Processes - outline

1. Processes - outline • Process & thread basics • Multi-threaded clients • Server basics • Server design issues • Code Migration Distributed Systems - Comp 655

2. If you remember one thing… • Threads execute • Processes don’t • A thread owns • Id • Program counter • Scheduling priority • Stack • Local storage • A process owns • Id • Thread table • Page tables • Authenticated identity • IO resources Distributed Systems - Comp 655

3. Threads in Java Threads are objects • … like almost everything in Java • To make one, • Define a class that • Extends java.lang.Thread, or • Implements java.lang.Runnable • At runtime, create and initialize an instance • Call the start method: • mythread.start(); or • new Thread(myrunnable).start(); Distributed Systems - Comp 655

4. Java thread example public class Worker implements Runnable { private long myInterval; private int myReps; public Worker(long interval, int repetitions) { myInterval = interval; myReps = repetitions; } public void run() { String me = Thread.currentThread().getName(); for( int repsDone=1; repsDone<=myReps; repsDone++) { System.out.println(me+" ["+(repsDone)+"]"); try{ Thread.sleep(myInterval); } catch(Exception e) {} } } } Distributed Systems - Comp 655

5. Starting the thread example Public static void main(String names[]) { for( int i=0; i<names.length; i++ ) { Worker w = new Worker(1000,7); Thread t = new Thread(w); t.setName(names[i]); t.start(); } } Distributed Systems - Comp 655

6. Possible output java ThreadExample hey whats up hey [1] whats [1] up [1] hey [2] whats [2] up [2] … Distributed Systems - Comp 655

7. Why threads? • Sometimes you have to block • usually for IO • Context switching (for processes) is expensive • Why is context switching expensive? • Memory management • Why do memory management? • Concurrency transparency Distributed Systems - Comp 655

8. The most expensive operations Context switching Distributed Systems - Comp 655

9. Lightweight processes • User-level thread packages can do context switching for threads MUCH less expensively • BUT, if one thread makes a blocking system call, all threads block • Solutions: • Lightweight processes • Scheduler activations Distributed Systems - Comp 655

10. Lightweight processes process thread table • Most thread switching is done in user space • LWP switching occurs only when a thread makes a blocking system call • Scheduling routine, thread table, and thread table mutexes are shared by LWPs and the user-level thread package Distributed Systems - Comp 655

11. User threads multiplexed

12. Processes - outline • Process & thread basics • Multi-threaded clients • Server basics • Server design issues • Code Migration Distributed Systems - Comp 655

13. Multi-threaded clients • Typical web browser • Main thread gets the page you requested • Additional threads are dispatched to fetch images, frames, scripts, etc • User can start viewing before all the data has been received • Parallelism can shorten elapsed download time when web server is replicated Distributed Systems - Comp 655

14. Processes - outline • Process & thread basics • Multi-threaded clients • Server basics • Server design issues • Code Migration Distributed Systems - Comp 655

15. Multi-threaded server Distributed Systems - Comp 655

16. Multi-threaded server – plan 1 Distributed Systems - Comp 655

17. Threads may need synchronization • Why synchronize? (preview of week 6): concurrent access to shared resources • Concurrent writers can corrupt data • Concurrent reader and writer can confuse the reader • Example: workers de-queue and dispatcher en-queues • All operating systems (and some languages) provide synchronization primitives • When you design a multi-threaded program you have to have a synchronization plan/design/architecture Distributed Systems - Comp 655

18. Multi-threaded server – plan 1 synchronization Distributed Systems - Comp 655

19. Multi-threaded server – plan 2 Compared to plan 1, no synchronization, but more overhead for starting and stopping threads Distributed Systems - Comp 655

20. Processes - outline • Process & thread basics • Multi-threaded clients • Server basics • Server design issues • Code Migration Distributed Systems - Comp 655

21. Typical server design issues • How to find it • While minimizing use of well-known endpoints (why?) • How to interrupt it • Stateful vs stateless • Object activation policy Distributed Systems - Comp 655

22. The super-server Distributed Systems - Comp 655

23. The super-server: inetd Distributed Systems - Comp 655

24. Interrupting a server that is processing a long-running request • Major options • Abrupt termination • Out-of-band control messages (see below) • In-band control messages with priority • Response chunking with option to quit after each chunk Client Out-of-band control messages Server Regular (data) messages Distributed Systems - Comp 655

25. Stateful vs stateless servers • Stateless server forgets everything after each request has been processed. E.g. HTTP server • Stateful servers can have better performance and be less annoying BUT • At the cost (usually) of losing • Relocation transparency • Failure transparency Distributed Systems - Comp 655

26. Object creation Per request Fixed-size pool Dynamic pool Threading Single Fixed-size thread pool Dynamic thread pool Thread per object Object activation policy options An object adapter is an implementation of an activation policy. Distributed Systems - Comp 655

27. For example, new object on a new thread for each request For example, single object on a single thread handles all requests Object adapters in an object server and two activation policies Distributed Systems - Comp 655

28. Code Migration • What it is: moving programs across a network on demand Distributed Systems - Comp 655

29. Why migrate code? • Performance improvement • Flexibility • Simplified administration Distributed Systems - Comp 655