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Programming in Occam-pi: A Tutorial

Programming in Occam-pi: A Tutorial. By: Zain-ul-Abdin Zain-ul-Abdin@hh.se. Outline. Occam-pi basics Networks and communication Types, channels, processes ... Primitive Processes Structured Processes Examples: Integration, Matrix Multiplication Mobility Mobile Semantics

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Programming in Occam-pi: A Tutorial

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  1. Programming in Occam-pi:A Tutorial By: Zain-ul-Abdin Zain-ul-Abdin@hh.se

  2. Outline • Occam-pi basics • Networks and communication • Types, channels, processes ... • Primitive Processes • Structured Processes • Examples: Integration, Matrix Multiplication • Mobility • Mobile Semantics • Mobile Assignment • Mobile Communication • FORKing Processes • Example: Dynamic Process Creation "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  3. CSP • CSP deals with processes, networks of processes and various forms of synchronisation / communication between processes. • A network of processes is also a process - so CSP naturally accommodates layered network structures (networks of networks). "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  4. Occam-pi Introduction • Named after Occam Razor ”If you have two equally likely solutions to a problem, choose the simplest” • Combines the concepts of CSP with pi-calculus • Parallel processing language that simplifies the description of systems • Allows composing simple components to build complex systems Semantics [A+B] = Semantics [A] + Semantics [B] • Built in semantics for concurrency • Processes • Channels (Unbuffered message passing) "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  5. myProcess Processes • A process is a component that encapsulates some data structures and algorithms for manipulating that data. • Both its data and algorithms are private. The outside world can neither see that data nor execute those algorithms! [It is not an object.] • The algorithms are executed by the process in its own thread (or threads) of control. • So, how does one process interact with another? "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  6. myProcess Processes • The simplest form of interaction is synchronised message-passing along channels. • The simplest forms of channel are zero-buffered and point-to-point (i.e. wires). "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  7. Synchronized Communication • Output valuedown the channel out • This operation does not complete until the process at the other end of the channel inputs the information • Until that happens, the outputting process waits out ! value "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  8. Synchronized Communication-cont’d • Input the next piece of information from channel ininto the variable x • This operation does not complete until the process at the other end of the channel outputs the information • Until that happens, the inputting process waits in ? x "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  9. x y A B c c ! x c ? y Synchronized Communication-cont’d • Amay write on c at any time, but has to wait for a read. • B may read from c at any time, but has to wait for a write. "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  10. Types • Primitive types • INT, BYTE, BOOL • INT16, INT32, INT64 • REAL32, REAL64 • Arrays types (indexed from 0) • [100]INT • [32][32][8]BYTE • [50]REAL64 "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  11. Precision must be matched Types must match Operators • +, -, *, /, \ PLUS, MINUS, TIMES • +, -, *, /, \ • <, <=, >=, > • =, <> • /\(and), \/(or), >< (xor), >>(rshift), <<(lshift) • ~ (not) INTxx, INTxx → INTxx BYTE, BYTE → BYTE REALxx, REALxx → REALxx INTxx, INTxx → BOOL BYTE, BYTE → BOOL REALxx, REALxx → BOOL *, * → BOOL INTxx, INTxx → INTxx BYTE, BYTE → BYTE INTxx → INTxx BYTE → BYTE "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  12. Expressions • No auto-coercionhappens between types: if xis a REAL32and iis an INT, then x + i is illegal ... • Where necessary, explicit casting between types must be programmed: e.g. x + (REAL32 i)... • No precedence is defined between operators, we must use brackets: e.g. a + (b*c) ... • The operators +, -, *and /trigger run-time errors if their results overflow. "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  13. Declarations • All declarations end in a colon • A declaration cannot be used before it is made ... • Declarations are aligned at the same level of indentation ... • Variable declarations • Channel declarations INT a, b: [max]INT c: [max]BYTE d: CHAN BYTE p: [max<<2]CHAN INT q: "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  14. An Occam-pi Process • Syntactically, an occam-pi process consists of: • an optional sequence of declarations (e.g. values, variables, channels) • a single executable process • All the declarations – and the executable – are aligned at the same level of indentation. • The executable process is either primitiveor structured. "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  15. Process Abstractions in PROC foo (VAL []BYTE s, VAL BOOL mode, INT result, CHAN INT in?, out!, CHAN BYTE pause?) ... : foo(s, mode, result) pause out "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  16. Primitive Processes • Assignment a := c[2] + b • Input (synchronising) in ? a • Output (synchronising) out ! a + (2*b) • Null (do nothing) SKIP • Timer (increments every millisec.) TIMER tim "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  17. Processes can run in any order, No data race hazards Structured Processes-SEQ and PAR SEQ in ? sum in ? x sum := sum + x out ! Sum PAR in0 ? a in1 ? b out ! a + b c := a + (2*b) "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  18. Structured Processes-IF and WHILE IF x > 0 screen ! 'p‘ x < 0 screen ! 'n' TRUE screen ! 'z' WHILE TRUE INT x: SEQ in ? x out ! 2*x "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  19. Structured Processes-PROC instance PROC octople (CHAN INT in?, out!) CHAN INT a, b: PAR double (in?, a!) double (a?, b!) double (b?, out!) : "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  20. Example - Integrate PROC integrate (CHAN INT in?, out!) INT total: SEQ total := 0 WHILE TRUE INT x: SEQ in ? x total := total + x out ! total : • Sequential implementation "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  21. Example - Integrate PROC integrate (CHAN INT in?, out!) CHAN INT a, b, c: PAR delta (a?, out!, b!) prefix (0, b?, c!) plus (in?, c?, a!) : • Parallel implementation "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  22. Example – Matrix Multiplication "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  23. Tool Set • KROC- Kent Retargetable Occam Compiler "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  24. Mobility in occam-pi

  25. B A y x c c ! x c ? y SAFE but SLOW Copy Semantics • Classical occam: data is copied from the workspace of A into the workspace of B. Subsequent work by A on its x variable and B on its y variable causes no mutual interference. "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  26. { <x , v >, <x , v>, … }x0 := x1{ <x , v >, <x , v >, … } 0 0 1 1 0 11 1 Copy Assignment • As assignment changes the state of its environment, which we can represent by a set of ordered pairs mapping variables into data values. • Inoccam, because of its zero-tolerance of aliasing, assignment semantics is what we expect: • [Inall other languages with assignment, the semantics are much more complex - since the variablex0 may be aliased to other variables … and the values associated with those aliases will also have changed tov.] "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  27. B A y x c c ! x c ? y UNSAFE but FAST Reference Semantics • Java/JCSP:references to data (objects) are copied from the workspace of A into B. Subsequent work by A on its x variable and B on its y variable causes mutual interference (race hazard). "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  28. B A c c ! x c ? y Reference Semantics HEAP before x y "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  29. B A c c ! x c ? y Reference Semantics HEAP after x y "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  30. Movement Semantics Consider copy and move operations on files … • copy duplicates the file, placing the copy in the target directory under a (possibly) new name. • move moves the file to the target directory, possibly renaming it: • the original file can no longer be found at the source address; • it has moved. "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  31. { <x , v >, <x , v>, … }x0 := x1{ <x , v >, <x , ??>, … } 0 0 1 1 0 11 Mobile Assignment Mobile semantics differs in one crucial respect: • The value of the variable at the source of the assignment has become undefined - its value has moved to the target variable. • It must be noted: mobile semantics is strictly weaker than copy semantics. "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  32. CHAN OF FOO c: equals PAR c ! x1 c ? x0 Mobile Communication • The semantics for assignment and communication are directly related. Inoccam, communication is just a distributed form of assignment - a value computed in the sending process is assigned to a variable in the receiving one (after the two processes have synchronised). • For example, ifx0andx1were of typeFOO, we have the semantic equivalence: x0 := x1 "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  33. Mobile data Syntax • Occam-pi has a new keyword – a MOBILE qualifier for data types/variables • The MOBILE qualifier doesn’t change the semantics of types as types. For example, MOBILE types are compatible with ordinary types in expressions. • But it imposes the mobile semantics on assignment and communication between MOBILE variables. • The MOBILE qualifier need not be burnt into the type declaration - it can be associated just with particular variables. MOBILE INT x0, x1: "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  34. Mobile data – cont’d • Mobiles are safe references • Assignment and communication with reference semantics • Only one process may hold a given mobile "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  35. Mobile Channel Type • In occam programs, channels are fixed in place at compile time • What if we want to reconnect the process network at runtime? • A channel type is a bundle of one or more related channels: for example, the set of channels connecting a client and a server • Note this has to be a CHAN TYPE, else you can’t put channels in it • Channel direction specifiers are mandatory "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  36. Mobile Channel Bundles • Defined as ‘client’ and ‘server’ ends of a “mobile channel-type”, e.g.: • Directions specified in the type are server-relative • Channel ends inside channel type ends can be used like regular channels CHAN TYPE INT.IO MOBILE RECORD CHAN INT in?: CHAN INT out!: : INT.IO! cli: SEQ cli[in] ! 42 cli[out] ? r INT.IO? svr: SEQ svr[in] ? x svr[out] ! x+1 "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  37. Communicating Mobile Channels • The main use of mobile channel bundles is to support the run-time reconfiguration of process networks • P and Q are now directly connected • Process networks may be arbitrarily reconfigured using mobile channels • dynamic process creation makes this much more interesting P c ? a c a Generator b d Q d ? b "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  38. Dynamic Process Creation • N-replicated PAR, where the replicator count is a constant • The creating process has to wait for them all to terminate before creating process has to wait for them all to terminate before it can do anything else. VAL N IS 10: SEQ PAR i = 1 FOR N ... "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  39. Dynamic Process Creation • Asynchronous process invocation using FORK • Essentially a procedure instance that runs in parallel with the invoking process • Parameter passing is strictly uni-directional and uses a communication semantics • occam-pi introduces two new keywords – FORKING and FORK • Inside a FORKING block, you can use FORK at any time to spawn a new process • When the FORKING block exits, it’ll wait for all the spawned processes to finish "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  40. Dynamic Process Creation PROC A () ... local state SEQ ...... FORKING SEQ ...... FORK P(n, svr, cli) ... ... : VAL data is copied into a FORKed process MOBILE data and channel-ends are moved into a FORKed process "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  41. Client-Server Application response S C request P "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

  42. Client-Server Application • Fault-tolerance by Replication response S C request P B S "Programming in Occam-pi: A Tutorial", Zain-ul-Abdin

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