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Concurrency and parallelism Friday, October 11, 13

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Core Concepts • Concurrency • Parallelism • Shared State • Mutable Data Structures • Immutability • Shared Mutable State Friday, October 11, 13

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Concurrency Hazards Friday, October 11, 13

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Concurrency Hazards • Race Conditions • Deadlocks • Livelocks • Starvation Friday, October 11, 13

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Threads Friday, October 11, 13

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Starting a thread (.start (Thread. (fn [] ...))) Friday, October 11, 13

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Executors Friday, October 11, 13

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When a single thread won’t do (import '[java.util.concurrent Executors ExecutorService Callable]) (let [^ExecutorService pool (Executors/newFixedThreadPool 16) ^Callable callable (cast Callable (fn [] (reduce + (range 0 10000))))] (.submit pool callable)) Friday, October 11, 13

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Approaches to concurrency Friday, October 11, 13

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Approaches to Concurrency • Locking • Synchronisation • Transactions Friday, October 11, 13

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Epochal Time Model Friday, October 11, 13

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$100 $40 $50 -60$ +10$ Consumer Consumer Consumer Consumer Identity Time Dereferencing Mutation Friday, October 11, 13

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STM atom Friday, October 11, 13

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(def a (atom {})) a {} @reference value Friday, October 11, 13

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(def a (atom {})) (swap! a assoc :b 1) a {} @reference value {:b 1} Friday, October 11, 13

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(def a (atom {})) (swap! a assoc :b 1) a {} @reference value {:b 1} Friday, October 11, 13

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(def a (atom {})) Initial Value Declaration Friday, October 11, 13

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(swap! a assoc :b 1) Attributes Mutating Function Friday, October 11, 13

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(def a (atom {})) (swap! a assoc :b 1) (swap! a assoc :c 2) a @reference value {:b 1} {:b 1 :c 2} Friday, October 11, 13

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(def a (atom {})) (swap! a assoc :b 1) (swap! a assoc :c 2) a @reference value {:b 1} {:b 1 :c 2} Friday, October 11, 13

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{:b 1} {:b 1 :c 2} Structural Sharing Friday, October 11, 13

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Operations on Atoms Friday, October 11, 13

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(deref my-atom) @my-atom Dereferences and returns current value of atom Friday, October 11, 13

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(swap! my-atom ) Swaps current atom value by applying with to it. Friday, October 11, 13

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(reset! my-atom ) Resets current atom value to . Friday, October 11, 13

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Update Types • Coordinated/Uncoordinated • Synchronous/Asynchronous Friday, October 11, 13

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STM • reference, not value • refs / agents / atoms / vars • transactional memory access yes, with retries • no user locks, no deadlocks • mutation through pure function • coordinated • readers can read value at any point in time Friday, October 11, 13

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STM • guarantees of “clear” values for readers and writers • atomic changes • changes are guaranteed • dereferenced values won’t change over the time Friday, October 11, 13

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STM • There are also Refs • synchronous, coordinated • manual transaction control • evaluated in `dosync` block • supports transactional mutations via `alter` • support commutative operations via `commute` Friday, October 11, 13

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STM • and Agents • uncoordianted, asynchronous updates • use their own thread pool • `send` for fixed thread pool • `send-off` for growing thread pool Friday, October 11, 13

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Futures future Friday, October 11, 13

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(def ft (future (+ 1 2 3 4 5 6))) ;; 㱺 #'user/ft ft ;; 㱺 # @ft ;; 㱺 21 Friday, October 11, 13

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(def ft (future (reduce + (range 0 10000)))) ;; 㱺 #'user/ft (realized? ft) ;; 㱺 true @ft ;; 㱺 49995000 Friday, October 11, 13

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Futures • Work with java.util.concurrent.Future • Perform job in background, return control immediately • Dereferencing/get blocks • Dereferencing is possible with timeout Friday, October 11, 13

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Locking locking Friday, October 11, 13

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(let [l (java.util.ArrayList.)] (locking l (.add l 10)) l) ;; 㱺 # Friday, October 11, 13

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Locking • Usually not required, because of immutable nature of DSs • May be needed in cases with heavy Java Interop Friday, October 11, 13

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java.util.concurrent Friday, October 11, 13

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Latches Friday, October 11, 13

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(let [cnt (atom []) n 5 latch (java.util.concurrent.CountDownLatch. n)] (doseq [i (range 0 n)] (.start (Thread. (fn [] (swap! cnt conj i) (.countDown latch))))) (.await latch) @cnt) Friday, October 11, 13

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Latches • Very useful for tests • Useful for understanding parallel execution flows Friday, October 11, 13

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Atomic Vars Friday, October 11, 13

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(let [l (AtomicLong.)] (dotimes [i 50] (.start (Thread. (fn [] (.incrementAndGet l))))) (.get l)) ;; 㱺 49 Friday, October 11, 13

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Atomic Vars • Atomic values without arbitration • In majority of cases you can (should) use atoms Friday, October 11, 13