Outline �� Please check the web page about your recitation group � s meeting place. � First group meeting: this Thursday (13:15) � Terminology � Exceptions � Control flow -- the order of execution � � “throw” and “catch” � – 3 :: 2 – � 0024 Spring 2010 �
Terminology � � � Program -> sequence of operations � � � Translation is done by the compiler � � � We may later see exactly what operations are executed � � � We are for now interested in atomic operations � – 3 :: 3 – � 0024 Spring 2010 �
Java model � – 3 :: 4 – � 0024 Spring 2010 �
Java rules � Simple � “print" – 3 :: 5 – � 0024 Spring 2010 �
Java rules � – 3 :: 6 – � 0024 Spring 2010 �
Java model � – 3 :: 7 – � 0024 Spring 2010 �
Simple Thread class � class TwoThreads { public static void main(String args[]) { SimpleThread st1 = new SimpleThread(); SimpleThread st2 = new SimpleThread(); st1.start(); st2.start(); } /* main */ } – 3 :: 8 – � 0024 Spring 2010 �
Simple Thread class, part 2 � class SimpleThread extends Thread { public void run() { System.out.println(“Started.”); } /* run */ } – 3 :: 9 – � 0024 Spring 2010 �
Variant � class TwoThreads_2 { public static void main(String args[]) { Thread t1 = new Thread(new MyRunnable()); Thread t2 = new Thread(new MyRunnable()); t1.start(); t2.start(); } /* main */ } – 3 :: 10 – � 0024 Spring 2010 �
Variant, part 2 � class MyRunnable implements Runnable { public void run() { System.out.println(“Started.”); } /* run */ } – 3 :: 11 – � 0024 Spring 2010 �
Atomic operations � An activity is called atomic if it completes without the possibility of interruption � int j, k; j = k + 1; � Is this (Java) statement executed atomically? � int j, k; j = j + 1; � Is this (Java) statement executed atomically? � – 3 :: 12 – � 0024 Spring 2010 �
Atomic operations � int j, k; j = j++; � Is this (Java) statement executed atomically? � Who decides what operations are done atomically? � � Ideally, the language designers have specified exactly what operations must be executed atomically. � – 3 :: 13 – � 0024 Spring 2010 �
Atomic operations � For now, we accept that reading or writing an int value is done atomically. � And nothing else. � � � Not completely true. All 32-bit quantities must be written atomically. � � � Some details are missing. � – 3 :: 14 – � 0024 Spring 2010 �
Primitive data types in Java � Java provides a rich set of primitive types � We � ve seen int. Variables, stored in 32 bits. � int j, k; There are “long int” and “double” -- both require 64 bits of storage. � – 3 :: 15 – � 0024 Spring 2010 �
Machine instruction not atomic � (Normal) machine instruction are not atomic � � � Several operations � � � E.g., addl 4(%ebp), %eax // add register and memory � � � Compute effective address (4+ebp) � � � Get value from memory � � � Get value from %eax � � � Add � � � Update %eax � � � Interruption possible after each step � � � Possibility of interruption implies that the machine instruction does not execute atomically. � – 3 :: 16 – � 0024 Spring 2010 �
Why looking at details? � We � ll stay at as high a level as possible but sometimes we must look “under the hood” � � � Most of the time source statements (or even methods) are a good abstraction level � � � When we want to understand parallel execution then we must understand which operations are executed atomically. � � � We � ll later see how we can ensure the atomic execution of sequences of operations (even methods) � – 3 :: 17 – � 0024 Spring 2010 �
Terminology, continued � � � Thread � � � Model of execution of a sequence of operations � � � “Program counter” (PC) + state � � � Program counter - tells us what operation to do next � � � State = local variables (sometimes in registers) � � � Address (data) space � � � Sequence of memory locations � � � Abstract memory cells � � � No information about access time � � � All memory locations have the same protection properties � � � Access rights, kind of address space (read-only, execute, ….) � � � Threads work with one (or more) address spaces � � � Each thread has a private address space for its private data � � � Some of the address spaces may be shared among threads � – 3 :: 18 – � 0024 Spring 2010 �
Terminology, continued � � � Process consists of one (or more) threads and one (or more address spaces) � � � Job: collection of processes � � � (Essential) Operating System (OS) services � � � Thread management � � � Creation, execution, interruption, termination, … � � � Address space management � � � Creation, mapping to real storage, setting/revoking access rights, space reclamation, … � � � Process management � � � Inter-process communication (IPC) � � � Threads communicate via shared address space(s) � – 3 :: 19 – � 0024 Spring 2010 �
Threads and processes � A simple model (but good for many scenarios) � � � 1 thread (“init thread” -- started by OS) � � � 1 address space for private data (i.e. stack) (for init thread) � � � 1 address space for thread (private) instructions � � � 1 address space for shared libraries � � � 1 address space for object instances � A slightly more sophisticated setup � � � 1 thread (“init thread”) -- started by OS � � � 1 .. N-1 threads started by init thread � � � N address spaces for thread-private data � � � N address spaces for thread (private) instructions � � � 1 address space for shared libraries � � � 1 address space for object instances � OS may not use (full) address space but just use different segments � – 3 :: 20 – � 0024 Spring 2010 �
Thread � � � Thread � � � Model of execution of a sequence of operations � � � “Program counter” (PC) + state � � � Program counter - tells us what operation to do next � � � State = local variables � class T1 { � public static void main(String args[]) { int k; k = 0; k = k + 1; } } (at least) 4 Operations: update k, read k, add one, update k � – 3 :: 21 – � 0024 Spring 2010 �
Detour: static methods � class S1 { public int static foo () { return 1; } } class S2 { public void static main(String args[ ]) { System.out.println(S1.foo()); } } � Static methods can be invoked with class name. � � � You don � t need an object instance � – 3 :: 22 – � 0024 Spring 2010 �
Why bother? � Java knows constructors -- special method to produce an instance of a given class. � � � Great if you need one instance. � � � What if you need two instances. Or two instances from different classes. � � � Constructor(s) won � t do the job. � Static methods help in such a situation. � � � Can impose rules on the usage/construction of object instances. � � � Also useful at the start of a program -- when there are no objects yet. � – 3 :: 23 – � 0024 Spring 2010 �
Example revisited � class S1 { public int foo () { /* not static */ return 1; } } class S2 { public void static main(String args[ ]) { int j; S1 sref = new S1(); j = sref.foo() System.out.println(j); } } � – 3 :: 24 – � 0024 Spring 2010 �
Comments � Default constructor -- S1(); � Reference variable to instance of S1 -- sref � “new” operator with constructor X() yields a reference to an instance of class X. � “j” and “sref” are local variables (local to method main) � – 3 :: 25 – � 0024 Spring 2010 �
Operations in example � 1 (or more) to get reference to instance of S1 � � � Plus whatever it takes to produce an instance of S1 � 1 (or more) to access sref � 1 (or more) to invoke foo() � 1 (or more) to get the result from foo() � 1 to update j � 1 (or more) to access System � 1 (or more) to access field out � 1 (or more) to invoke println() � 1 (or more) to return � – 3 :: 26 – � 0024 Spring 2010 �
More comments � Our goal is not to count operations. � Want to understand that many operations are not atomic and that a thread can be interrupted often. � – 3 :: 27 – � 0024 Spring 2010 �
Terminology, continued � � � Address (data) space � � � Sequence of memory locations � � � Abstract memory cells � � � Can store values � » � Don � t worry for now about details � � � No information about access time � � � All memory locations have the same protection properties � � � Access rights, kind of address space (read-only, execute, ….) � � � Not important for us right now -- just for the record � public void static main(String args[ ]) { int j; S1 sref … } Space for j and sref � – 3 :: 28 – � 0024 Spring 2010 �
Data space � S1 sref = new S1(); System will find space to store instance of S1. � sref can be stored, passed as an argument (to a method), or copied. � No other manipulation is allowed. � – 3 :: 29 – � 0024 Spring 2010 �
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