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Thread basics

Thread basics. A computer process. Every time a program is executed a process is created It is managed via a data structure that keeps all things memory related Global address space File-handle table Heap data. A computer process. When swapping processes for execution:

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Thread basics

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  1. Thread basics

  2. A computer process • Every time a program is executed a process is created • It is managed via a data structure that keeps all things memory related • Global address space • File-handle table • Heap data

  3. A computer process • When swapping processes for execution: • All things in that d.s. have to be brought to disk • Large chunks of memory contents • Process lots of memory • Swapping processes is expensive, lots of memory have to be moved around. • Swapping (context swapping) is measured in seconds

  4. Thread • Sequence of byte code instructions executed by the JVM; thus a thread consists of two things: • A sequence of code • Its execution • This sequence forms a path of code through a program sequence • No notion of objects or methods here • Sequences of instructions (threads) can overlap, also they can execute simultaneously on multiple threads

  5. Thread data structure • It contains data to keep track of this sequence and its execution • Contents of the registers • Position of execution in the instruction sequence • Run-time stack (LIFO) made up of activation records (created for methods to contain local variables, parameters, return pointer) • Life-time: threads are created and terminated.

  6. Swapping threads • OS typically swaps a thread • Push registers on the thread’s stack • Add thread d.s. to an OS list • Pull a different thread’s d.s off that list • Pop thread’s local stack into register set. • Thread swapping is measured in milliseconds. • Thus: threads are lightweight processes • A process is equivalent to Java’s virtual machine • A thread is process state, ie a virtual machine state.

  7. Threads and Processes • Important: threads share the memory allocated to the process. • Thus, given two threads: • May share instruction sequence • May share memory • As a consequence: • Two threads may share program’s data and methods to modify it • This is what makes threads hard to program. You need to carefully design for: • The commands executed by possibly multiple threads • The queries executed by possibly multiple threads • The time when that execution must occur. • How to design threaded programming: • By synchronizing (locking) the actions caused by each thread that shares data with other threads.

  8. Managing threads • If a thread manipulates data that is not shared by any other thread, no need to manage this sequence of execution, no need for synchronization. • Example of such case. Given two threads, each has its own stack: • Two threads do not share methods local variables and parameters. • If a method does not access any heap data ( thus no access to objects and their fields), it can be executed simultaneously by multiple threads without explicit synchronization.

  9. Multi-threaded programming • Modern OS allow multiple threads to be running concurrently within a process. • When JVM is started • A new process is created • Many threads can be created (spawned) in it. • One thread spawned is the thread having as execution sequence given by the “main”.

  10. Default threads in JVM • The “main” thread: • Spawned by the JVM when the process is created. • Beginning sequence is given by the main(). • Executes all statements in main() • Dies when main() completes execution.

  11. Default threads in JVM • The “garbage collection” thread: • Spawned by the JVM when the process is created. • The sequence of code it executes is the code written to discard objects and reclaim memory. • Thus any Java program has at least two threads executing: we say that a Java program runs in a multi-threaded environment.

  12. Default threads in JVM • If the program contains a GUI interface a new thread is created: The “event-handling” thread: The sequence of instructions it executes is • the code associated with listeners. • Events coming from the OS. • This means that if this thread is executing code of a listener, any mouse click or key press is put on hold until this thread terminates what is doing. • As a consequence we have an unresponsive user interface: one that appears to hang.

  13. Thread safety • The phrase “thread safe” is used to describe a method that can run safely in a multi-threaded environment • Accesses process-level data (shared memory by several processes) in a safe and efficient way. • The goal in designing multi-threaded programs is to achieve thread safety among other properties. • This is usually a hard goal to achieve, one you must design up-front about, and one whose bugs are not only hard to pin-point but difficult to remove.

  14. Why multi-threaded programming? • Better interaction with user • Simulation of simultaneous activities in one processor machine • Exploitation of multi-processor machines • Execution of computation code while doing I/o. Specially useful in network programming. • Allow multiple user to execution same code simultaneously . Specially useful in network programming. • Better Object oriented design.

  15. Why understanding and using threads • All nontrivial applications for the Java platform are multithreaded, whether you like it or not. • It's not ok to have an unresponsive UI. • It’s not ok for a server to reject requests.

  16. When multi-threaded programming might not be good • Threads carry overhead both in space and time. • Thread management. • All this cost must be considered when considering designing for threads.

  17. Thread behavior is platformdependent! • You need to use the OS threading system to get parallelism (vs. concurrency) • Different operating systems use different threading models (more in a moment). • Behavior often based on timing. • Multi-threaded apps can be slower than single-threaded apps (but be better organized)

  18. Priorities • The Java™ programming language has 10 levels • The Solaris™ OS has 2 31 levels • NT™ offers 5 (sliding) levels within 5 "priority classes." • NT priorities change by magic. • After certain (unspecified) I/O operations priority is boosted(by an indeterminate amount) for some (unspecified) time. • Stick to Thread.MAX_PRIORITY, • Thread.NORM_PRIORITY,Thread.MIN_PRIORITY)

  19. Threading models • Cooperative (Windows 3.1) • A Thread must voluntarily relinquish control of the CPU. • Fast context swap, but hard to program and can’t leveragemultiple processors. • Preemptive (NT) • Control is taken away from the thread at effectively random times. • Slower context swap, but easier to program and multiple threads can run on multiple processors. • Hybrid (Solaris™ OS, Posix, HPUX, Etc.) • Simultaneous cooperative and preemptive models are supported.

  20. Do not assume a particular environment • Assume both of these rules, all the time: • A thread can prevent other threads from running if it doesn't occasionally yield • by calling yield(), performing a blocking I/O operation, etc. • A thread can be preempted at any time by another thread • even by one that appears to be lower priority than the current one.

  21. Conclusion • Easy to start using threads, VERY tough to master. • Designing classes that contain methods that are thread safe is a challenge. • Read: javadoc for the class java.lang.Thread

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