A Whirlwind Tour through Concurrency

1. A Whirlwind Tour through Concurrency Kedar Namjoshi Bell Labs Columbia University, Jan. 2008

2. Outline • Concurrency • Common Concurrency Patterns • The Plan9 Thread Model • Reasoning about Concurrency • The “Session Data Type” Model

3. Concurrency • Processes which overlap in time and exchange information • The world is concurrent. Purely sequential behavior is often artificial (“like a machine”) • However, writing concurrent programs is viewed as a hard problem. Why?

4. Concurrency in programming • For a long time, concurrency has been a concern mainly for designers and implementers of • operating systems • supercomputers • processor chips • database systems • and phone networks … • This is about to change! • Faster Chips Are Leaving Programmers In Their Dust • (The New York Times, December 17, 2007)

5. Multi-Core Processors (from http://www.amd.com) Dual-core already common; Quad-core on its way; expect more!

6. Shared-Memory Concurrency • initially, account x holds $0 • P1: x := x+50; // deposit $50 to x • P2: x := x+100; // deposit $100 to x • What is the amount in x after P1||P2? • It should be $150 --- but it might be $50 ! • How could this happen?

7. Shared-Memory II • Let’s take a closer look • P1: t1 := x; t1 := t1+50; x := t1; • P2: t2 := x; t2 := t2+100; x := t2; • For the schedule P2;P1;P2;P1;P2;P1, the final value of x is $50! • Mistake: implicit assumption that the deposit action is atomic.

8. Shared-Memory III • Atomicity must be enforced. • P1’: lock x; deposit $50; unlock x • P2’: lock x; deposit $100; unlock x • Locks are difficult to get right. • programmers forget to place them (“data races”) • or forget to release locks, or do so in the wrong order (“deadlock”) • or put too many locks (minimal concurrency) • Locking could result in priority inversion (a higher priority process gets locked out) --- the Mars Pathfinder failure was traced back to priority inversion • Locks often don’t match up well to the structure of a problem.

9. Shared Memory IV • Compilers can get in the way, too! • P1: P2: • x := 0; y := 0; • y := 1; x := 1; • On termination, we may expect anything but x=0,y=0. However, the compiler (or the processor) is free to reorder instructions! • P1’: P2’: • y := 1; x := 1; • x := 0; y := 0; • Now x=0,y=0 is a possibility! • This leads to considerations of a weak consistency model.

10. CSP [Hoare 1978] • CSP: Communicating Sequential Processes • Three key concepts • No shared variables • Processes communicate through synchronized send-receive actions • A process may respond to one of a set of communication actions • Simple. Clean. Elegant. • Hoare received the Turing Award in 1980 for • this and many other contributions to computing

11. Communication Patterns • Often, the most natural description of a concurrent program is through its communication pattern. • We’ll see a number of very common patterns.

12. 1. The knock on the door … • … also known as a “real-time interrupt” • C + Unix : signals Java: Thread.interrupt() • “As captain of the crew I had had extensive experience (dating back to 1958) in making basic software dealing with real time interrupts and I knew by bitter experience that as a result of the irreproducibility of the interrupt moments, a program error could present itself misleadingly like an occasional machine malfunctioning. As a result I was terribly afraid. Having fears regarding the possibility of debugging we decided to be as careful as possible and —prevention is better than cure!— to try to prevent bugs from entering the construction.” • -- Edsger W. Dijkstra, The Structure of the “THE”–multiprogramming System (1968) EWD 196 • Dijkstra received the Turing Award in 1972 • for his work on concurrency and structured • programming

13. How does one prevent bugs? • The non-determinism introduced by the real-time interrupt implies that a program execution may be stopped at any point, in any intermediate state. • The key is to write down the minimal set of facts that should be invariant (i.e., unchanged) across an interrupt, for the main program to work correctly. • Then write the interrupt handling routine to respect the invariant, or disable interrupts over a short execution sequence.

14. 2. Copy • process copy(channel in, channel out) • var x; • *[in?x  out!x] • ? is the input action • ! is the output action • *[] means repeat forever

15. 3. Pipeline • A chain of N (N >= 1) copy processes: • copy(c0,c1) || copy(c1,c2) || … || copy(c(N-1),c(N)) • If the i’th process is altered to • [in?x  out!f(i)(x)]* • the pipeline computes the function • f(N) * f(N-1) * … * f(2) * f(1)

16. 4. Delegation: Taxi Rank • All taxis follow the same protocol. • process Taxi[i]: • taxi-rank! i; *[dispatch[i]?p  drive(p); taxi-rank! i] • process Dispatcher: • *[queue?p  (taxi-rank?i  dispatch[i]!p)] • Also known as a “thread pool”

17. 5. Delegation: Futures • process A: • toB ! (“x+2”,x:10); • …. stuff …. • fromB ? y; • Process B: • *[toB? (expr, binding)  fromB! eval(expr,binding)] • Introduced in Scheme (1975?) as delay and force

18. 6. Co-authoring a paper • process Kedar: • toDennis! token; • *[fromDennis? token  write; toDennis! token] • process Dennis: • *[toDennis? token  write; fromDennis! token] • Also known as a “co-routine” .

19. 7. Callback • K: • C! “let me know if you can’t pick up the kids today”; • [work interrupt (C? “can’t do it”  pick up kids)] • Best understood as an expected interrupt. • All warnings regarding interrupt processing apply.

20. 8. Multiplexing • Merge messages from two channels into a single channel • process mux: • *[ • | channel1? x  out! x • | channel2? y  out! y • ] • The operator “|” defines alternatives. If both alternatives are available, the choice is made non-deterministically. • The result may not be a fairmerge!

21. 9. De-Multiplexing • Copy input from one channel into two • process demux: • *[ input? x  out1! x; out2! x] • Or, perhaps: • process demux: • *[input? X  if (x>0) out1!x; else out2!x]

22. 10. On Friday … • K: • C! “let me know if you can’t pick up the kids today”; • [work interrupt • ( • | C? “can’t do it”  pick up kids • | Al? “would you like to teach a class?”  chat • | Dennis? “lunch?”  go to lunch • | … • ) • ] • All warnings regarding interrupt processing apply.

23. The Plan9 Thread Model • Plan9 is an operating system developed at Bell Labs from about 1990 onwards. • Lots of interesting features. • In particular, the OS thread model influenced by CSP. • CSP also influenced two languages at Bell Labs • newsqueak (Pike, Cardelli) Rob Pike’s talk • Alef (Winterbottom) Russ Cox’s CSP-Plan9 page • and, in the wider world … • The design of the Ada programming language • The Occam programming language • The Transputer chip • The BPEL web services language • The CSP book is available.

24. What is the Plan9 thread model? • Two kinds of entities: processes and threads. • Threads are co-routines within a process (i.e., scheduling is cooperative) • Processes are pre-emptively scheduled • Channels are used for communication and synchronization. They can be synchronous (as in CSP) or buffered (with finite capacity, essentially a pipeline) • Further reading: these lecture notes (Sape Mullender) and a description of the structure of the Plan9 window system (Pike)

25. How is it used? • Typically, shared data is held in one process, and accessed through commands sent through a channel (e.g., read, write) • Hence, no locks. Synchronization is enforced through channels. Hence, no race conditions • Clean fit to the design • Communication deadlock is possible, but it can often be detected early by analyzing the communication “skeleton”

26. Programming Languages: Shared-variable concurrency • High-Performance Fortran (HPF), OpenMP, X10, C*: designed for high-performance, scientific computing. “forall” construct, data layout specification • Java: Threads, synchronized methods, external locks. Also, lock-free implementations of basic data structures in java.util.concurrent • C/C++: concurrency provided by external thread libraries (e.g., pthreads). C++ will include language constructs for concurrency in its next version • OCaml/F#: OCaml and F# use external thread libraries. F# has a concurrent garbage collector

27. Programming Languages: Message-passing concurrency • Ada: processes (called tasks) communicating through “rendezvous” operations (like CSP ! and ?) • Erlang: processes communicating though unbounded message queues. Used by Ericsson for programming telephony switches • MPI: Message-passing interface. Used for wide-area and supercomputer applications

28. Bell Labs Research Session Data Types: Simple Management of the Life-cycle of Session-oriented Services Al Aho*, Dennis Dams, RickHull, John Letourneau, Kedar Namjoshi, Frans Panken, Mans van Tellingen Alcatel-Lucent and *Columbia University

29. Session Data Types • A new project at Bell Labs • a simple model and a (fairly simple) API for programming real-time communications • Targets applications that mix web-based setup and control with real-time communications via telephony, video, and IM • Model based around the key concept of a session.

30. The “impedance mismatch” Device OS’s and platforms • Programming of converged services: programmers still think in multiple paradigms • Web: HTTP, SOAP, WSDL, XML • All-IP Telecom: SIP, IMS, Parlay/ParlayX, AIN, PacketTV, XMPP, … Devices Service Developer/ Blender Eclipse-based SCE SIP, SIP Servlets, SB Steplets, … SIP App Server (including SB) Web Services App Server Web Services, SOAP, WSDL, … Other? . . .

31. SDT Model • Three basic building blocks • Party: e.g., kedar@cellphone, dennis@IMserver, aho@laptop • Bubble: a group of parties in conversation. The group could be empty, or have a single person in it (usually a temporary situation) • Session: a group of bubbles, related in some way • And three key concepts • Application: a control program • State Machine: Each party is limited to behavior specified by a state machine over events such as “pickup”, “drop”, “mute” • Notify-Respond: the application is notified for events it registers for. It can trigger changes to a state machine or to a session in response

32. SDT Model Session Manager Session 1 Bubble 2 Bubble 1 Global Address Book / Social Network Bob [ people, devices, friends ] Alice Deborah Charles

33. “Hello World” in SDT • // register as a client for a session manger • ISessionManager sm = SessionManager.register(“config.properties”, this); • // start a session with a predefined audio bubble • AudioSDT s = new AudioSDT(sm); • AudioBubble b = s.getBubble(); • // add yourself and a friend • b.addParty(new PartyID(“Kedar”, “19085821891”), Role.ReadWrite); • b.addParty(new PartyID(“Dennis”, “19085828859”), Role.ReadWrite); • // …… talk …… • System.out.println(“Hello World!”); • Thread.sleep(whatever); • // exit: sm shuts down sessions and bubbles

34. A bit more … • // set up a handler for pickup actions • public class pickupHandler implements IHandler { • public Response run(TriggerBinding[] b) { • num_pickup++; • return Response.Continue; • } • } • // link the handler to the pickup action • PartyVar who = new PartyVar(“who”); • Expr e = b.mkEvent(AudioBubble.SM.Events.pickup(),who); • b.addTrigger(e, new pickupHandler());

35. And that’s it! • The session/bubble model is simple, yet very flexible, thanks to the notify-respond mechanism. • Typically, the application program sets up sessions and bubbles and waits for events to happen • The response to an event may create (or delete) sessions and bubbles, add (or drop) parties, or create new event triggers, all at run-time • This results in an incredible amount of flexibility, out of a very few basic primitives.

36. What we have done • We have an implementation of this model, running on an externally available server • We have written sample applications, including: • A customizable, network-hosted, speed-conference Through a web interface, set your phone buttons to perform SDT actions. As the service is network-hosted, these bindings work from anywhere. [subject to somefine print] • A conference system with the ability to create “whisper” sub-sessions

37. What you might do • Design a scripting language that hides some of the Java intricacies • Design a graphical, web-based language to implement your own view of speed dial • Create a conferencing application which you can distribute through Facebook • … or something far more interesting!  • We are available throughout the semester for help with the API and the system.