduling algorithms • Multi-lev
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Multi-processor scheduling • Flyn
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Layered file I/O (1) Application Di
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Disk access • Disks are slow ◾
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Disk scheduling • So, the order i
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Disk scheduling algorithms (2) •
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Concurrent Systems II - Bad Request - Trinity College Dublin

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