What if notifyAll is called at the start of critical section where lock is not released and released at the end of cs(critical section). What will happen to waiting threads?
Do the notifyAll will get heard by all threads and all thread will try to get lock and get unsuccessful(since notify/notifyAll do not release lock) and keep on trying until they get the lock?
So, what difference it makes to call notify at the start of critical section or at the end ?why it is recommended to call notifyAll at the end of cs?
When you call notifyAll() all waiting threads are reactivated and try to acquire the lock on the object on which you called notifyAll().
IMHO the correctness of your code is not sacrificed - no other thread can acquire the lock as long as it is held by the current thread (the one calling notifyAll()).
But the performance will suffer - waking up threads takes processor resources, having them try to acquire the lock takes processor resources, waiting for the release of the lock takes processor resources.
Often the advice to call notifyAll() at the end of a critical section is fulfilled trivially because your thread is doing some processing in a critical section and at the end of that processing it notices that it should notify waiting threads that they can continue. In that case notifying other threads that they can continue before being sure that this is really the case doesn't make any sense.
Related
From the JAVA docs for Object notify()
The awakened thread will not be able to proceed until the current
thread relinquishes the lock on this object.
This means that unless the Thread which notifes, its synchronized block is complete and it releases the lock, the waiting thread cannot proceed. If that's the case then whats the point of having notify() if the sync block is going to be executed anyway? What's the actual use of notify() if it doesn't wake up the waiting thread and let it do its job?
Good question. Will point you to take a look at the Thread State Class.
A thread that calls the Object.notify method enables a thread that previously called Object.wait is now enabled to be scheduled by the thread scheduler. In parlance, the thread that was waiting is now "runnable". Although it is "runnable", it is not "running".
It can only continue running when the thread invoking notify releases the lock - one way is when it exits out of the synchronized block.
There are a lot of schematics on the web on the Thread States. Some of them are completely incorrect or confusing since they introduce terminology not in the official docs. Here is one that makes sense to me.
Strictly speaking, we don't: we could have the waiting thread run a loop where it re-acquires the lock, checks the condition, and sleeps for a short amount of time. But using wait() and notify() is much more efficient, because then the waiting thread doesn't keep waking up and tying up CPU (and tying up the lock).
notify() and notifyAll() are used to wake up thread(s) that called wait() on the same object on which notify() or notifyAll() is called.
Without call to notify() those "waiting" threads will wait forever (although JVM spec says that threads may sometime wake up without call to notify).
Also because call to notify() doesn't releases the lock associated with the object itself that call usually is the last statement in a synchronized block.
So notify() is used together with wait() and not by itself.
Usually the use case is like the following (blocking queue with limited size).
Method that adds element to queue (some pseudo code)
synchronized(lockObject) {
if (size < LIMIT) {
addElement();
lockObject.notifyAll(); //notifying threads that are waiting to get element from empty queue
} else {
lockObject.wait(); // waiting for other thread to get element from queue and make room for new element
}
}
Method that gets element
synchronized(lockObject) {
if (size > 0) {
getElement();
lockObject.notifyAll(); // notify threads that there is a room for new element
} else {
lockObject.wait(); // waiting for other thread to put element into the queue
}
}
Also calling lockObject.wait() releases lock on lockObject. More details regarding that could be found here: Java : Does wait() release lock from synchronized block
Notifying is what wakes up a thread that is waiting. If you remove the notify then waiting threads stay waiting (barring spurious wakeups but let’s not go there for now).
(Interrupting wakes up the thread but the guidance is to use it for cancellation only. Interruption targets a specific thread, where notifying lets the scheduler decide which threads are affected.)
When a thread calls wait it has to have the lock, then the wait method lets go of the lock.
When a thread calls notify it has to have the lock.
As a practical matter the notify can’t take effect on any waiting thread until the notifying thread relinquishes the lock. The first thing the notified thread is going to need to do anyway is to try to acquire the lock. All the passage you're quoting is trying to say is that the wakeup doesn't occur instantaneously when a thread calls notify.
So what happens here is that the notifying thread lets go of the lock and sends the notify to the scheduler, the scheduler decides which thread to notify, then the notified thread wakes up and contends for the lock in order to leave the wait method.
Imagine if you need a thread to wait for another thread to do something that it may or may not even currently be actively working on. For example, a thread that's waiting for a job to do may need to wait until another thread has put a job on the list of jobs it should do if that list is empty. How would you do this?
You can't just use some form of mutual exclusion. There may be long periods of time when there's no work to do and not thread holds any lock on the queue. There may just not be any work to do right now. The thread that does work needs to wait, without holding any lock, until another thread has given it some work to do.
So somewhere, there's a thread that does something like this:
Acquire the lock that protects some shared state that another thread might be waiting for a change to. (In this case, the job queue.)
Change the shared state to reflect the fact that the thing a thread might need to wait for has happened. (That is, put a job on the queue.)
Release the lock and let any waiting thread(s) know that the thing has happened.
So what could our code to wait look like? Perhaps:
Acquire the lock that protects the shared state.
Check whether we need to wait or not. (Is there a job on the queue?)
If we need to wait, wait. (If not, wait for a job to be placed on the queue.)
...
Oops, we have a problem. The thing we're waiting for can't happen because we hold the lock. No other thread can change the shared state. (Our thread to put a job on the queue can't touch the queue until we release the lock we acquired in step 1.)
Let's try it again:
Acquire the lock that protects the shared state.
Check whether we need to wait or not. (Is there a job on the queue?)
If we don't need to wait, exit this algorithm. (If there's a job, take it off the queue, release the lock, and do it.)
Release the lock. (So another thread can put a job on the queue.)
Wait for the thing to happen.
...
Oops, we have another problem. What if the thing we're waiting for happens after step 4 but before step 5. Since the lock has been released, the thing we're waiting for can happen. We can't check again because we don't hold the lock. How can we ensure we don't wait for something that has already happened, which may mean waiting forever?
To solve this, we need an atomic "unlock and wait" operation. That's what wait does. And we also need some operation that can end this wait that can be called by the thread that changed the shared state so that we no longer need to wait. That's what notify does.
I read in a Java textbook the following pertaining to multi-threading.
For a thread to call wait() or notify(), the thread has to be the owner of the lock for that object. When
the thread waits, it temporarily releases the lock for other threads to use, but it will need
it again to continue execution.
I'm confused about what is meant by the clause
When the thread waits, it temporarily releases the lock for other
threads to use
I don't get what that clause is talking about. Is it saying that when the wait() method is called it is actually releasing the lock before the wait() returns (i.e. this happens without caller knowing)? Or is it just alluding to wait(timeout) releasing the lock when the timeout elapses? If it is the former why would it release the lock before notify()? This seems like a vague and poorly explained statement.
For a thread to call wait() or notify(), the thread has to be the owner of the lock for that object.
Otherwise, a runtime error occur and the rest of code is not executed.
When the thread waits, it temporarily releases the lock for other threads to use
In more details, call to wait() does the following:
the lock is released
current thread is registered as waiting in the monitor
processor switches to some other thread ready for execution
Then, some thread calls notify() or notifyAll(), which causes one or all threads which are registered as waiting at this monitor to be moved from the wait set to the ready set, waiting for a free processor to execute.
but it will need it again to continue execution.
This means the execution of the thread is continued with executing synchronized statement to regain the lock. After the lock is aquired, then the wait() method returns. wait(timeout) differs in that except for notify() or notifyAll(), it also can return upon the timeout.
In sum, you need to understand how a thread switches between following 4 states:
running on a processor
blocked on synchronized statement
waiting for notification
ready to execute and waiting for a free processor
When a thread calls wait, the thread releases the lock right away and then goes dormant until either the timeout expires, if any, or until it receives a notification, which occurs when another thread acquires the same lock that the waiting thread gave up and calls notify on it (also the scheduler has to pick the waiting thread from among any other waiting threads; calling notify doesn’t notify a given thread, it tells the scheduler to pick a thread from a given lock’s wait set to notify).
Once the thread is woken up by a notify, it has to reacquire the lock in order to leave the wait method, because the thread is still inside of a synchronized method or block. That is what the quote means when it says the thread will need the lock to resume execution.
When a thread calls wait(), it's temporarily releasing the monitor (lock) of the object until it receives a notification from another thread. This way, a thread can willingly give control (that it has, in the first place) of the object's monitor to another thread. Take a look at the docs:
The invocation of wait() does not return until another thread has
issued a notification that some special event may have occurred —
though not necessarily the event this thread is waiting for (so always
invoke wait() inside a loop that tests for the condition being
waited for).
...
When wait() is invoked, the thread releases the lock and suspends
execution. At some future time, another thread will acquire the same
lock and invoke Object.notifyAll, informing all threads waiting on
that lock that something important has happened.
Suppose d is the object we're using to invoke wait. When a thread invokes d.wait, it must own the intrinsic lock for d — otherwise an error is thrown. Invoking wait inside a synchronized method is a simple way to acquire the intrinsic lock.
so is this means that two threads cannot invoke wait() at the same time? what do intrinsic lock here mean [mentioned it as Monitor]? but how 's monitor implemented to achive mutual exclusion?
once the thread invokes wait does it holds object forever?
if so how about other thread using that lock for notifyAll()?
if we need to acquire object during notifyall, then why all waiting threads notified?
shouldn't it notify threads waiting on that object alone?
Any code to explain is appreciated.
so is this means that two threads cannot invoke wait() at the same
time?
Correct two thread cannot invoke wait() at the same time. However, once one thread is in wait(), another thread can acquire the same lock and enter a wait() state soon after. You can have any number of threads WAITING on the same object, but only one really holds the lock.
what do intrinsic lock here mean [mentioned it as Monitor]? but
how 's monitor implemented to achive mutual exclusion?
Only one thread can be running while holding a object. Other thread can be blocking trying to acquire the lock and more can be wait()ing on it.
once the thread invokes wait does it holds object forever?
The opposite, it gives it up or another thread can acquire it almost immediately.
if so how
about other thread using that lock for notifyAll()?
If you call notifyAll() on the object, all the wait()ing thread are woken in turn. Those threads can only acquire the lock one at a time and will re-acquire the lock as soonas they can.
if we need to acquire object during notifyall, then why all waiting
threads notified?
That is what notifyAll does, it is considered safer than using notify, which wakes a random one as it is less prone to coding errors.
shouldn't it notify threads waiting on that object alone?
That is what it does.
You should note that;
before you notify()/notifyAll() you should perform a state change. You should also wait() inside a loop which checks that state change. You need to do this because a) wait() can miss a notify(), b) it can wake spuriously c) another thread might grab whatever you ahve done and it might need to wait again.
over the last 9 years, there has been greater use of High Level concurrency classes. Using these classes mean you don't need to work with Threads, Queues, wait() and notify() directly.
Invoking wait inside a synchronized method is a simple way to acquire
the intrinsic lock.
Wait does not provide the lock on an object rather it makes the thread to wait to listen about the lock release when other thread calls notify. Thread gets the lock when it enters the guarded//synchronized block. Synchronzied block/method allows to take the lock if available otherwise thread cannot enter those code block.
Locks are not held forever, according to the javadoc:
The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
When you call wait(), you release the intrinsic lock on that object, until another thread calls notify() or notifyAll() on it. At that point, the JVM will wake one of the threads waiting, and automatically reacquire the lock on that object.
So to answer your question, yes, multiple threads can wait() on the same object.
According to a colleague, JVM does not guarantee that when calling "notify" on an object, the correct "wait" will be notified at that time. He says there can be a case when a previous notify which is not valid anymore is delivered at an invalid time.
Is this true? If so, how/why is this, and what use is the wait/notify mechanism if you cannot assume something as basic as this will work?
For java.lang.Object.notify, The Javadoc says:
Wakes up a single thread that is waiting on this object's monitor. If
any threads are waiting on this object, one of them is chosen to be
awakened. The choice is arbitrary and occurs at the discretion of the
implementation. A thread waits on an object's monitor by calling one
of the wait methods.
Here is a pattern to wait for a particular condition:
synchronized( lock ) {
while( conditionEvaluation( data )) {
lock.wait();
}
}
The counterpart should use java.lang.Object.notifyAll() to ensure the vivacity of the application. Even if today, it's only one waiter, after many evolutions of the software, it may be several waiters in the future, so notifyAll() is more robust than notify().
Each object that waits on an intrinsic lock will enter the lock's wait set. When you invoke notify on the lock object, one of the threads in its wait set will be chosen to resume work. The only guarantee that the JVM offers is that the waiting threads will be eventually notified. One of the main reasons for this non-deterministic behavior is the way suspended threads are chosen to run by the JVM, which is arbitrary. In addition however, locks in java implement a non-fair locking policy which permits thread barging. This simply means that it is permissible for new lock requests to jump ahead of the lock's wait set, it the lock is available at the time of the request. The justification behind this is that given substantial contention, there might be some (potentially significant) delay before choosing and resuming a suspended thread in the wait set and the time it actually runs. Any incoming lock request from a thread could therefore utilize this time delay to immediately run, in the hope that it will have released the lock by the time the resumed thread is ready to run. For example consider the following sequence of events:
Thread A that previously has acquired monitor X calls notify()
Thread B waiting on monitor X has chosen to be suspended (arbitrarily).
Thread C tries to acquire monitor X, sees that it is available and acquires it.
Thread C runs (despite thread B is currently in the process of being resumed)
Thread C finishes execution and releases monitor X, just before thread B is actually run.
Thread B is ready to run so it acquires the lock and starts execution.
It should be evident that between step 2 and 6 there exists some time interval where no threads are actually using the lock. Thread C barges in and utilizes the time interval as an optimization. The downside of this of course is the risk of not releasing the lock at the time thread B is ready to run, which at that time thread B will notice that the lock is unavailable and will enter the wait set again. Statistically however it can be proven that non-fair locking offers better performance in most situations.
As an aside note, you could use fair locks where waiting threads are resumed in the order they acquired the lock, but in practice this offers worse performance. Read more about this here: http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/locks/ReentrantLock.html
I hope this answers your question.
No it's not true. When a thread invokes notify, one waiting thread is awakened (if such a thread exists, otherwise notification is lost). Probably your colleague had in mind "spurious notify", which can awake a thread when in fact no other thread invoked notify or notifyAll. To filter "spurious notify", each notify invocation should accompanied with some change in monitored object state, and the waiting threads should check that state:
synchronized void up() {
counter++;
notify();
}
synchronized void down() {
while (counter==0) {
wait();
}
counter--;
}
Note checking state in down() is done before call to wait(), as it could be changed before the invocation and the notification is lost. In other words, the real information is passed with object's state, and wait/notify only help to avoid polling. Never rely on notifications without changing an object's state.
some comments here have confused me! I thought I knew this and god knows I've written a ton of MT code, but its been a while and so....
FWIK notify/notifyall
notify: one thread is selected from the waitset and moved to the entryset to acquire monitor lock
notifyall : all threads are "notified" -
are they all moved to the entryset ?
Does this mean that they will all reacquire the lock as the lock holder releases it ? no more notify necessary ?
-all answers are just rephrasing what I've read elsewhere. I understand that only one of them can acquire the lock etc., my question this: once a thread is notified, it gets to wait on the monitor. right ? so need not be notified again if the thread holding the lock calls notify
For a precise specification of what happens, refer to the JLS section 17.8.2.
When a thread performs a notifyAll on a lock object, all other threads that are currently waiting are removed from the lock's waitset; i.e. they become runable. Each one then tries to reacquire the lock, and when that succeeds it returns from the wait(...) call.
Of course, the threads only succeed in acquiring the lock one at a time, and there are no guarantees of fairness. But eventually, they will all have acquired the lock.
Notification of waiting threads occur when notifyAll is called. All the waiting threads are removed from the object's wait set. Only one of the thread in the wait set is selected, of which there is no guarantee as to which thread is selected
The langspec has a section on Thread Notification
When notifyAll is called, all threads waiting on that lock are woken up, and one of them acquires the lock. The rest goes back to wait.
This may sound like notifyAll is a waste of resources, but AFAIR there used to be special tricky cases when calling notify may wake up the wrong thread, which can not handle the situation, resulting in a deadlock. Therefore it was recommended to use notifyAll always.
Since Java5 it is rarely necessary to worry about things like this, because the new concurrency utilities almost always handle these tasks better than wait and notify*, rendering them almost obsolete. The typical use of wait and notify* is in blocking queues, and now we have several ready made implementations of these - such as LinkedBlockingQueue and PriorityBlockingQueue - available in the class library.