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

Joe Bowbeer joe.bowbeer at gmail.com
Sat Dec 20 19:39:25 EST 2014


I think PriorityQueue more clearly represents the function than Map does in
this case.

A Queue should also retain less memory. A Map will tend to accumulate both
Entry and Integer instances over time.

The ConcurrentMap does have the advantage of lock striping, which might be
more performant if there is high contention.

I think this is an interesting problem because there are quite a few
different solutions, each with advantages and disadvantages in terms of
performance and maintainability.

If there were a PriorityQueue version of AbstractQueuedSynchronizer, then
we could have yet another solution using the AQS's state to represent the
next available order.

On Sat, Dec 20, 2014 at 3:59 PM, Hanson Char <hanson.char at gmail.com> wrote:
>
> This one looks interesting and educational.  The use of condition enables
> us to suspend and resume specifically targeted threads without resorting to
> the use of LockSupport.{part, unpark}.  Instead of using a PriorityQueue,
> perhaps we can use a simple Map<Integer, Condition> instead so we can get
> away without the extra Waiter inner class?
>
> Regards,
> Hanson
>
> On Sat, Dec 20, 2014 at 3:12 PM, Joe Bowbeer <joe.bowbeer at gmail.com>
> wrote:
>
>> To round out the selection, here's an implementation using a
>> PriorityQueue of Conditions:
>>
>> class OrderedExecutor<T> {
>>
>>   static class Waiter implements Comparable<Waiter> {
>>
>>     final int order;
>>     final Condition condition;
>>
>>     Waiter(int order, Condition condition) {
>>       this.order = order;
>>       this.condition = condition;
>>     }
>>
>>     @Override
>>     public int compareTo(Waiter waiter) {
>>       return order - waiter.order;
>>     }
>>   }
>>
>>   final Lock lock = new ReentrantLock();
>>   final Queue<Waiter> queue = new PriorityQueue<>();
>>   int nextOrder; // 0 is next
>>
>>   public T execCallableInOrder(int order, Callable<T> callable) throws
>> Exception {
>>     assert order >= 0;
>>     awaitTurn(order);
>>     try {
>>       return callable.call();
>>     } finally {
>>       signalNext(order + 1);
>>     }
>>   }
>>
>>   void awaitTurn(int order) {
>>     lock.lock();
>>     try {
>>       Condition condition = null;
>>       while (nextOrder != order) {
>>         if (condition == null) {
>>           condition = lock.newCondition();
>>           queue.add(new Waiter(order, condition));
>>         }
>>         condition.awaitUninterruptibly();
>>       }
>>     } finally {
>>       lock.unlock();
>>     }
>>   }
>>
>>   void signalNext(int nextOrder) {
>>     lock.lock();
>>     try {
>>       this.nextOrder = nextOrder;
>>       Waiter waiter = queue.peek();
>>       if (waiter != null && waiter.order == nextOrder) {
>>         queue.remove();
>>         waiter.condition.signal();
>>       }
>>     } finally {
>>       lock.unlock();
>>     }
>>   }
>> }
>>
>> On Sat, Dec 20, 2014 at 11:21 AM, Joe Bowbeer <joe.bowbeer at gmail.com>
>> wrote:
>>>
>>> Suman,
>>>
>>> I would advise against using notify/wait.  It raises a red flag for a
>>> lot of reviewers, including me.
>>>
>>> The problems I see in this implementation are:
>>>
>>> 1. Pre-allocation of locks is prohibited by (revised) problem statement.
>>>
>>> Note that if pre-allocation were allowed, then an array would be more
>>> efficient than a Map.
>>>
>>> 2. Access to currentAllowedOrder is not thread-safe but should be.
>>>
>>>
>>> On Sat, Dec 20, 2014 at 6:04 AM, suman shil <suman_krec at yahoo.com>
>>> wrote:
>>>>
>>>> I have modified my solution to avoid notifyAll(). Let me know your
>>>> feedback.
>>>>
>>>>
>>>> public class OrderedExecutor
>>>> {
>>>> private int maxOrder;
>>>> private int currentAllowedOrder;
>>>> private Map<Integer, Object> map = new HashMap<Integer, Object>();
>>>>     public OrderedExecutor(int n)
>>>>     {
>>>>          this.maxOrder = n;
>>>>          this.currentAllowedOrder = 0;
>>>>     for(int i = 0 ; i < this.maxOrder ; i++)
>>>>     {
>>>>     map.put(i, new Object());
>>>>     }
>>>>     }
>>>>
>>>>     public  Object execCriticalSectionInOrder(int order,
>>>>                                               Callable<Object> callable)
>>>>                                               throws Exception
>>>>     {
>>>> if (order >= this.maxOrder)
>>>> {
>>>> throw new Exception("Exceeds maximum order "+ maxOrder);
>>>> }
>>>>
>>>> while(order != currentAllowedOrder)
>>>> {
>>>> synchronized (this.map.get(order))
>>>> {
>>>> this.map.get(order).wait();
>>>> }
>>>> }
>>>>  try
>>>> {
>>>> return callable.call();
>>>> }
>>>>        finally
>>>> {
>>>>             currentAllowedOrder = currentAllowedOrder+1;
>>>> synchronized (this.map.get(order+1))
>>>>             {
>>>>                 this.map.get(order+1).notify();
>>>>             }
>>>>        }
>>>>     }
>>>> }
>>>>
>>>>
>>>>   ------------------------------
>>>>  *From:* Joe Bowbeer <joe.bowbeer at gmail.com>
>>>> *To:* concurrency-interest <concurrency-interest at cs.oswego.edu>
>>>> *Sent:* Friday, December 19, 2014 3:33 AM
>>>>
>>>> *Subject:* Re: [concurrency-interest] Enforcing ordered execution of
>>>> critical sections?
>>>>
>>>> That would be "Tom" Cargill; link to paper:
>>>>
>>>>
>>>> http://www.profcon.com/profcon/cargill/jgf/9809/SpecificNotification.html
>>>>
>>>>
>>>>
>>>> On Thu, Dec 18, 2014 at 1:32 PM, Joe Bowbeer <joe.bowbeer at gmail.com>
>>>> wrote:
>>>>
>>>> I frown on use of notify[All]/wait because they make the code hard to
>>>> maintain.
>>>>
>>>> In this case, with potentially lots of waiting threads, I would check
>>>> out the "Specific Notification" pattern if I were determined to go the
>>>> wait/notify route:
>>>>
>>>> Tim Cargill's paper is dated but still worth reading.
>>>>
>>>> Also see chapter 3.7.3 Specific Notifications in Doug Lea's CPiJ and
>>>> Peter Haggar's article:
>>>>
>>>> http://www.ibm.com/developerworks/java/library/j-spnotif.html
>>>>
>>>> On Thu, Dec 18, 2014 at 1:02 PM, Oleksandr Otenko <
>>>> oleksandr.otenko at oracle.com> wrote:
>>>>
>>>>  Yes, no one said it is a good idea to always do that. When it is
>>>> contended, most of the threads will wake up to only go back to sleep.
>>>>
>>>> The pattern you are after is usually called sequencer. You can see it
>>>> used in TCP. I am not sure why it wasn't implemented in j.u.c. - maybe not
>>>> that popular.
>>>>
>>>> The best solution will be lock-like, but the waiter nodes will contain
>>>> the value they are waiting for - so only the specific threads get woken up.
>>>> The solution with concurrent map is very similar, only with larger overhead
>>>> from storing the index the thread is waiting for.
>>>>
>>>>
>>>> Alex
>>>>
>>>>
>>>>
>>>> On 18/12/2014 20:21, Hanson Char wrote:
>>>>
>>>> Less overhead and simpler are a nice properties, even though at the
>>>> expense of having to wake up all waiting threads just to find out the one
>>>> with the right order to execute.  Still, this seems like a good tradeoff.
>>>>
>>>>  Thanks,
>>>> Hanson
>>>>
>>>> On Wed, Dec 17, 2014 at 11:43 PM, Peter Levart <peter.levart at gmail.com>
>>>> wrote:
>>>>
>>>> On 12/17/2014 08:15 PM, Oleksandr Otenko wrote:
>>>>
>>>> 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
>>>>
>>>>
>>>> You're right, Alex. I'm so infected with park/unpark virus that I
>>>> missed that ;-)
>>>>
>>>> Peter
>>>>
>>>>
>>>> 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
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