How to kill a specific thread from an array of threads

Question

Hi. I am creating an array of threads based on the number of records in a database. Each thread then polls an ipaddress and then sleeps for a time and then repolls again. I periodically check the database for any change in the number of hosts. If there are more hosts I start another thread. If there are less hosts I need to kill the specific thread that was monitoring that host. How do i kill the specific thread.

enter code here protected static void GetThreads()
    {
        Thread[] threads;
        do
        {
            dt = getIP_Poll_status();
            threads = new Thread[dt.Rows.Count];
            Console.WriteLine(dt.Rows.Count + " Threads");
            for (int i = 0; i < threads.Length; ++i)
            {
                string ip = dt.Rows[i][0].ToString();
                int sleep = Convert.ToInt32(dt.Rows[i][1].ToString());
                string status = dt.Rows[i][2].ToString();
                string host = dt.Rows[i][3].ToString();
                Hosts.Add(host);
                string port = dt.Rows[i][4].ToString();
                //Console.WriteLine("starting on " + ip + " delay  " + sleep+".current status "+status);
                threads[i] = new Thread(PollingThreadStart);
                threads[i].Start(new MyThreadParameters(ip, sleep, status, host, port));
                threads[i].Name = host;

            }
            Thread.Sleep(50000);
        }

        while (true);
    }

Answer 1

Given that most of your threads will spend the majority of their time doing nothing, your design might be better realised as a single thread that keeps a list of ip addresses and the time they're due to be polled next. Keep it sorted in order of next poll time.

Pseudocode:

What time does the next ip address need to be polled?
Sleep till then
Poll the address.
Update the poll time for that address to now + interval.
Resort the list
Repeat.

Whenever you have a DB update, update the list and then order the thread to re-evaluate when it needs to stop.

Answer 2

Killing threads forcibly is a bad idea. It can leave the system in an indeterminate state.

You should set a flag (in a thread-safe way) so that the thread will terminate itself appropriately next time it checks. See my threading article for more details and sample code.

I would add that using Sleep is almost always the wrong thing to do, by the way. You should use something which allows for a graceful wake-up, such as Monitor.Wait. That way when there are changes (e.g. the polling thread should die) something can wake the thread up if it's waiting, and it can notice the change immediately.

Answer 3

You don't specify the language you are targetting, but in general you use the same method regardless of the language. Simple use a shared variable that is used to signal the thread when it is time to stop running. The thread should periodically check the value and if it is set it will stop in a graceful fashion. Typically, this is the only safe method to stop a thread.

Answer 4

I would say:

1. Don't kill threads. Ask them to die, nicely (vie Events or some shared flag).
2. Be careful when creating exactly one thread per DB entry. This could mean that some unexpected DB activity where suddenly you have many rows translates into killing the OS with too many threads. Definitely have a limit on the number of threads.

Answer 5

You could send an interrupt signal to the thead you want to end. The thread that has been signalled would need to catch the ThreadInterruptedException that will be raised, and exit gracefully.

There are other, possibly better, ways to achieve what you want, but they are more complicated...

Answer 6

What you are trying to do is a bad idea because of the reasons mentioned above. However you can still give the thread a name which can be the host name. You may find out the thread by checking the name and kill it.