Use of the Watchdog Timer in Java

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Example Use of the Watchdog Timer
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Listing 103 illustrates the use of the watchdog timer Listing 103 Watchdog
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import javaioIOException; import comdalsemisystemTINIOS; class Watchdog { boolean feedDog; int interval; // Create a thread for timer maintainence Thread feeder = new Thread(new Runnable() { public void run() { while (feedDog) { try { Threadsleep(interval); } catch (InterruptedException ie) {} Systemoutprintln("Feeding the dog!"); TINIOSfeedWatchdog(); } } });
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Accessing System Resources
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Watchdog(int timeout, int frequency) { // # of milliseconds between watchdog feedings interval = timeout / frequency; feedDog = true; feederstart(); // Set watchdog timeout value, this also starts the timer TINIOSsetWatchdogTimeout(timeout); } public static void main(String[] args) throws IOException { // Set the timeout for 8 seconds and knock back the // timer 2 times during each timeout period (ie every // 4 seconds) Watchdog wd = new Watchdog(8000, 2); Systemoutprintln("Hit <ENTER> and die!!!"); int c = Systeminread(); Systemoutprintln("Shutting down Watchdog thread"); // Allow Feeder thread to fall out of its run loop wdfeedDog = false; // Now we re just waiting to die! while (true) { Systemoutprintln("Still breathing "); try { Threadsleep(2000); } catch (InterruptedException ie) {} } } }
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Watchdog creates a thread to feed the watchdog, feeder, that keeps the system from resetting The watchdog time-out is set to 8 seconds As soon as the setWatchdogTimeout method is invoked, the timer starts running The timer maintenance thread resets the watchdog timer twice per time-out period So every 4 seconds feeder wakes up, knocks back the watchdog, and goes back to sleep As long as the feedDog boolean remains true, feeder will keep the system from resetting The program s other thread of execution, the primordial thread, blocks waiting for user input Once user input has been received, the primordial thread sets the feedDog boolean to false The next time feeder wakes up, it detects that feedDog is false and falls out of the run loop At this point, there is nothing running in the system that can stop the watchdog timer from expiring From that point, the system is 8 seconds from a hard reset Watchdog should be run from a serial session to view the boot progress messages just shown Also, since a watchdog reset is abrupt, the OS doesn t have any time to perform an orderly system shutdown, closing network connections and open les So, before running Watchdog, be sure to exit any Telnet or FTP sessions, and kill all processes other than slush and the garbage collector Here is a sample output of Watchdog
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The External Interrupt
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TINI /> java Watchdogtini Hit <ENTER> and die!!! Feeding the dog! Feeding the dog! Feeding the dog! Shutting down Watchdog thread Still breathing Still breathing Still breathing Still breathing ----> TINI Boot <---TINI OS 102 API Version 8009
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Here we can see that the watchdog timer was knocked back three times before user input was received So the application ran peacefully for about 12 seconds At the point user input was received, the warning message Shutting down Watchdog thread was displayed, and the in nite loop in the primordial thread began executing Somewhere between the fourth and fth iteration of the would be in nite loop, the watchdog timer expired, rebooting the system
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The watchdog is a very powerful and important tool in helping to ensure reliable operation for systems that use TINI technology However, since a watchdog reset is a very abrupt action, it should be used only as a last resort to regain control of a completely unresponsive system If an application is executing well enough to detect fatal errors, it should use the reboot method, provided in TINIOS, instead of forcing a watchdog reset To avoid extraneous resets, use long time-out values, and feed the watchdog at least a couple of times per time-out interval
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