Concurrent Systems II - Bad Request - Trinity College Dublin

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Page replacement policy • Least Recently Used (LRU) (Cont’d) ◾ Counter: • Associate a timestamp variable with each page table entry • Implement a logical clock that is incremented on every memory access • Whenever we access a memory page, we update the timestamp in the page table entry with the current logical clock value • To replace a page we need to search the page tables to find the least recently used page • Updates are cheap but searching for a replacement is very expensive ◾ Stack • Maintain a stack as a doubly linked list of page table entries • When a page is referenced, move it from its current position in the stack to the top of the stack • Always replace the page at the bottom of the stack • Updates are expensive but searching for a victim page is cheap 176 Trinity College Dublin © Mike Brady 2007–2009
Page replacement policy • First-In-First-Out (FIFO) ◾ Replace pages in a round-robin order ◾ Very efficient to implement – maintain a pointer to the next page to be replaced ◾ Assumes that the page loaded into memory furthest back in time is also the page accessed furthest back in time ◾ Bad assumption, we might have a page that is accessed frequently for a long period of time • This page will be repeatedly loaded and unloaded • Clock algorithm (or second-chance algorithm) ◾ Performs better than FIFO but is not as inefficient to implement as LRU ◾ Conceptually, candidate pages for replacement are arranged in a circular list ◾ Each page has an associated use bit (also called reference bit) • Set to 1 when page is loaded or subsequently accessed 177 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
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
Page 163 and 164: E.g. Virtual memory implementation
Page 165 and 166: Multi-level page tables Virtual add
Page 167 and 168: Typical process memory layout memor
Page 169 and 170: Translation look aside buffer • E
Page 171 and 172: Page fault handling Start Access pa
Page 173 and 174: Fetch policy • When should a page
Page 175: Page replacement policy • A page
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
Page 209 and 210: Scheduling algorithms • Multi-lev
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 and 218: Disk scheduling • So, the order i
Page 219 and 220: Disk scheduling algorithms (2) •
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