[concurrency-interest] Enforcing ordered execution of critical sections?

Peter Levart peter.levart at gmail.com
Sun Dec 14 12:31:15 EST 2014


On 12/14/2014 06:11 PM, Hanson Char wrote:
> Hi Peter,
>
> Thanks for this proposed idea of using LockSupport. This begs the 
> question: which one would you choose if you had all three (correct) 
> implementation available?  (Semaphore, CountDownLatch, or LockSupport)?
>
> Regards,
> Hanson

The Semaphore/CountDownLatch variants are equivalent if you don't need 
re-use. So any would do. They lack invalid-use detection. What happens 
if they are not used as intended? Semaphore variant acts differently 
than CountDownLatch variant. The low-level variant I proposed detects 
invalid usage. So I would probably use this one. But the low level 
variant is harder to reason about it's correctness. I think it is 
correct, but you should show it to somebody else to confirm this.

Another question is whether you actually need this kind of synchronizer. 
Maybe if you explained what you are trying to achieve, somebody could 
have an idea how to do that even more elegantly...

Regards, Peter

>
> On Sun, Dec 14, 2014 at 9:01 AM, Peter Levart <peter.levart at gmail.com 
> <mailto:peter.levart at gmail.com>> wrote:
>
>     Hi Hanson,
>
>     This one is more low-level, but catches some invalid usages and is
>     more resource-friendly:
>
>
>     public class OrderedExecutor {
>
>         public <T> T execCriticalSectionInOrder(
>             final int order,
>             final Supplier<T> criticalSection
>         ) throws InterruptedException {
>             if (order < 0) {
>                 throw new IllegalArgumentException("'order' should be
>     >= 0");
>             }
>             if (order > 0) {
>                 waitForDone(order - 1);
>             }
>             try {
>                 return criticalSection.get();
>             } finally {
>                 notifyDone(order);
>             }
>         }
>
>         private static final Object DONE = new Object();
>         private final ConcurrentMap<Integer, Object> signals = new
>     ConcurrentHashMap<>();
>
>         private void waitForDone(int order) throws InterruptedException {
>             Object sig = signals.putIfAbsent(order,
>     Thread.currentThread());
>             if (sig != null && sig != DONE) {
>                 throw new IllegalStateException();
>             }
>             while (sig != DONE) {
>                 LockSupport.park();
>                 if (Thread.interrupted()) {
>                     throw new InterruptedException();
>                 }
>                 sig = signals.get(order);
>             }
>         }
>
>         private void notifyDone(int order) {
>             Object sig = signals.putIfAbsent(order, DONE);
>             if (sig instanceof Thread) {
>                 if (!signals.replace(order, sig, DONE)) {
>                     throw new IllegalStateException();
>                 }
>                 LockSupport.unpark((Thread) sig);
>             } else if (sig != null) {
>                 throw new IllegalStateException();
>             }
>         }
>     }
>
>
>     Regards, Peter
>
>
>     On 12/14/2014 05:08 PM, Peter Levart wrote:
>>
>>     On 12/14/2014 04:20 PM, Hanson Char wrote:
>>>     Hi Peter,
>>>
>>>     Thanks for the suggestion, and sorry about not being clear about
>>>     one important detail: "n" is not known a priori when
>>>     constructing an OrderedExecutor.  Does this mean the use of
>>>      CountDownLatch is ruled out?
>>
>>     If you know at least the upper bound of 'n', it can be used with
>>     such 'n'. Otherwise something that dynamically re-sizes the array
>>     could be devised. Or you could simply use a ConcurrentHashMap
>>     instead of array where keys are 'order' values:
>>
>>
>>     public class OrderedExecutor<T> {
>>
>>         private final ConcurrentMap<Integer, CountDownLatch> latches
>>     = new ConcurrentHashMap<>();
>>
>>         public T execCriticalSectionInOrder(final int order,
>>                                             final Supplier<T>
>>     criticalSection) throws InterruptedException {
>>             if (order > 0) {
>>                 latches.computeIfAbsent(order - 1, o -> new
>>     CountDownLatch(1)).await();
>>             }
>>             try {
>>                 return criticalSection.get();
>>             } finally {
>>                 latches.computeIfAbsent(order, o -> new
>>     CountDownLatch(1)).countDown();
>>             }
>>         }
>>     }
>>
>>
>>     Regards, Peter
>>
>>>
>>>     You guessed right: it's a one-shot object for a particular
>>>     OrderedExecutor instance, and "order" must be called indeed at
>>>     most once.
>>>
>>>     Regards,
>>>     Hanson
>>>
>>>     On Sun, Dec 14, 2014 at 2:21 AM, Peter Levart
>>>     <peter.levart at gmail.com <mailto:peter.levart at gmail.com>> wrote:
>>>
>>>         Hi Hanson,
>>>
>>>         I don't think anything like that readily exists in
>>>         java.lang.concurrent, but what you describe should be
>>>         possible to achieve with composition of existing primitives.
>>>         You haven't given any additional hints to what your
>>>         OrderedExecutor should behave like. Should it be a one-shot
>>>         object (like CountDownLatch) or a re-usable one (like
>>>         CyclicBarrier)? Will execCriticalSectionInOrder() for a
>>>         particular OrderedExecutor instance and 'order' value be
>>>         called at most once? If yes (and I think that only a
>>>         one-shot object makes sense here), an array of
>>>         CountDownLatch(es) could be used:
>>>
>>>         public class OrderedExecutor<T> {
>>>             private final CountDownLatch[] latches;
>>>
>>>             public OrderedExecutor(int n) {
>>>                 if (n < 1) throw new IllegalArgumentException("'n'
>>>         should be >= 1");
>>>                 latches = new CountDownLatch[n - 1];
>>>                 for (int i = 0; i < latches.length; i++) {
>>>                     latches[i] = new CountDownLatch(1);
>>>                 }
>>>             }
>>>
>>>             public T execCriticalSectionInOrder(final int order,
>>>         final Supplier<T> criticalSection) throws InterruptedException {
>>>                 if (order < 0 || order > latches.length)
>>>                     throw new IllegalArgumentException("'order'
>>>         should be [0..." + latches.length + "]");
>>>                 if (order > 0) {
>>>                     latches[order - 1].await();
>>>                 }
>>>                 try {
>>>                     return criticalSection.get();
>>>                 } finally {
>>>                     if (order < latches.length) {
>>>         latches[order].countDown();
>>>                     }
>>>                 }
>>>             }
>>>         }
>>>
>>>
>>>         Regards, Peter
>>>
>>>
>>>         On 12/14/2014 05:26 AM, Hanson Char wrote:
>>>>         Hi, I am looking for a construct that can be used to
>>>>         efficiently enforce ordered execution of multiple critical
>>>>         sections, each calling from a different thread. The calling
>>>>         threads may run in parallel and may call the execution
>>>>         method out of order. The perceived construct would
>>>>         therefore be responsible for re-ordering the execution of
>>>>         those threads, so that their critical sections (and only
>>>>         the critical section) will be executed in order. Would
>>>>         something like the following API already exist? /** * Used
>>>>         to enforce ordered execution of critical sections calling
>>>>         from multiple * threads, parking and unparking the threads
>>>>         as necessary. */ public class OrderedExecutor<T> { /** *
>>>>         Executes a critical section at most once with the given
>>>>         order, parking * and unparking the current thread as
>>>>         necessary so that all critical * sections executed by
>>>>         different threads using this executor take place in * the
>>>>         order from 1 to n consecutively. */ public T
>>>>         execCriticalSectionInOrder(
>>>>         final int order,
>>>>                      final Callable<T> criticalSection) throws InterruptedException;
>>>>         }
>>>>
>>>>         Regards,
>>>>         Hanson
>>>>         _______________________________________________
>>>>         Concurrency-interest mailing list
>>>>         Concurrency-interest at cs.oswego.edu  <mailto:Concurrency-interest at cs.oswego.edu>
>>>>         http://cs.oswego.edu/mailman/listinfo/concurrency-interest
>>>
>>
>

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