What causes spurious wakeup?

What causes spurious wakeup?

It’s called spurious because the thread has seemingly been awakened for no reason. But spurious wakeups don’t happen for no reason: they usually happen because, in between the time when the condition variable was signaled and when the waiting thread finally ran, another thread ran and changed the condition.

How do I stop spurious wakeup?

To avoid spurious wakeups, you should ensure that the predicate did not hold before the wakeup and does hold afterwards. You can use preconditions and monitor invariants to guarantee this, but if you do, be sure to document them clearly!

What is spurious wakeup in C++?

Spurious wakeup is a situation when a thread waiting on a condition variable is resumed (aka awoken) although the condition actually doesn’t hold.

How do condition variables avoid the lost wakeup problem?

To avoid lost-wakeups, the common advice is that all notifiers must hold the lock while updating their condition states. However, if you were to guard the WaitForMessage() call with work_to_do_lock you could prevent other signals from waking up the event loop.

How often do spurious Wakeups happen?

In busy periods, up to 20% of the wakeups are spurious (ie when it wakes there is nothing in the queue).

How do you wake up a thread?

Waking up Wait and Sleep We can wake the thread by calling either the notify() or notifyAll() methods on the monitor that is being waited on. Use notifyAll() instead of notify() when you want to wake all threads that are in the waiting state.

What does condition variable wait do?

std::condition_variable A condition variable is an object able to block the calling thread until notified to resume. It uses a unique_lock (over a mutex ) to lock the thread when one of its wait functions is called.

How do condition variables work?

Condition variables are synchronization primitives that enable threads to wait until a particular condition occurs. Condition variables support operations that “wake one” or “wake all” waiting threads. After a thread is woken, it re-acquires the lock it released when the thread entered the sleeping state.

What is lost wakeup problem?

A lost wake-up occurs when all of the following conditions are in effect: A thread calls pthread_cond_signal() or pthread_cond_broadcast() Another thread is between the test of the condition and the call to pthread_cond_wait() No threads are waiting. The signal has no effect, and therefore is lost.

What is Pthread_cond_signal?

The pthread_cond_signal() function wakes up at least one thread that is currently waiting on the condition variable specified by cond. If no threads are currently blocked on the condition variable, this call has no effect.

What is difference between sleep () and wait ()?

Java sleep() and wait() – Discussion The major difference is that wait() releases the lock or monitor while sleep() doesn’t releases the lock or monitor while waiting. wait() is used for inter-thread communication while sleep() is used to introduce pause on execution, generally. Thread.

When does a spurious wake up occur in Solaris?

In the Solaris implementation of condition variables, a spurious wakeup may occur without the condition being signaled if the process is signaled; the wait system call aborts and returns EINTR. The Linux pthread implementation of condition variables guarantees it will not do that.

What do you need to know about spurious wakeup?

Spurious wakeup describes a complication in the use of condition variables as provided by certain multithreading APIs such as POSIX Threads and the Windows API . Even after a condition variable appears to have been signaled from a waiting thread’s point of view, the condition that was awaited may still be false.

Why does pthread _ cond _ wait have spurious wakeups?

The more common scenario of a “spurious wakeup” is most likely the side-effect of a call to pthread_cond_broadcast(). Let’s say you have a pool of 5 threads, two wake up to the broadcast and do the work. The other three wake up and find the work has been done.

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