iterative computations concurrency

ITERATIVE COMPUTATIONSCONCURRENCY

ID1218 Lecture 04 2009-11-04

Christian Schultecschulte@kth.se

Software and Computer SystemsSchool of Information and Communication

TechnologyKTH – Royal Institute of Technology

Stockholm, Sweden

A Fourth Look

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A Better Length?

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l([]) -> 0;l([_|Xr]) -> 1+l(Xr).

l([],N) -> N;l([_|Xr],N) -> l(Xr,N+1).

Two different functions: l/1 and l/2 Which one is better?

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Running l/1l([1,2,3]) ;

→ [1,2,3]CALL(l/1) ; → CALL(l/1) ; [1,2,3]→ 1+l([2,3]) ; → 1l([2,3])ADD ; → l([2,3])ADD ; 1→ [2,3]CALL(l/1)ADD ; 1→ CALL(l/1)ADD ; [2,3]1→ 1+l([3])ADD ; 1→ 1l([3])ADDADD ; 1→ l([3])ADDADD ; 11→ …

requires stack space in the length of list

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Running l/2l([1,2,3],0) ;

→ [1,2,3]0 CALL(l/2) ; → 0 CALL(l/2) ; [1,2,3]→ CALL(l/2) ; 0[1,2,3]→ l([2,3],0+1) ; → [2,3]0+1 CALL(l/2) ; → 0+1 CALL(l/2) ; [2,3]→ 01ADDCALL(l/2) ; [2,3]→ 1ADDCALL(l/2) ; 0[2,3]→ ADDCALL(l/2) ; 10[2,3]→ CALL(l/2) ; 1[2,3]→ l([3],1+1) ; → …

requires constant stack space!

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Appending Two Lists

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app([],Ys) -> Ys;app([X|Xr],Ys) -> [X|app(Xr,Ys)].

How much memory needed: easy! Stack space… in the length of the first

list CONS accumulate on the stack

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Iterative Computations Iterative computations run with

constant stack space

Make use of last optimization call correspond to loops essentially

Tail recursive procedures are computed by iterative computations

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Concurrency

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ID1218, Christian Schulte

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The World Is Concurrent! Concurrent programs

several activities execute simultaneously (concurrently)

Most of the software you use is concurrent

operating system: IO, user interaction, many processes, …

web browser, Email client, Email server, … telephony switches handling many calls …

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Why Should We Care? Software must be concurrent…

… for many application areas Concurrency can be helpful for

constructing programs organize programs into independent parts concurrency allows to make them independent

with respect to how they execute essential: how do concurrent programs

interact?

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Concurrent Programming Is Easy…

Erlang has been designed to be very good at concurrency…

Essential for concurrent programming here

message passingvery simple interaction betweenconcurrent programs

light-weight processes no shared data structures

independence

Concurrency in Erlang Concurrent programs are composed of

communicating processes each process has a unique id: PID processes are spawned to execute functions send messages to PIDs receive messages messages are Erlang data structures

Erlang processes are not OS processes one Erlang OS process can host lots of Erlang

processes create by spawning they are independent of the underlying OS

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Creating Processes Spawning a process:

takes a function as input creates a new concurrently running process

executing the function returns the PID of the newly created process

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Our First Processloop() -> receive kill -> io:format("Aargh: dead...~n");

Other -> io:format("Yummy: ~p~n",[Other]),

loop() end. L04, 2009-11-04ID1218, Christian Schulte

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Running the Process> P=spawn(fun loop/0).<0.62.0>> P ! apple.… Yummy: apple> P ! bogey.… Yummy: bogey> P ! kill.… Aaargh: dead> P ! ham.…

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Processes Run Forever… Why does process not run out of

memory property of loop/0

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Primitives Creating processes

spawn(F) for function value F spawn(M,F,As) for function F in module M

with argument list As Sending messages

PID ! message Receiving messages

receive … end with clauses Who am I?

self() returns the PID of the current processL04, 2009-11-04ID1218, Christian Schulte

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Processes Each process has a mailbox

incoming messages are stored in order of arrival

sending puts message in mailbox Processes are executed fairly

if a process can receive a message or compute……eventually, it will

It will pretty soon… Simple priorities available (low)

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Message Sending Message sending P ! M is asynchronous

the sender does not wait until message has been processed

continues execution immediately evaluates to M

When a process sends messages M1 and M2 to same PID, they arrive in order in mailbox

FIFO ordering When a process sends messages M1 and M2

to different processes, order of arrival is undefined

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Message Receipt Only receive inspects mailbox

all messages are put into the mailbox Messages are processed in order of arrival

that is, receive processes mailbox in order If the receive statement has a matching

clause for the first message remove message and execute clause always choose the first matching clause

Otherwise, continue with next message Unmatched messages are kept in original order

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Receive Example

1. receive c -> … end2. receive d -> …; b -> … end3. receive M -> … end

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abcd

abd

ad

dmailbox head

1. 2. 3.

Receiving Multiple Messagesseq() -> receive a -> receive b -> … end; c -> … end. With other words: processes can use different receive

statements What does it mean

is a sent before b? is a received before b?

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Receive With Timeoutsreceive …after Time -> Exprend If no matching message arrived within Time milliseconds, evaluate Expr

If only after clause present, process sleeps for Time milliseconds

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Flushing the Mailboxflush() -> receive _ -> flush() after 0 -> true end.

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Priority Receiptpriority() -> receive alarm -> … after 0 -> receive M -> …, priority() end end.

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Timed Repeaterstart(T,F) -> spawn(fun() -> rep(T,F) end).stop(PID) -> PID ! stop.

rep(T,F) -> receive stop -> true after T -> F(), rep(T,F) end.

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Different Message Typesreceive {a, … } -> … ; {b, … } -> … …end

Use tuples as messages first field of tuple describes message type

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Client Server Architectures

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ID1218, Christian Schulte

Client Server Single server processing requests

wait for request perform request reply to request (ok or result)

Multiple clients sending requests send request wait for reply

Very common architecture WWW, RPC, RMI, … example: RPC

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How to Reply: RPC Server must know how to reply to client

client sends request……plus its own PID

PID of process available via self() After server has fulfilled request

sends back reply to sender's PID RPC is synchronous

client must wait until reply received

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RPC Serverserve() -> receive {Client,Request} -> Response = process(Request), Client ! Response, serve() end.

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RPC Clientrpc(Server,Request) -> Server ! {self(), Request}, receive Response -> Response end. This is easy… but wrong…

assumption: first message in mailbox is from server

but: can be from anybody!

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Who Talks To Me? If we only want to receive messages from

process PID, messages must include PID SendingP ! {self(), … }

ReceiptPID= …,receive {P,…} when P==PID -> …end

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Scoping in Patterns Revisited The following

PID= …,receive {P,…} when P==PID -> … end

can be rewritten toPID= …,receive {PID,…} -> …end

Variables already introduced are not pattern variables but the values they are assigned to

whoa, this is ugly (my personal taste)

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A Working RPC Clientrpc(Server,Request) -> Server ! {self(),Request}, receive {Server,Response} -> Response end. This is still easy… but correct…

but why: there can only be one pending reply not so easy to see

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The Registry Register processes under names (atoms)

for example: clock, logger, … Operations

register(Name,Pid) unregister(Name) whereis(Name) returns PID or undefined registered() returns all registered names

Exampleregister(a,PID), a ! M

As always: the registry is scary…L04, 2009-11-04ID1218, Christian Schulte

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Summary & Outlook

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ID1218, Christian Schulte

Summary: Concurrency Processes communicate by message

sending feature ordered mailbox execute selective receive statements messages buffered until removed by receive are scheduled fairly can use timeouts

Simple concurrency pattern client – server request – reply

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Outlook: L05 How can concurrent computations

synchronize with each other cooperate

What are the properties of programs with and without

message sending and message receipt

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