Hi, I come across this question about multi task in operating system:let's say I use semaphore to control the multi task read/write toshared resource. When one thread or process does not get the lock, itsuspends itself, basically it calls sleep(). Then how is it awakened? I can think of 2 implementations: 1. semaphore maintains a list which has all the threads or processeswhich did not get the lock and once the lock is available, thesemaphore wakens them. 2. When the thread/process calls sleep(), it puts a timer, say,every 1ms, to waken itself to check the lock. What is the real solution then?

Hi,First of all ,the task which did not get the lock goes to pended stateand not delayed state as mentioned by you.How ever it may opt to pendfor a specific period of time(pending with timeout option).In thatcase if the task does not get lock with in specific period of time itwould return error and move out of pended state.Your implementation number 1 is the appropriate way to invoke ablocked task how ever may not be the only may be OS specific.In the case of pending with time out,once timeout happens timerexpires and theres no re running of it.All the above said are specific to corresponding OS even though can bethought of as generic.If you mention the OS you are referring to ,youmay get correct answer to your question.Regards,s.subbarayan

how is the thread/process awakened after it slept?

Oct 09, 2007 21 Replies

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Clifford Heath 18 years ago

That's what I meant by a ticket. I was assuming you were referring to the count of a counting semaphore. In this case, you need both. And you still need to search...

And no, 1000 threads isn't too many. Many server architectures support that many, and some have ten times more. It's a bit out of the embedded league though.

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ssubbarayan 18 years ago

Why do you think timer need to rerun once it has done a timeout by expiring?I dont believe its really needed to rerun timer when you are pending with a delay.If this is true theres no need for have a pending with a time delay.Try using semshow command in vxworks,it will display the tasks waiting for this semaphore.How else this would be possible if semaphore does not maintain the task list?How ever I am not aware about the way of implementation in other RTOS.This is my understanding. Regards, s.subbarayan

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