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

Oleksandr Otenko oleksandr.otenko at oracle.com
Wed Dec 17 14:15:36 EST 2014


No, there is no difference. Peter didn't spot your entire method is 
synchronized, so spurious wakeup won't make progress until the owner of 
the lock exits the method.

You could split the synchronization into two blocks - one encompassing 
the wait loop, the other in the finally block; but it may make no 
difference.

Alex

On 17/12/2014 18:36, suman shil wrote:
> Thanks peter for your reply. You are right. I should have incremented 
> currentAllowedOrder in finally block.
>
> Suman
> ------------------------------------------------------------------------
> *From:* Peter Levart <peter.levart at gmail.com>
> *To:* suman shil <suman_krec at yahoo.com>; Oleksandr Otenko 
> <oleksandr.otenko at oracle.com>; Concurrency-interest 
> <concurrency-interest at cs.oswego.edu>
> *Sent:* Wednesday, December 17, 2014 11:54 PM
> *Subject:* Re: [concurrency-interest] Enforcing ordered execution of 
> critical sections?
>
> On 12/17/2014 06:46 PM, suman shil wrote:
>> Thanks for your response. Will notifyAll() instead of notify() solve the problem?
>
> It will, but you should also account for "spurious" wake-ups. You 
> should increment currentAllowedOrder only after return from 
> callable.call (in finally block just before notifyAll()).
>
> Otherwise a nice solution - with minimal state, providing that not 
> many threads meet at the same time...
>
> Regards, Peter
>
>> RegardsSuman
>>        From: Oleksandr Otenko<oleksandr.otenko at oracle.com>  <mailto:oleksandr.otenko at oracle.com>
>>   To: suman shil<suman_krec at yahoo.com>  <mailto:suman_krec at yahoo.com>; Concurrency-interest<concurrency-interest at cs.oswego.edu>  <mailto:concurrency-interest at cs.oswego.edu>  
>>   Sent: Wednesday, December 17, 2014 9:55 PM
>>   Subject: Re: [concurrency-interest] Enforcing ordered execution of critical sections?
>>     
>>   There is no guarantee you'll ever hand over the control to the right thread upon notify()
>>   
>>   Alex
>>   
>>   
>>
>> On 17/12/2014 14:07, suman shil wrote:
>>    
>>    Hi, Following is my solution to solve this problem. Please let me know if I am missing something.
>>    public class OrderedExecutor {  private int currentAllowedOrder = 0;  private int maxLength = 0;  public OrderedExecutor(int n)  {          this.maxLength = n;  }      public synchronized Object execCriticalSectionInOrder(                                   int order,                                   Callable<Object> callable)                                 throws Exception  {  if (order >= maxLength)  {  throw new Exception("Exceeds maximum order "+ maxLength);  }    while(order != currentAllowedOrder)  {  wait();  }    try  {  currentAllowedOrder = currentAllowedOrder+1;  return callable.call();  }  finally  {  notify();  }  } }
>>    Regards Suman
>>        From: Peter Levart<peter.levart at gmail.com>  <mailto:peter.levart at gmail.com>
>>   To: Hanson Char<hanson.char at gmail.com>  <mailto:hanson.char at gmail.com>  
>>   Cc: concurrency-interest<concurrency-interest at cs.oswego.edu>  <mailto:concurrency-interest at cs.oswego.edu>  
>>   Sent: Sunday, December 14, 2014 11:01 PM
>>   Subject: Re: [concurrency-interest] Enforcing ordered execution of critical sections?
>>     
>>     
>>   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 listConcurrency-interest at cs.oswego.edu  <mailto:Concurrency-interest at cs.oswego.edu>  http://cs.oswego.edu/mailman/listinfo/concurrency-interest  
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