Concurrency: what, why, how Oleg Batrashev February 10, 2014 Course description Course ■ description what this course is about? Overview What is ◆ additional experience with concurrency concurrency ◆ try out some language concepts and techniques Reasons for concurrency ■ java.util.concurrent.Future? Classifications of concurrency ■ agents, software transactional memory (STM) Languages for Concurrent ■ programming Grading ◆ ~5 lab assignments: 20% (1 Java + 4 Clojure) ◆ home assignments: 2×15% (2 Clojure) ■ declarative concurrency/agents... ■ STM/distributed program... ◆ exam: 50% (5-10 questions/exercises for 1-2 hours) ■ Languages: Clojure, Oz, Haskell, Erlang, Scala, Go Concurrency: what, why, how 2 / 35 Overview Course ■ description Lecture about everything and nothing in particular Overview ■ Explain concurrency What is concurrency ◆ basic idea Reasons for concurrency ◆ concurrency (pseudoparallelism) vs. parallelism Classifications of concurrency ■ List reasons for using concurrency Languages for Concurrent ■ Present briefly different programming ◆ classifications ◆ approaches ■ Concurrent programming ◆ models and languages Concurrency: what, why, how 3 / 35 Course description Overview What is concurrency Basic idea Dependency Some terms (1) Some terms (2) Some terms (3) Reasons for What is concurrency concurrency Classifications of concurrency Languages for Concurrent programming Concurrency: what, why, how 4 / 35 Basic idea Course ■ description intuitively Overview What is ◆ simultaneous execution of concurrency Basic idea ■ instructions (with CPU pipelines) Dependency Some terms (1) ■ actions (functions within a program) Some terms (2) ■ Some terms (3) programs (distributed application) Reasons for concurrency ■ what is simultaneous? Classifications of concurrency ◆ physically at the same time? Languages for ◆ Concurrent “nearly” at the same time? programming ■ 2 threads on a one single-core CPU? Concurrency: what, why, how 5 / 35 Dependency Course description If 2 actions Overview ■ What is do not need result of each other (independent) concurrency ■ do not interfere otherwise Basic idea Dependency Some terms (1) ◆ e.g. do not write to the same variable/file Some terms (2) Some terms (3) then the order of their execution does not matter: Reasons for concurrency Classifications of a = c+d concurrency b = c+e Languages for Concurrent programming ■ may be run in parallel (concurrent) ■ excessive optimism (about action dependency/interference) leads to troubles ■ are a and e aliased? Language semantics do matter! Concurrency: what, why, how 6 / 35 Some terms (1) Course description Not a rule, just to extend the understanding of concurrency. Overview What is ■ concurrency Parallel - execute simultaneously Basic idea ■ Concurrent - the order of execution does not matter Dependency Some terms (1) Some terms (2) From wikipedia: Some terms (3) Reasons for Parallel computing is a form of computation in concurrency which many calculations are carried out simultaneously. Classifications of concurrency Concurrent computing is a form of computing in Languages for Concurrent which programs are designed as collections of programming interacting computational processes that may be executed in parallel. Concurrent sometimes referred to as pseudoparallel. Concurrency: what, why, how 7 / 35 Some terms (2) Course description Haskell community variants. Overview What is ■ concurrency Parallel - deterministic “data crunching” simultaneous Basic idea execution of the same type tasks Dependency ■ Some terms (1) Concurrent - non-deterministic execution of unrelated Some terms (2) communicating processes Some terms (3) Reasons for concurrency From Chapter 24. Concurrent and multicore programming: Classifications of concurrency A concurrent program needs to perform several Languages for Concurrent possibly unrelated tasks at the same time. programming In contrast, a parallel program solves a single problem. Concurrency: what, why, how 8 / 35 Some terms (3) Course description “Programming Clojure”, 5.1 State, Concurrency, Parallelism, Overview and Locking What is concurrency Basic idea A concurrent program models more than one thing Dependency Some terms (1) happening simultaneously. Some terms (2) Some terms (3) A parallel program takes an operation that could be Reasons for sequential and chooses to break it into separate pieces concurrency that can execute concurrently to speed overall Classifications of concurrency execution. Languages for Concurrent programming Concurrency: what, why, how 9 / 35 Course description Overview What is concurrency Reasons for concurrency List of reasons Some real life “analogies” Faster programs Reasons for concurrency Hiding latency Better structure Classifications of concurrency Languages for Concurrent programming Concurrency: what, why, how 10 / 35 List of reasons Course ■ description Faster programs Overview What is ◆ running on several cores/cpus/computers concurrency Reasons for ■ More responsive programs concurrency List of reasons Some real life ◆ GUI interface “analogies” Faster programs ◆ hiding disk/network latency Hiding latency Better structure ■ Programs with natural concurrency Classifications of concurrency ◆ Languages for distributed programs (client-server, etc) Concurrent programming ■ Fault tolerant programs ◆ using redundancy ■ Better structured programs Concurrency: what, why, how 11 / 35 Some real life “analogies” Course ■ description Speed of a “process” Overview What is ◆ With 1 axe one friend can chop wood and the other concurrency collect it (conveier) Reasons for concurrency ◆ With 2 axes both friends can chop wood in parallel List of reasons Some real life “analogies” ■ Hiding latency Faster programs Hiding latency ◆ Better structure When we turn on a kettle we do not wait until it boils Classifications of concurrency ■ e.g. we go and take out cups from cupboard Languages for ■ then return to the kettle Concurrent programming ■ Better structured ◆ doing ironing and cooking concurrently is messy ■ assign to different people Concurrency: what, why, how 12 / 35 Faster programs Course ■ description Calculate elements of an array in parallel Overview ■ Perform calculations on several processors/nodes What is ■ concurrency Serving youtube videos from multiple servers Reasons for concurrency End of Moore’s law List of reasons Some real life “analogies” The number of transistors that can be placed Faster programs Hiding latency inexpensively on an integrated circuit has increased Better structure exponentially, doubling approximately every two years Classifications of concurrency ■ Languages for Every new laptop comes with (at least) dual core technology Concurrent programming ◆ usually stuck with 50% CPU usage Concurrency: what, why, how 13 / 35 Hiding latency Course ■ description disk/network take time Overview What is ◆ if a thread blocks while waiting for HTTP response, no concurrency other action can happen Reasons for concurrency List of reasons ■ no other network request Some real life “analogies” ■ not even UI events Faster programs Hiding latency ■ either Better structure Classifications of concurrency ◆ work asynchronously Languages for Concurrent ■ leads to sliced programs and “pyramids of doom” programming ◆ dedicated thread ■ asking for synchronization troubles.. leads to race conditions Concurrency: what, why, how 14 / 35 Better structure Course ■ description Assign different threads to unrelated tasks (if reasonable) Overview What is Data sharing server concurrency Reasons for ■ vertically, one thread per request concurrency List of reasons Some real life ◆ typical web processing (excl. database access) “analogies” Faster programs ■ Hiding latency horizontally (conveier) Better structure Classifications of ◆ dedicated thread(s) for reading requests concurrency ◆ dedicated thread(s) for searching data Languages for Concurrent ◆ new thread for sending data programming Mixing tasks of all threads in one thread ■ asynchronous behavior ■ structural nightmare Concurrency: what, why, how 15 / 35 Course description Overview What is concurrency Reasons for concurrency Classifications of concurrency Task and data parallelism Classifications of concurrency Coarse and fine grained parallelism High and low level parallelism Explicit and implicit parallelism (1) Explicit and implicit parallelism (2) Formalizations By application areas By computation model Languages for Concurrent programming Concurrency: what, why, how 16 / 35 Task and data parallelism Course ■ description Task parallelism: different operations concurrently Overview What is ◆ calculate g and h in f(g(x),h(y)) concurrently concurrency ◆ threads in the same program Reasons for concurrency ◆ several programs running on the same computer Classifications of concurrency ■ Task and data Data parallelism: same operation for different data (SIMD) parallelism Coarse and fine ◆ forall i=1..N do a[i]=a[i]+1 grained loop operations: parallelism ◆ vectorised operations: MMX, SSE, etc High and low level parallelism Explicit and implicit A program may benefit from both! parallelism (1) Explicit and implicit parallelism (2) Formalizations By application areas By computation model Languages for Concurrent programming Concurrency: what, why, how 17 / 35 Coarse and fine grained parallelism Course description Ratio of computation and communication Overview ■ What is coarse-grain parallel programs compute most of the time concurrency Reasons for ◆ e.g distribute data, calculate, collect result (Google concurrency MapReduce) Classifications of concurrency Task and data