Networking, Java threads and synchronization. PRIS lecture 4 Fredrik Kilander

Transcription

1 Networking, Java threads and synchronization PRIS lecture 4 Fredrik Kilander

2 OSI Application Presentation Session Transport Network Data link Physical TCP/IP Application Transport Internet Host-to-network A. S. Tanenbaum, Computer Networks, 3rd Ed.

3 OSI Application Presentation Session Transport Network Data link Physical TCP/IP Application Middleware Transport Internet Host-to-network A. S. Tanenbaum, Computer Networks, 3rd Ed.

4 HTTP BitTorrent SMTP SSH DNS Application Protocols TCP UDP Transport IP Network Networks WAN MAN LAN WiFi 3G PPP Data link + physical Adapted from A. S. Tanenbaum, Computer Networks, 3rd Ed.

5 Application Remote Procedure Call Application SOAP Message SOAP XML Message XML HTTP Connection HTTP Transport Connection Transport Network Packet Network Network Data link Frame Data link Data link Physical Waves Physical Physical

6 Communication Layers, interfaces, and protocols in the OSI (Open Systems Interconnection) reference model. Divided into 7 layers each deals with one specific aspects of the communication

7 reliable Transport Transport unreliable Network Network reliable Data link Data link unreliable Physical Physical

8 Reliable data transmission Divide data into packets Add error-detection/correction to payload Add sequence numbers Add a timer for each packet sent Keep resending packets until they are ack d. Acknowledge received packets

9 Point-to-point connection Transport Transport Network Network Data link Data link Physical Wire or EM beam Physical

10 Common media, no longer point-to-point Transport Transport Transport Network Network Network Data link Data link Data link Physical Physical Physical Coaxial cable or radio channel (wave propagation medium)

11 Medium ACcess Layer OSI Application Presentation Session Transport Network Data link Physical MAC

12 MAC layer negotiates access to shared medium Transport Transport Transport Network Network Network Data link Data link Data link MAC MAC MAC Physical Physical Physical

13 Ethernet history thick cable

14 Ethernet history thinwire

15 Ethernet history CAT6/WiFi

16 Stations share the broadcast medium Only one station may send - all listen Ada Bea Cia Didi Eve MAC MAC MAC MAC MAC Transmissions are addressed to an interface (unicast) or to a group (multicast) or to all (broadcast)

17 Stations share the broadcast medium The MAC-address depends on the medium 00:1F:3B:BF:CA:35 Ada Bea Cia Didi Eve MAC MAC MAC MAC MAC Ethernet 48 bits vv:vv:vv:ss:ss:ss BlueTooth 48 bits NAP(16)UAP(8)LAP(24) NAP:Non-significant address portion LAP:Lower address portion UAP:Upper address portion

18 Stations share the broadcast medium Simultaneous broadcasts leads to collisions Ada Bea Cia Didi Eve MAC MAC MAC MAC MAC

19 Stations share the broadcast medium Simultaneous broadcasts leads to collisions Ada Bea Cia Didi Eve MAC MAC MAC MAC MAC Interference where frames overlap

20 OSI Application Presentation Session Transport Network Data link Physical TCP/IP Application Transport Internet Host-to-network A. S. Tanenbaum, Computer Networks, 3rd Ed.

21 Data network: routers and links

22 Network/Internet layer routing of packets Each router selects the best output line for the packet Packets may be Lost (router memory is full) Reordered (go separate paths) Delayed (queued)

23 OSI Application Presentation Session Transport Network Data link Physical TCP/IP Application Transport Internet Host-to-network A. S. Tanenbaum, Computer Networks, 3rd Ed.

24 Transport layer virtual connection Endpoints simulate a continuous stream of bytes TCP: packet sizes depend on delay Fixes lost and reordered packets

25 Multithreading Concurrent computing

26 Threads Threads allow parallel execution in one pgm Threading is a programming language construct

27 Threads Threads allow parallel execution in one pgm The main thread is issued by the operating system. It enters the main routine of the program. When it exits the program ends.

28 Threads Threads allow parallel execution in one pgm Other threads are issued from the main thread. They enter other routines of the same program. Several threads can enter the same routine. When the entry point is exited, the thread ends.

29 Threads Threads allow parallel execution in one pgm The main thread always enters the main program. int foo() { a = b; c = d;... void bar (int x) { x = u * b;... void main (String argv[]) { boolean o; double p;... Other threads execute with other routines as their main programs.

30 Threads Threads allow parallel execution in one pgm All threads share global variables. Threads do not share local variables because each thread has its own stack. int a = 0; boolean b; void foo() { int a;... void bar (int x) { float y;... void main (String argv[]) { a = 42;... Threads Threads can can be be implemented implemented in in several several ways: ways: Virtual Virtual threads threads by by interpreter interpreter Parallel Parallel processes processes from from OS OS Threads Threads supported supported by by OS OS

31 Mental models Single-threaded whole program Multi-threaded several small programs Where and how can and should threads interact in my program?

32 Surreptious multi-threading: Callback APIs GUI events Messaging systems Discovery systems Timers RPC server

33 Race condition Insert presentation here

34 Threads in Java A class that supports threading implements interface Runnable. Interface Runnable has one method: run(). This is a thread s entry point. import java.lang.runnable; import java.lang.thread; class MyServer implements Runnable { // In interface Runnable: public void run () {... A new thread is created just like any other object. It is given the object where to execute and told to start running. public void main (String argv[]) {... new Thread (this).start ();...

35 Threads in Java A class that supports threading implements interface Runnable. Interface Runnable has one method: run(). This is a thread s entry point. class class MyServer MyServer implements implements Runnable Runnable Thread Thread t t = = new new Thread Thread (this); (this); public public void void run() run() {... {... A new thread is created just like any other object. It is given the object where to execute and told to start running. Thread Thread t t public public void void start() start()

36 Threads in Java class class MyServer MyServer Thread Thread t t = = new new Thread Thread (new (new Worker()); Worker()); t.start t.start (); (); class class Worker Worker implements implements Runnable Runnable public public void void run() run() {... {... Thread Thread t t public public void void start() start()

37 Threads in Java In Java you must use method Runnable.run(). Where is the diversity? import java.lang.runnable; import java.lang.thread; class MyServer implements Runnable { int h; Programmatic or computed choice. Race condition on on variable h! h! // In interface Runnable: public void run () { switch (h) { case 1: foo(); break; case 2: bar(); break; public void main (String argv[]) {... h = 1; new Thread (this).start (); h = 2;

38 Threads in Java In Java you must use method Runnable.run(). Where is the diversity? Separate out methods into their own classes. import java.lang.runnable; class Foo implements Runnable { public void run () {... import java.lang.runnable; class Bar implements Runnable { public void run () {... import java.lang.runnable; import java.lang.thread; class MyServer { public void main (String argv[]) { new Thread (new Foo ()).start (); new Thread (new Bar ()).start ();

39 Threads in Java In Java you must use method Runnable.run(). Where is the diversity? import java.lang.runnable; import java.lang.thread; class MyServer { protected void dowork () {... public void main (String argv[]) {... new Thread (new Runnable () { public void run () { dowork (); ); Anonymous subclassing

40 Threads in Java Wait for a thread to die: Thread.join() import java.lang.runnable; import java.lang.thread; class MyServer { public void main (String argv[]) {... // Create thread t. Thread t = new Thread (new Foo ()).start ();... // Main thread waits for t to die. t.join ();...

41 Threads in Java Thread control sleep (long ms) suspend caller for at least ms milliseconds join (Thread t) suspend caller until other thread t has exited wait() suspend until notified (in a synchronized block) notify() release a wait()ing thread (in a synchronized block)

42 Synchronization in Java Threads that access common variables together can seriously mess up the state of the program. Synchronization is achieved by monitors. A monitor is a non-sharable entity associated with a Java object (choosen by the programmer). Synchronized code is locked by the object. A thread must have the monitor to be able to execute the synchronized code.

43 Synchronization in Java When a method is declared as synchronized the monitor is retrieved from the method s object. Potential inefficiency. public synchronized void enqueue() {... When a block of code is synchronized, any object s monitor can be used: synchronized (myqueue) {...

44 Synchronization in Java A thread that attempts to enter a synchronized method or block must wait in a queue for the monitor. When the monitor is released the thread continues to execute. While the thread is inside the synchronized section it can release the monitor and wait(). synchronized (myqueue) {... wait (); // Nothing to do... When some other thread has the monitor it can notify the waiting thread, giving back the monitor and allowing it to continue: synchronized (myqueue) {... notify (); // Work is ready for you...

45 Synchronization in Java Producer Producer thread thread Adds Adds items items to to the the queue queue myqueue myqueue myqueue myqueue Consumer Consumer thread thread Removes Removes items items from from the the queue queue myqueue myqueue head myqueue tail

46 Synchronization in Java Qmobj Producer Producer thread thread Adds Adds items items to to the the queue queue myqueue myqueue myqueue myqueue Consumer Consumer thread thread Removes Removes items items from from the the queue queue myqueue myqueue Producer is waiting for the monitor Producer asks for myqueue s monitor Producer receives myqueue s monitor Consumer asks for myqueue s monitor Synchronized code Producer enqueues items Producer returns myqueue s monitor Consumer receives myqueue s monitor Consumer is waiting for the monitor Consumer dequeues items Synchronized code Consumer returns myqueue s monitor

47 Synchronization in Java Qmobj Producer Producer thread thread Adds Adds items items to to the the queue queue myqueue myqueue myqueue myqueue Consumer Consumer thread thread Removes Removes items items from from the the queue queue myqueue myqueue Producer is waiting for the monitor Synchronized code Producer asks for myqueue s monitor Consumer asks for myqueue s monitor Producer receives myqueue s monitor Producer enqueues items Producer returns Both Both threads myqueue s threads are are monitor waiting waiting for for Consumer receives myqueue s monitor the the monitor: monitor: The The winner winner selection selection is is undefined undefined Consumer dequeues items Consumer returns myqueue s monitor Consumer is waiting for the monitor Synchronized code

48 Synchronization in Java A typical application of the wait()/notify() mechanism is a consumer producer pair with a queue between them. Access to the queue must be synchronized. While the queue is empty the consumer does not execute. synchronized (myqueue) { myqueue.enqueue (x); synchronized (qmobj) { notify (); myqueue M // In qmobj: for (;;) { while (!myqueue.isempty()) { synchronized (myqueue) { e = myqueue.dequeue (); synchronized (this) { wait ();

49 synchronized (myqueue) { myqueue.enqueue (x); synchronized (qmobj) { notify (); // In qmobj: myqueue M for (;;) { while (!myqueue.isempty()) { synchronized (myqueue) { e = myqueue.dequeue (); synchronized (this) { wait ();

50 Stuff we did not mention... Avoid Thread. stop() suspend() resume() (deprecated) Class Threadgroup : handle threads as a unit Class ThreadLocal : thread-specific data Thread intercommunication with pipes. Thread priorities.

51 The End