ITERATIVE COMPUTATIONS CONCURRENCY ID1218 Lecture 042009-11-04 Christian Schulte [email protected]...

ITERATIVE COMPUTATIONSCONCURRENCY

ID1218 Lecture 04 2009-11-04

Christian [email protected]

Software and Computer SystemsSchool of Information and Communication

TechnologyKTH – Royal Institute of Technology

Stockholm, Sweden

mailto:[email protected]

A Fourth Look

L04, 2009-11-04ID1218, Christian Schulte

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A Better Length?


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/1

l([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/2

l([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


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 programsseveral 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 Process

loop() -> 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 process


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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

orderL04, 2009-11-04ID1218, Christian Schulte

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Receive Example

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


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a

b

c

d

a

b

d

a

d

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 Timeouts

receive …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 Mailbox

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


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Priority Receipt

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


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Timed Repeater

start(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 Types

receive {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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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 Server

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


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RPC Client

rpc(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 Client

rpc(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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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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ITERATIVE COMPUTATIONS CONCURRENCY ID1218 Lecture 042009-11-04 Christian Schulte [email protected]...

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Transcript of ITERATIVE COMPUTATIONS CONCURRENCY ID1218 Lecture 042009-11-04 Christian Schulte [email protected]...