On 6/15/07, Xiao-Feng Li <[EMAIL PROTECTED]> wrote:
Very interesting study. This situation happens not only here but also finalizer threads shutdown. We have test case that creates infinite loop execution in a finalizer (or waiting on a lost socket), requiring the system can shutdown correctly by sort of figuring out this situation and not waiting for the (dead) finalizer's finish. At the same time, we have test case that lets the finalizer to run lots of heavy duty work, and requiring the system to figure out this situation and waiting for the finalizer's finish. In GCv5, we solved the problem (or passed the tests anyway) by letting the system to timed wait on the finalizers. If at the timeout event we detect there is at least one finalizer is executed, we will loop back timed waiting again, since in this case it means the finalizers are still making progress. If at a timeout event we find the finalizers number is unchanged, we decide the finalizers are dead and will go on to exit.
For the finalizers that we want to allow to complete, we can reasonably expect that they will all end at some point( except for those in error situations like infinite loops ). For shutdown, there is no contract like runFinalizersOnExit etc. that requires us to allow all the daemon threads to finish. So we attach a callback that fires at the next thread safepoint and expect the thread to correctly release all locks, memory etc. resources before exiting. This is hard to do( eg., in our implementation, at the next safepoint callback, we just exit ). This means that currently every time a thread exits and joins successfully, we are sort of increasing the probability that some other thread(s) would be hung. So I am not sure how to converge using an interative approach. I can try a couple of ways. That's why we need the realistic scenarios.
The problem is, we don't know which timeout value is reasonable, 1ms or 1s. In this case, I personally think a bigger value makes more sense. Since in our case, the timed wait doesn't need to wait for timeout, it can also be waken up by the finalizers once they are finished, so a longer timeout value does not impact the performance normally. I guess this is the same case for the thread joining timed wait?
It is a little different ( as above ), in the timeout we are trying to guess when the running thread's next safepoint will be taken. The blocked threads of course cannot execute the callbacks. In theory if all the threads that are running, correctly release resources and exit at the next safepoint, the blocked threads should become unblocked and also exit correctly unless there are deadlocks already. So some form of iteration could help, but for this the safepoint callback has to be really good.
Thanks, xiaofeng
