Simple Blocking and Waiting

Question

hello,
I am studying concurrency, and was hoping some could confirm I have got this right so far...
1) When a thread waits() or sleeps(...), or sits around for an acquire()..they are all the same in terms of the CPU cycles...ie. none, they are not busy waiting?
2) Also, if I put a wait(time) at the start of a Critical Section, when the time expires, it will enter the subsequent Critical section regardless? I.e a notify would just mean it gets there sooner...
thanks
julian

Answer 1

vijaykandy wrote:
it makes no sense to put a wait(time) call in a while(condition) loop. either the condition will be met and the object notified, or the wait call will timeout, in which case you will just loop around and wait again. Maybe so. The poster who asked the question needed to understand wait(timeout)/notify() scenario hence the example.only, your example didn't actually help them understand why you would wait with a timeout, which is what i was pointing out. as dannyyates subsequently pointed out, in your example loop, the timeout was useless.
not to mention that you don't usually notify before you enter the critical section. you notify when you are leaving the critical section. also, you would set notAvailable to true when you enter the critical section. in short, that example is completely broken.It's not broken. If you think it is, can you provide an explanation, a problem scenario? Please don't confuse readers with blanket statements.
For any one interested - Java monitors are called "Signal and Continue" (search on google) because they can signal other threads and still continue to hold the lock. Besides, always remember that a.notify() must be called only when thread acquires a's lock; otherwise an exception is thrown. The problem in the example is not with the notify() but with where the condition is toggled. If there are threads that call methods on another class based on the condition (notAvailable), they might run into an exception condition because condition was toggled prematurely. Since the original question did not address this scenario, this example will suffice.thanks for the tutorial on wait/notify. but, i wasn't confused at how they worked, just at your example. actually, your explanation here is broken, because no other thread (assuming correct syncronization) would ever see the new value of notAvailable until the current thread exited the sync block. you are correct that it doesn't "technically" matter where the notify is located, however, there's no sense waking the other threads before you do the actual work, you're just wasting other thread resources. however, your followup example is more what i would expect in terms of normal sync handling, so i guess you get the general idea (other than the timeout).
again, my point was that your original example was not necessarily useful for understanding how sync worked. you made a couple of followup posts which seem to indicate that you basically know how it all, which leads me to wonder why you didn't include all that knowledge in your original example.
Edited by: jtahlborn on Apr 20, 2008 9:00 PM