Question

I understand that a lost wake-up is when a thread misses a wake-up signal because it had not yet gone to sleep, but i dont exactly understand what this means. Could I a more in-depth explaination of lost wake-up?

Answer 1

In general, there maybe some scheduled wake-up signals that might be sent through some daemon jobs or cron jobs etc. This can be for various reasons, making sure that some process is properly carried out etc.

Now, if the thread has not yet gone to sleep, it will miss the wake-up call, as it'll be ignored by the operating system. Thus, now when it does go to sleep, it'll have to wait for the next scheduled wake-up call. This can be very long.

Take for an example, a multithreaded queue with a producer and two consumers. Say the consumer threads are waiting for the queue to fill up to pop an element out of the queue, both the threads are on sleep right now. Now, the producer thread pushes and item and sends a wake-up call to both the consumers, but only one woke up and by that time the producer thread took over due to scheduling, which leads to the second consumer thread never waking up. This is clearly a bug.

Comment in case of any doubts.