Concurrent Systems II - Bad Request - Trinity College Dublin

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Scheduling algorithms • First-Come First-Served (FCFS) (continued) ◾ Now consider the same processes, but in the order P2, P3, P1 on the ready queue P 2 P 3 0 3 6 P 1 ◾ Average waiting time = (0 + 3 + 6) / 3 = 3 ms ◾ Another example: one CPU bound process and many I/O bound processes: • The CPU bound process will obtain and hold the CPU for a long time, during which the I/O bound processes complete their I/O and enter the ready queue • The CPU bound process eventually blocks on an event, the I/O bound processes are dispatched in order and block again on I/O • The CPU now remains idle until the CPU bound process is again scheduled and the I/O bound processes again spend a long time waiting 198 Trinity College Dublin © Mike Brady 2007–2009
Scheduling algorithms • Shortest-Job-First [preemptive or non-preemptive] ◾ Given a set of processes, the process with the shortest next CPU burst is selected to execute on the CPU ◾ Optimizes minimum average waiting time ◾ There is an obvious problem with this – how do we know how long the next CPU burst will be for each process? ◾ We can use past performance to predict future behaviour • Estimate the next CPU burst time as an exponential average of the lengths of previous CPU bursts • Let tn be the length of the n th CPU burst for a process and let τn+1 be our estimated next CPU burst time τn+1 = αtn+(1- α)τn , α ≤ 0 ≤ 1 • If α = 0, the current prediction is equal to the last prediction • if α = 1, then τn+1 = αtn and the next CPU burst time is estimated to be the length of the last CPU burst time • We need to set τ0 to a constant value 199 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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Page 169 and 170: Translation look aside buffer • E
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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
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Page 201 and 202: Scheduling algorithms • Priority
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Page 213 and 214: Layered file I/O (1) Application Di
Page 215 and 216: Disk access • Disks are slow ◾
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