Programming Paradigms for Concurrency Pavol Cerny Vasu Singh Thomas Wies Part III – Message Passing Concurrency

The Downsides of Classical Shared Memory Concurrency shared memory typically implies physically shared memory locks: the “goto statements” of concurrency OS threads are resource-hungry and context-switching is expensive number of threads = number of available cores ≠ number of logical tasks number of threads = number of available cores ≠ number of logical tasks no natural adaptation to distributed settings reasoning about programs is (even more) difficult

Overview of Part III 1.Message Passing Programming Paradigms  actors (Scala)  first-class synchronous events (Concurrent ML) 2.MP Programming Techniques – client/server programming – process networks 3.Formal Semantics of MP Programs – process calculi 4.Formal Reasoning about MP Programs  behavioral equivalences  verifying safety and liveness properties

Actors object-oriented approach to concurrency “actor = object + logical thread” [Hewitt, Bishop, Steiger 1973, Agha 1986]

Actors actors perform local computations and communicate via MP communication is – asynchronous – buffered (in practice FIFO) – over unique-receiver channels (mailboxes) computation is – even-driven: react to incoming messages – dynamically create other actors – send messages to other actors – dynamically change behavior languages supporting actor-based concurrency – Erlang – Salsa – Scala – many implementations in form of libraries A A A A B B

Example: Ping-Pong Actors Ping Pong pong PingMsg

Example: Ping-Pong Actors Ping Pong pong PingMsg sender PongMsg

Example: Concurrent Merge Sort Sorter 1 523

Example: Concurrent Merge Sort Sorter

Example: Concurrent Merge Sort Sorter

Example: Concurrent Merge Sort Sorter

Example: Concurrent Merge Sort Sorter

Example: Concurrent Merge Sort Sorter

Example: Concurrent Merge Sort Sorter

CCS - A Process Calculus P,Q ::= 0 | A(a 1,…,a n ) | ¿.P | a?P | a!P | P “|” Q | P + Q | ( º a) P terminated process process identifier internal action input action output action parallel composition choice channel restriction

CCS - A Process Calculus Processes are defined by systems of recursive equations Ping(ping, pong)= Msg(ping) | (pong ? Ping(ping, pong)) Formal semantics given in terms of labeled transition systems ) enables formal reasoning about MP programs Diverge() = ¿.Diverge() Diverge() ´ ( º ping, pong) ((Ping(ping, pong) | Pong (ping, pong))) Pong(ping, pong) = ping ? (Msg(pong) | Pong (ping, pong)) Msg(a) = a ! 0

Project Proposals 1.Performance/Conciseness Evaluation of Concurrent Programming Paradigms Compare idiomatic implementations of the same algorithm in Scala using different programming paradigms: – a sequential implementation – a concurrent implementation based on actors – a concurrent implementation based on shared memory

Project Proposals 2.First-Class Synchronous Events in Scala – Take inspiration from the Scala Actor Library and the CML library – Implement a Scala library supporting first-class synchronous events.

Project Proposals 3.Verification of Scala Actor Programs –Implement a non-trivial Scala Actor program or take an existing one –Manually compute finite-state abstractions of all actors –Verify properties of the abstracted program using a formal verification tool for MP programs Please drop by or send me an to discuss concrete projects!