Metered concurrency: Difference between revisions
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=={{header|Logtalk}}== |
=={{header|Logtalk}}== |
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Using Logtalk's multi-threading notifications, which use a per-object FIFO message queue, thus avoiding the need of idle-loops |
Using Logtalk's multi-threading notifications, which use a per-object FIFO message queue, thus avoiding the need of idle-loops. Works when using SWI-Prolog, XSB, or YAP as the backend compiler. |
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<lang logtalk> |
<lang logtalk> |
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:- object(metered_concurrency). |
:- object(metered_concurrency). |
Revision as of 19:29, 25 October 2013
![Task](http://static.miraheze.org/rosettacodewiki/thumb/b/ba/Rcode-button-task-crushed.png/64px-Rcode-button-task-crushed.png)
You are encouraged to solve this task according to the task description, using any language you may know.
The goal of this task is to create a counting semaphore used to control the execution of a set of concurrent units. This task intends to demonstrate coordination of active concurrent units through the use of a passive concurrent unit. The operations for a counting semaphore are acquire, release, and count. Each active concurrent unit should attempt to acquire the counting semaphore before executing its assigned duties. In this case the active concurrent unit should report that it has acquired the semaphore. It should sleep for 2 seconds and then release the semaphore.
Ada
The interface for the counting semaphore is defined in an Ada package specification: <lang ada>package Semaphores is
protected type Counting_Semaphore(Max : Positive) is entry Acquire; procedure Release; function Count return Natural; private Lock_Count : Natural := 0; end Counting_Semaphore;
end Semaphores;</lang> The Acquire entry has a condition associated with it. A task can only execute the Acquire entry when Lock_Count is less than Max. This is the key to making this structure behave as a counting semaphore. This condition, and all the other aspects of Counting_Semaphore are contained in the package body. <lang ada>package body Semaphores is
------------------------ -- Counting_Semaphore -- ------------------------
protected body Counting_Semaphore is
------------- -- Acquire -- -------------
entry Acquire when Lock_Count < Max is begin Lock_Count := Lock_Count + 1; end Acquire;
----------- -- Count -- -----------
function Count return Natural is begin return Lock_Count; end Count;
------------- -- Release -- -------------
procedure Release is begin if Lock_Count > 0 then Lock_Count := Lock_Count - 1; end if; end Release;
end Counting_Semaphore;
end Semaphores;</lang> We now need a set of tasks to properly call an instance of Counting_Semaphore. <lang ada>with Semaphores; with Ada.Text_Io; use Ada.Text_Io;
procedure Semaphores_Main is
-- Create an instance of a Counting_Semaphore with Max set to 3 Lock : Semaphores.Counting_Semaphore(3);
-- Define a task type to interact with the Lock object declared above task type Worker is entry Start (Sleep : in Duration; Id : in Positive); end Worker;
task body Worker is Sleep_Time : Duration; My_Id : Positive; begin accept Start(Sleep : in Duration; Id : in Positive) do My_Id := Id; Sleep_Time := Sleep; end Start; --Acquire the lock. The task will suspend until the Acquire call completes Lock.Acquire; Put_Line("Task #" & Positive'Image(My_Id) & " acquired the lock."); -- Suspend the task for Sleep_Time seconds delay Sleep_Time; -- Release the lock. Release is unconditional and happens without suspension Lock.Release; end Worker;
-- Create an array of 5 Workers type Staff is array(Positive range 1..5) of Worker; Crew : Staff;
begin
for I in Crew'range loop Crew(I).Start(2.0, I); end loop;
end Semaphores_Main;</lang>
ALGOL 68
<lang algol68>SEMA sem = LEVEL 1;
PROC job = (INT n)VOID: (
printf(($" Job "d" acquired Semaphore ..."$,n)); TO 10000000 DO SKIP OD; printf(($" Job "d" releasing Semaphore"l$,n))
);
PAR (
( DOWN sem ; job(1) ; UP sem ) , ( DOWN sem ; job(2) ; UP sem ) , ( DOWN sem ; job(3) ; UP sem )
)</lang> Output:
Job 3 acquired Semaphore ... Job 3 releasing Semaphore Job 1 acquired Semaphore ... Job 1 releasing Semaphore Job 2 acquired Semaphore ... Job 2 releasing Semaphore
BBC BASIC
In BBC BASIC concurrency can only be achieved by timer events (short of running multiple processes). <lang bbcbasic> INSTALL @lib$+"TIMERLIB"
DIM tID%(6) REM Two workers may be concurrent DIM Semaphore%(2) tID%(6) = FN_ontimer(11, PROCtimer6, 1) tID%(5) = FN_ontimer(10, PROCtimer5, 1) tID%(4) = FN_ontimer(11, PROCtimer4, 1) tID%(3) = FN_ontimer(10, PROCtimer3, 1) tID%(2) = FN_ontimer(11, PROCtimer2, 1) tID%(1) = FN_ontimer(10, PROCtimer1, 1) ON CLOSE PROCcleanup : QUIT ON ERROR PRINT REPORT$ : PROCcleanup : END sc% = 0 REPEAT oldsc% = sc% sc% = -SUM(Semaphore%()) IF sc%<>oldsc% PRINT "Semaphore count now ";sc% WAIT 0 UNTIL FALSE DEF PROCtimer1 : PROCtask(1) : ENDPROC DEF PROCtimer2 : PROCtask(2) : ENDPROC DEF PROCtimer3 : PROCtask(3) : ENDPROC DEF PROCtimer4 : PROCtask(4) : ENDPROC DEF PROCtimer5 : PROCtask(5) : ENDPROC DEF PROCtimer6 : PROCtask(6) : ENDPROC DEF PROCtask(n%) LOCAL i%, temp% PRIVATE delay%(), sem%() DIM delay%(6), sem%(6) IF delay%(n%) THEN delay%(n%) -= 1 IF delay%(n%) = 0 THEN SWAP Semaphore%(sem%(n%)),temp% delay%(n%) = -1 PRINT "Task " ; n% " released semaphore" ENDIF ENDPROC ENDIF FOR i% = 1 TO DIM(Semaphore%(),1) temp% = TRUE SWAP Semaphore%(i%),temp% IF NOT temp% EXIT FOR NEXT IF temp% THEN ENDPROC : REM Waiting to acquire semaphore sem%(n%) = i% delay%(n%) = 200 PRINT "Task "; n% " acquired semaphore" ENDPROC DEF PROCcleanup LOCAL i% FOR i% = 1 TO 6 PROC_killtimer(tID%(i%)) NEXT ENDPROC</lang>
Output:
Task 1 acquired semaphore Task 2 acquired semaphore Semaphore count now 2 Task 1 released semaphore Task 3 acquired semaphore Task 2 released semaphore Task 4 acquired semaphore Task 3 released semaphore Task 5 acquired semaphore Task 4 released semaphore Task 6 acquired semaphore Task 5 released semaphore Semaphore count now 1 Task 6 released semaphore Semaphore count now 0
C
<lang c>#include <semaphore.h>
- include <pthread.h>
- include <stdlib.h>
- include <stdio.h>
- include <unistd.h>
sem_t sem; int count = 3;
/* the whole point of a semaphore is that you don't count it:
* p/v are atomic. Unless it's locked while you are doing * something with the count, the value is only informative */
- define getcount() count
void acquire() { sem_wait(&sem); count--; }
void release() { count++; sem_post(&sem); }
void* work(void * id) { int i = 10; while (i--) { acquire(); printf("#%d acquired sema at %d\n", *(int*)id, getcount()); usleep(rand() % 4000000); /* sleep 2 sec on average */ release(); usleep(0); /* effectively yield */ } return 0; }
int main() { pthread_t th[4]; int i, ids[] = {1, 2, 3, 4};
sem_init(&sem, 0, count);
for (i = 4; i--;) pthread_create(th + i, 0, work, ids + i); for (i = 4; i--;) pthread_join(th[i], 0); printf("all workers done\n");
return sem_destroy(&sem); }</lang>
C#
C# has built in semaphore system where acquire is called via Wait(), release with Release() and count with semaphore.CurrentCount. <lang csharp>using System; using System.Threading; using System.Threading.Tasks;
namespace RosettaCode {
internal sealed class Program { private static void Worker(object arg, int id) { var sem = arg as SemaphoreSlim; sem.Wait(); Console.WriteLine("Thread {0} has a semaphore & is now working.", id); Thread.Sleep(2*1000); Console.WriteLine("#{0} done.", id); sem.Release(); }
private static void Main() { var semaphore = new SemaphoreSlim(Environment.ProcessorCount*2, int.MaxValue);
Console.WriteLine("You have {0} processors availiabe", Environment.ProcessorCount); Console.WriteLine("This program will use {0} semaphores.\n", semaphore.CurrentCount);
Parallel.For(0, Environment.ProcessorCount*3, y => Worker(semaphore, y)); } }
}</lang>
D
<lang d>module meteredconcurrency ; import std.stdio ; import std.thread ; import std.c.time ;
class Semaphore {
private int lockCnt, maxCnt ; this(int count) { maxCnt = lockCnt = count ;} void acquire() { if(lockCnt < 0 || maxCnt <= 0) throw new Exception("Negative Lock or Zero init. Lock") ; while(lockCnt == 0) Thread.getThis.yield ; // let other threads release lock synchronized lockCnt-- ; } void release() { synchronized if (lockCnt < maxCnt) lockCnt++ ; else throw new Exception("Release lock before acquire") ; } int getCnt() { synchronized return lockCnt ; }
}
class Worker : Thread {
private static int Id = 0 ; private Semaphore lock ; private int myId ; this (Semaphore l) { super() ; lock = l ; myId = Id++ ; } override int run() { lock.acquire ; writefln("Worker %d got a lock(%d left).", myId, lock.getCnt) ; msleep(2000) ; // wait 2.0 sec lock.release ; writefln("Worker %d released a lock(%d left).", myId, lock.getCnt) ; return 0 ; }
}
void main() {
Worker[10] crew ; Semaphore lock = new Semaphore(4) ; foreach(inout c ; crew) (c = new Worker(lock)).start ; foreach(inout c ; crew) c.wait ;
}</lang>
Phobos with tools
Using the scrapple.tools extension library for Phobos .. <lang d>module metered;
import tools.threads, tools.log, tools.time, tools.threadpool;
void main() {
log_threads = false; auto done = new Semaphore, lock = new Semaphore(4); auto tp = new Threadpool(10); for (int i = 0; i < 10; ++i) { tp.addTask(i /apply/ (int i) { scope(exit) done.release; lock.acquire; logln(i, ": lock acquired"); sleep(2.0); lock.release; logln(i, ": lock released"); }); } for (int i = 0; i < 10; ++i) done.acquire;
}</lang>
E
This semaphore slightly differs from the task description; the release operation is not on the semaphore itself but given out with each acquisition, and cannot be invoked too many times.
<lang e>def makeSemaphore(maximum :(int > 0)) {
var current := 0 def waiters := <elib:vat.makeQueue>() def notify() { while (current < maximum && waiters.hasMoreElements()) { current += 1 waiters.optDequeue().resolve(def released) when (released) -> { current -= 1 notify() } } } def semaphore { to acquire() { waiters.enqueue(def response) notify() return response } to count() { return current } } return semaphore
}
def work(label, interval, semaphore, timer, println) {
when (def releaser := semaphore <- acquire()) -> { println(`$label: I have acquired the lock.`) releaser.resolve( timer.whenPast(timer.now() + interval, fn { println(`$label: I will have released the lock.`) }) ) }
}
def semaphore := makeSemaphore(3) for i in 1..5 {
work(i, 2000, semaphore, timer, println)
}</lang>
Erlang
In this implementation the semaphore is handled as its own process. Taking advantage of erlang's receive queues, which act as a FIFO queue for 'acquire' requests. As workers come online and request the semaphore they will receive it in order. 'receive' has the effect of pausing the process until a message is matched, so there's no idle looping. <lang erlang> -module(metered). -compile(export_all).
create_semaphore(N) ->
spawn(?MODULE, sem_loop, [N,N]).
sem_loop(0,Max) ->
io:format("Resources exhausted~n"), receive {release, PID} -> PID ! released, sem_loop(1,Max); {stop, _PID} -> ok end;
sem_loop(N,N) ->
receive {acquire, PID} -> PID ! acquired, sem_loop(N-1,N); {stop, _PID} -> ok end;
sem_loop(N,Max) ->
receive {release, PID} -> PID ! released, sem_loop(N+1,Max); {acquire, PID} -> PID ! acquired, sem_loop(N-1,Max); {stop, _PID} -> ok end.
release(Sem) ->
Sem ! {release, self()}, receive released -> ok end.
acquire(Sem) ->
Sem ! {acquire, self()}, receive acquired -> ok end.
start() -> create_semaphore(10).
stop(Sem) -> Sem ! {stop, self()}.
worker(P,N,Sem) ->
acquire(Sem), io:format("Worker ~b has the acquired semaphore~n",[N]), timer:sleep(500 * random:uniform(4)), release(Sem), io:format("Worker ~b has released the semaphore~n",[N]), P ! {done, self()}.
test() ->
Sem = start(), Pids = lists:map(fun (N) -> spawn(?MODULE, worker, [self(),N,Sem]) end, lists:seq(1,20)), lists:foreach(fun (P) -> receive {done, P} -> ok end end, Pids), stop(Sem).
</lang>
Euphoria
<lang euphoria>sequence sems sems = {} constant COUNTER = 1, QUEUE = 2
function semaphore(integer n)
if n > 0 then sems = append(sems,{n,{}}) return length(sems) else return 0 end if
end function
procedure acquire(integer id)
if sems[id][COUNTER] = 0 then task_suspend(task_self()) sems[id][QUEUE] &= task_self() task_yield() end if sems[id][COUNTER] -= 1
end procedure
procedure release(integer id)
sems[id][COUNTER] += 1 if length(sems[id][QUEUE])>0 then task_schedule(sems[id][QUEUE][1],1) sems[id][QUEUE] = sems[id][QUEUE][2..$] end if
end procedure
function count(integer id)
return sems[id][COUNTER]
end function
procedure delay(atom delaytime)
atom t t = time() while time() - t < delaytime do task_yield() end while
end procedure
integer sem
procedure worker()
acquire(sem) printf(1,"- Task %d acquired semaphore.\n",task_self()) delay(2) release(sem) printf(1,"+ Task %d released semaphore.\n",task_self())
end procedure
integer task
sem = semaphore(4)
for i = 1 to 10 do
task = task_create(routine_id("worker"),{}) task_schedule(task,1) task_yield()
end for
while length(task_list())>1 do
task_yield()
end while</lang>
Output:
- Task 1 acquired semaphore. - Task 2 acquired semaphore. - Task 3 acquired semaphore. - Task 4 acquired semaphore. + Task 1 released semaphore. - Task 5 acquired semaphore. + Task 4 released semaphore. - Task 6 acquired semaphore. + Task 3 released semaphore. - Task 7 acquired semaphore. + Task 2 released semaphore. - Task 8 acquired semaphore. + Task 7 released semaphore. - Task 9 acquired semaphore. + Task 6 released semaphore. - Task 10 acquired semaphore. + Task 5 released semaphore. + Task 8 released semaphore. + Task 10 released semaphore. + Task 9 released semaphore.
Go
Buffered channel
Recommended solution for simplicity. Acquire operation is channel send, release is channel receive, and count is provided with len(channel). Also WaitGroup used as a completion checkpoint. <lang go>package main
import (
"log" "os" "sync" "time"
)
// log package serializes output var fmt = log.New(os.Stdout, "", 0)
// library analogy per WP article const nRooms = 10 const nStudents = 20
func main() {
// buffered channel used as a counting semaphore rooms := make(chan int, nRooms) for i := 0; i < nRooms; i++ { rooms <- 1 } // WaitGroup used to wait for all students to have studied // before terminating program var studied sync.WaitGroup studied.Add(nStudents) // nStudents run concurrently for i := 0; i < nStudents; i++ { go student(rooms, &studied) } studied.Wait()
}
func student(rooms chan int, studied *sync.WaitGroup) {
<-rooms // acquire operation // report per task descrption. also exercise count operation fmt.Printf("Room entered. Count is %d. Studying...\n", len(rooms)) // len function provides count operation time.Sleep(2 * time.Second) // sleep per task description rooms <- 1 // release operation studied.Done() // signal that student is done
}</lang> Output for this and the other Go programs here shows 10 students studying immediately, about a 2 second pause, 10 more students studying, then another pause of about 2 seconds before returning to the command prompt.
Sync.Cond
A more traditional approach implementing a counting semaphore object with sync.Cond. It has a constructor and methods for the three operations requested by the task. <lang go>package main
import (
"log" "os" "sync" "sync/atomic" "time"
)
var fmt = log.New(os.Stdout, "", 0)
type countSem struct {
c int32 cond *sync.Cond
}
func newCount(n int) *countSem {
return &countSem{int32(n), sync.NewCond(new(sync.Mutex))}
}
func (cs *countSem) count() int {
return int(atomic.LoadInt32(&cs.c))
}
func (cs *countSem) acquire() {
if atomic.AddInt32(&cs.c, -1) < 0 { atomic.AddInt32(&cs.c, 1) cs.cond.L.Lock() for atomic.AddInt32(&cs.c, -1) < 0 { atomic.AddInt32(&cs.c, 1) cs.cond.Wait() } cs.cond.L.Unlock() }
}
func (cs *countSem) release() {
atomic.AddInt32(&cs.c, 1) cs.cond.Signal()
}
func main() {
librarian := newCount(10) nStudents := 20 var studied sync.WaitGroup studied.Add(nStudents) for i := 0; i < nStudents; i++ { go student(librarian, &studied) } studied.Wait()
}
func student(studyRoom *countSem, studied *sync.WaitGroup) {
studyRoom.acquire() fmt.Printf("Room entered. Count is %d. Studying...\n", studyRoom.count()) time.Sleep(2 * time.Second) studyRoom.release() studied.Done()
}</lang>
Monitor
Monitor-style solution implements counting semaphore as a monitor goroutine encapsulating the count. It implements semaphore operations with separate Go channels. <lang go>package main
import (
"log" "os" "sync" "time"
)
var fmt = log.New(os.Stdout, "", 0)
func main() {
// three operations per task description acquire := make(chan int) release := make(chan int) count := make(chan chan int)
// library analogy per WP article go librarian(acquire, release, count, 10) nStudents := 20 var studied sync.WaitGroup studied.Add(nStudents) for i := 0; i < nStudents; i++ { go student(acquire, release, count, &studied) } // wait until all students have studied before terminating program studied.Wait()
}
func librarian(a, r chan int, c chan chan int, count int) {
p := a // acquire operation is served or not depending on count for { select { case <-p: // acquire/p/wait operation count-- if count == 0 { p = nil } case <-r: // release/v operation count++ p = a case cc := <-c: // count operation cc <- count } }
}
func student(a, r chan int, c chan chan int, studied *sync.WaitGroup) {
cc := make(chan int) a <- 0 // acquire c <- cc // request count fmt.Printf("Room entered. Count is %d. Studying...\n", <-cc) time.Sleep(2 * time.Second) // sleep per task description r <- 0 // release studied.Done() // signal done
}</lang>
Haskell
The QSem (quantity semaphore) waitQSem and signalQSem functions are the Haskell acquire and release equivalents, and the MVar (synchronizing variable) functions are used to put the workers statuses on the main thread for printing. Note that this code is likely only compatible with GHC due to the use of "threadDelay" from Control.Concurrent.
<lang Haskell>import Control.Concurrent import Control.Monad
worker :: QSem -> MVar String -> Int -> IO () worker q m n = do
waitQSem q putMVar m $ "Worker " ++ show n ++ " has acquired the lock." threadDelay 2000000 -- microseconds! signalQSem q putMVar m $ "Worker " ++ show n ++ " has released the lock."
main :: IO () main = do
q <- newQSem 3 m <- newEmptyMVar let workers = 5 prints = 2 * workers mapM_ (forkIO . worker q m) [1..workers] replicateM_ prints $ takeMVar m >>= print</lang>
Icon and Unicon
Icon doesn't support concurrency. A Unicon solution is: <lang unicon>procedure main(A)
n := integer(A[1] | 3) # Max. number of active tasks m := integer(A[2] | 2) # Number of visits by each task k := integer(A[3] | 5) # Number of tasks sem := [: |mutex([])\n :] every put(threads := [], (i := 1 to k, thread every 1 to m do { write("unit ",i," ready") until flag := trylock(!sem) write("unit ",i," running") delay(2000) write("unit ",i," done") unlock(flag) }))
every wait(!threads)
end</lang>
Sample run:
->mc unit 2 ready unit 2 running unit 1 ready unit 1 running unit 3 ready unit 3 running unit 4 ready unit 5 ready unit 2 done unit 2 ready unit 5 running unit 1 done unit 2 running unit 1 ready unit 3 done unit 3 ready unit 4 running unit 5 done unit 5 ready unit 1 running unit 2 done unit 5 running unit 4 done unit 3 running unit 4 ready unit 1 done unit 4 running unit 5 done unit 3 done unit 4 done ->
Java
<lang java>public class CountingSemaphore{
private int lockCount = 0; private int maxCount;
CountingSemaphore(int Max){ maxCount = Max; } public synchronized void acquire() throws InterruptedException{ while( lockCount >= maxCount){ wait(); } lockCount++; } public synchronized void release(){ if (lockCount > 0) { lockCount--; notifyAll(); } } public synchronized int getCount(){ return lockCount; }
}
public class Worker extends Thread{
private CountingSemaphore lock; private int id;
Worker(CountingSemaphore coordinator, int num){ lock = coordinator; id = num; } Worker(){ } public void run(){ try{ lock.acquire(); System.out.println("Worker " + id + " has acquired the lock."); sleep(2000); } catch (InterruptedException e){ } finally{ lock.release(); } } public static void main(String[] args){ CountingSemaphore lock = new CountingSemaphore(3); Worker crew[]; crew = new Worker[5]; for (int i = 0; i < 5; i++){ crew[i] = new Worker(lock, i); crew[i].start(); }
}
}</lang>
Logtalk
Using Logtalk's multi-threading notifications, which use a per-object FIFO message queue, thus avoiding the need of idle-loops. Works when using SWI-Prolog, XSB, or YAP as the backend compiler. <lang logtalk>
- - object(metered_concurrency).
:- threaded.
:- public(run/2). run(Workers, Max) :- % start the semaphore and the workers threaded_ignore(semaphore(Max, Max)), forall( integer::between(1, Workers, Worker), threaded_call(worker(Worker)) ), % wait for the workers to finish forall( integer::between(1, Workers, Worker), threaded_exit(worker(Worker)) ), % tell the semaphore thread to stop threaded_notify(worker(stop, _)).
:- public(run/0). run :- % default values: 7 workers, 2 concurrent workers run(7, 2).
semaphore(N, Max) :- threaded_wait(worker(Action, Worker)), ( Action == acquire, N > 0 -> M is N - 1, threaded_notify(semaphore(acquired, Worker)), semaphore(M, Max) ; Action == release -> M is N + 1, threaded_notify(semaphore(released, Worker)), semaphore(M, Max) ; Action == stop -> true ; % Action == acquire, N =:= 0, threaded_wait(worker(release, OtherWorker)), threaded_notify(semaphore(released, OtherWorker)), threaded_notify(semaphore(acquired, Worker)), semaphore(N, Max) ).
worker(Worker) :- % use a random setup time for the worker random::random(0.0, 2.0, Setup), thread_sleep(Setup), threaded_notify(worker(acquire, Worker)), threaded_wait(semaphore(acquired, Worker)), write('Worker '), write(Worker), write(' acquired semaphore\n'), thread_sleep(2), threaded_notify(worker(release, Worker)), write('Worker '), write(Worker), write(' releasing semaphore\n'), threaded_wait(semaphore(released, Worker)).
- - end_object.
</lang> Output: <lang text> | ?- metered_concurrency::run. Worker 1 acquired semaphore Worker 6 acquired semaphore Worker 1 releasing semaphore Worker 2 acquired semaphore Worker 6 releasing semaphore Worker 5 acquired semaphore Worker 2 releasing semaphore Worker 7 acquired semaphore Worker 5 releasing semaphore Worker 3 acquired semaphore Worker 7 releasing semaphore Worker 4 acquired semaphore Worker 3 releasing semaphore Worker 4 releasing semaphore yes </lang>
Oz
Counting semaphores can be implemented in terms of mutexes (called "locks" in Oz) and dataflow variables (used as condition variables here). The mutex protects both the counter and the mutable reference to the dataflow variable. <lang oz>declare
fun {NewSemaphore N} sem(max:N count:{NewCell 0} 'lock':{NewLock} sync:{NewCell _}) end
proc {Acquire Sem=sem(max:N count:C 'lock':L sync:S)} Sync Acquired in lock L then if @C < N then C := @C + 1 Acquired = true else Sync = @S Acquired = false end end if {Not Acquired} then {Wait Sync} {Acquire Sem} end end
proc {Release sem(count:C 'lock':L sync:S ...)} lock L then C := @C - 1 @S = unit %% wake up waiting threads S := _ %% prepare for new waiters end end
proc {WithSemaphore Sem Proc} {Acquire Sem} try {Proc} finally {Release Sem} end end
S = {NewSemaphore 4}
proc {StartWorker Name} thread
for do {WithSemaphore S proc {$} {System.showInfo Name#" acquired semaphore"} {Delay 2000} end } {Delay 100} end
end end
in
for I in 1..10 do {StartWorker I} end</lang>
Perl
See Coro::Semaphore.
PicoLisp
<lang PicoLisp>(let Sem (tmp "sem")
(for U 4 # Create 4 concurrent units (unless (fork) (ctl Sem (prinl "Unit " U " aquired the semaphore") (wait 2000) (prinl "Unit " U " releasing the semaphore") ) (bye) ) ) )</lang>
PureBasic
This launches a few threads in parallel, but restricted by the counter. After a thread has completed it releases the Semaphore and a new thread will be able to start. <lang PureBasic>#Threads=10
- Parallels=3
Global Semaphore=CreateSemaphore(#Parallels)
Procedure Worker(*arg.i)
WaitSemaphore(Semaphore) Debug "Thread #"+Str(*arg)+" active." Delay(Random(2000)) SignalSemaphore(Semaphore)
EndProcedure
- Start a multi-thread based work
Dim thread(#Threads) For i=0 To #Threads
thread(i)=CreateThread(@Worker(),i)
Next Debug "Launcher done."
- Wait for all threads to finish before closing down
For i=0 To #Threads
If IsThread(i) WaitThread(i) EndIf
Next</lang> Sample output
Thread #0 active. Thread #2 active. Thread #4 active. Launcher done. Thread #1 active. Thread #3 active. Thread #5 active. Thread #7 active. Thread #9 active. Thread #6 active. Thread #8 active. Thread #10 active.
Python
Python threading module includes a semaphore implementation. This code show how to use it.
<lang python>import time import threading
- Only 4 workers can run in the same time
sem = threading.Semaphore(4)
workers = [] running = 1
def worker():
me = threading.currentThread() while 1: sem.acquire() try: if not running: break print '%s acquired semaphore' % me.getName() time.sleep(2.0) finally: sem.release() time.sleep(0.01) # Let others acquire
- Start 10 workers
for i in range(10):
t = threading.Thread(name=str(i), target=worker) workers.append(t) t.start()
- Main loop
try:
while 1: time.sleep(0.1)
except KeyboardInterrupt:
running = 0 for t in workers: t.join()</lang>
Racket
<lang racket>
- lang racket
(define sema (make-semaphore 4)) ; allow 4 concurrent jobs
- start 20 jobs and wait for all of them to end
(for-each
thread-wait (for/list ([i 20]) (thread (λ() (semaphore-wait sema) (printf "Job #~a acquired semaphore\n" i) (sleep 2) (printf "Job #~a done\n" i) (semaphore-post sema)))))
</lang>
Raven
Counting semaphores are built in:
<lang raven># four workers may be concurrent 4 semaphore as sem
thread worker
5 each as i sem acquire # tid is thread id tid "%d acquired semaphore\n" print 2000 ms sem release # let others acquire 100 ms
- start 10 threads
group
10 each drop worker
list as workers</lang>
Thread joining is automatic by default.
Tcl
Uses the Thread package, which is expected to form part of the overall Tcl 8.6 release. <lang tcl>package require Tcl 8.6 package require Thread
- Create the global shared state of the semaphore
set handle semaphore0 tsv::set $handle mutex [thread::mutex create] tsv::set $handle cv [thread::cond create] tsv::set $handle count 0 tsv::set $handle max 3
- Make five worker tasks
for {set i 0} {$i<5} {incr i} {
lappend threads [thread::create -preserved {
# Not bothering to wrap this in an object for demonstration proc init {handle} { global mutex cv count max set mutex [tsv::object $handle mutex] set cv [tsv::object $handle cv] set count [tsv::object $handle count] set max [tsv::get $handle max] } proc acquire {} { global mutex cv count max thread::mutex lock [$mutex get] while {[$count get] >= $max} { thread::cond wait [$cv get] [$mutex get] } $count incr thread::mutex unlock [$mutex get] } proc release {} { global mutex cv count max thread::mutex lock [$mutex get] if {[$count get] > 0} { $count incr -1 thread::cond notify [$cv get] } thread::mutex unlock [$mutex get] }
# The core task of the worker
proc run {handle id} { init $handle acquire puts "worker $id has acquired the lock" after 2000 release puts "worker $id is done" }
# Wait for further instructions from the main thread
thread::wait
}]
}
- Start the workers doing useful work, giving each a unique id for pretty printing
set i 0 foreach t $threads {
puts "starting thread [incr i]" thread::send -async $t [list run $handle $i]
}
- Wait for all the workers to finish
foreach t $threads {
thread::release -wait $t
}</lang>
UnixPipes
The number of concurrent jobs can be set by issuing that many echo '1s at the begining to sem.
<lang bash>rm -f sem ; mkfifo sem
acquire() {
x=;while test -z "$x"; do read x; done;
}
release() {
echo '1'
}
job() {
n=$1; echo "Job $n acquired Semaphore">&2 ; sleep 2; echo "Job $n released Semaphore">&2 ;
}
( acquire < sem ; job 1 ; release > sem ) & ( acquire < sem ; job 2 ; release > sem ) & ( acquire < sem ; job 3 ; release > sem ) &
echo 'Initialize Jobs' >&2 ; echo '1' > sem</lang>
Visual Basic .NET
This code shows using a local semaphore. Semaphores can also be named, in which case they will be shared system wide.
<lang vbnet>Dim sem As New Semaphore(5, 5) 'Indicates that up to 5 resources can be aquired sem.WaitOne() 'Blocks until a resouce can be aquired Dim oldCount = sem.Release() 'Returns a resource to the pool 'oldCount has the Semaphore's count before Release was called</lang>