THE MYSTERY OF THE 20GB LOG FILE. Instructor: Prasun Dewan (FB 150,
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1 THE MYSTERY OF THE 20GB LOG FILE Instructor: Prasun Dewan (FB 150,
2 PROBLEM? The server the grader is running on ran out of disk space so I'm currently compressing the 20GB of debug log files so there is space again. I'll post when it is done. 2
3 GRADER AT FAULT? Timeout the solution? Timeout is > 10 minutes A polling process can write lots of data in 10 minutes 3
4 POLLING SELECT A series of select called and select unblocked calls They occur after an exception 4
5 CONNECT CONCURRENT MODIFICATION java.util.concurrentmodificationexception at java.util.arraylist$itr.checkforcomodification(arraylist.java:909) at java.util.arraylist$itr.next(arraylist.java:859) at inputport.aconnectregistrarandnotifier.notifyconnect(aconnectregistrarandnotifier.java:29) at inputport.datacomm.simplex.buffer.agenericsimplexbufferclientinputport.notifyconnect(agenericsimplexbufferclientinputport.java:158) at inputport.datacomm.simplex.buffer.agenericsimplexbufferclientinputport.connected(agenericsimplexbufferclientinputport.java:187) at inputport.datacomm.simplex.buffer.nio.anniosimplexbufferclientinputportdriver.connected(anniosimplexbufferclientinputportdriver.java:89 at inputport.datacomm.duplex.buffer.nio.annioduplexbufferclientinputportdriver.connected(annioduplexbufferclientinputportdriver.java:24) at inputport.nio.manager.commands.classes.aconnectcommand.dogiveresponse(aconnectcommand.java:129) at inputport.nio.manager.commands.classes.aconnectcommand.processselectedoperation(aconnectcommand.java:156) at inputport.nio.manager.aselectionmanager.processselectedoperation(aselectionmanager.java:76) at inputport.nio.manager.aselectionmanager.run(aselectionmanager.java:111) at java.lang.thread.run(thread.java:748) 5
6 ACONNECTCOMMAND: TAKE 1 void dogivereponse() { SelectionKey selectionkey = socketchannel.keyfor(selectionmanager.getselector()); SocketChannelConnected.newCase(this, socketchannel, listeners); Tracer.info(this, "Registering read op for:" + socketchannel); for (SocketChannelConnectListener listener : listeners) listener.connected(socketchannel); changeinterestops(selectionkey); Listener throws exception Terminates method execution changeinterestops() not called to remove connect Connect is always enabled, causing select() unblocking 6
7 ACONNECTCOMMAND: TAKE 2 void dogivereponse() { SelectionKey selectionkey = socketchannel.keyfor(selectionmanager.getselector()); SocketChannelConnected.newCase(this, socketchannel, listeners); Tracer.info(this, "Registering read op for:" + socketchannel); changeinterestops(selectionkey); for (SocketChannelConnectListener listener : listeners) listener.connected(socketchannel); changeinterestops not hostage to listener code But calling method could get terminated prematurely 7
8 ACONNECTCOMMAND: TAKE 3 void dogivereponse() { SelectionKey selectionkey = socketchannel.keyfor(selectionmanager.getselector()); SocketChannelConnected.newCase(this, socketchannel, listeners); Tracer.info(this, "Registering read op for:" + socketchannel); changeinterestops(selectionkey); try { for (SocketChannelConnectListener listener : listeners) listener.connected(socketchannel); catch (Exception e) { e.printstacktrace(); Should calling method decide on what to do? Caller (lower-level layer) does not depend on or know about upcall and thus can ignore the exception and simply a print the message One listener hostage to code of another 8
9 ACONNECTCOMMAND: TAKE 4 void dogivereponse() { SelectionKey selectionkey = socketchannel.keyfor(selectionmanager.getselector()); SocketChannelConnected.newCase(this, socketchannel, listeners); Tracer.info(this, "Registering read op for:" + socketchannel); changeinterestops(selectionkey); for (SocketChannelConnectListener listener : listeners) { try { listener.connected(socketchannel); catch (Exception e) { e.printstacktrace(); Every upcall should be in a try-catch block (with a timer perhaps if thread creation is not an issue) 9
10 CONNECT CONCURRENT MODIFICATION java.util.concurrentmodificationexception at java.util.arraylist$itr.checkforcomodification(arraylist.java:909) at java.util.arraylist$itr.next(arraylist.java:859) at inputport.aconnectregistrarandnotifier.notifyconnect(aconnectregistrarandnotifier.java:29) at inputport.datacomm.simplex.buffer.agenericsimplexbufferclientinputport.notifyconnect(agenericsimplexbufferclientinputport.java:158) at inputport.datacomm.simplex.buffer.agenericsimplexbufferclientinputport.connected(agenericsimplexbufferclientinputport.java:187) at inputport.datacomm.simplex.buffer.nio.anniosimplexbufferclientinputportdriver.connected(anniosimplexbufferclientinputportdriver.java:89 at inputport.datacomm.duplex.buffer.nio.annioduplexbufferclientinputportdriver.connected(annioduplexbufferclientinputportdriver.java:24) at inputport.nio.manager.commands.classes.aconnectcommand.dogiveresponse(aconnectcommand.java:129) at inputport.nio.manager.commands.classes.aconnectcommand.processselectedoperation(aconnectcommand.java:156) at inputport.nio.manager.aselectionmanager.processselectedoperation(aselectionmanager.java:76) at inputport.nio.manager.aselectionmanager.run(aselectionmanager.java:111) at java.lang.thread.run(thread.java:748) But the exception occurred only in grader run It occurred in a higher level observable 10
11 ACONNECTNOTIFIERANDREGISTRAR public class AConnectRegistrarAndNotifier implements ConnectionRegistrarAndNotifier List<ConnectionListener> portconnectlisteners = new public void addconnectionlistener(connectionlistener portconnectlistener) { if (portconnectlisteners.contains(portconnectlistener)) { System.out.println ("Ignoring duplicate connect listnener"); return; public synchronized void notifyconnect (String remoteend, ConnectionType aconnectiontype) { Tracer.info(this, "Notifying connect to:" + remoteend); for (ConnectionListener portconnectlistener:portconnectlisteners) portconnectlistener.connected(remoteend, aconnectiontype); addconnectionlistener() not synchronized! Wish you could declare a class to be synchronized all public methods synchronized 11
12 ACONNECTNOTIFIERANDREGISTRAR: TAKE 2 public class AConnectRegistrarAndNotifier implements ConnectionRegistrarAndNotifier private List<ConnectionListener> portconnectlisteners = new public synchronized void addconnectionlistener(connectionlistener portconnectlistener) { if (portconnectlisteners.contains(portconnectlistener)) { System.out.println ("Ignoring duplicate connect listnener"); return; public synchronized void notifyconnect (String remoteend, ConnectionType aconnectiontype) { Tracer.info(this, "Notifying connect to:" + remoteend); for (ConnectionListener portconnectlistener:portconnectlisteners) portconnectlistener.connected(remoteend, aconnectiontype); Do not want subclass to allow concurrent modification Should provide synchronized protected getters and setters 12
13 MORAL With every mistake, we must surely be learning We help others by catching their mistakes! 13
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