Describe the concept of lightweight process (LWP) and the advantages to using LWPs

1. Describe the concept of lightweight process (LWP) and the advantages to using LWPs • Lightweight process (LWP) lies in a hybrid form of user-level & kernel-level threads. LWP runs in the context of a single (Heavy-weight) process, and there can be several LWPs per process. In addition to having LWPs, a system also offers a user-level thread package. • The combination of (user-level) threads and LWPs works as follows: • The thread package has a single routine to schedule the next thread • When LWP is created, it is given its own stack and is instructed to execute the scheduling routine in search of a thread to execute • Thread table is shared by LWPs. Synchronization is realized by mutexes implemented entirely in user space. • Advantages to using LWPs in combination with user-level thread • Creating, destroying, and synchronizing threads is relatively cheap and involves on kernel intervention at all • A blocking system call will not suspend the entire process, provided that a process has enough LWPs. • There is no need for an application to know about the LWPs. All it sees are user level threads. • LWPs canbe easily used in multiprocessing environments by executing different LWPs on different CPUs

2. Having only a single lightweight process per process is not such a good idea. Why not? • See page 140-141 of the core book (Tanebaum 2002).

3. Give an example to show the advantages to using multithreaded Clients • See page 142 of the core book (Tanebaum 2002).

4. Compare the multithreaded file server based on dispatcher/worker model and a single threaded file server introduced during the lecture time; tell the advantages to using the multithreaded technique in server designing. • See page 143-144 of the core book (Tanebaum 2002).

5. Explain what is so called stateless server, and stateful server? Give examples • A stateless server does not keep information on the sate of its clients, and can change its own state without having to inform any client, e.g. A Web Server. • A stateful server does maintain information on its clients, e.g. a file server that allows a client to keep a local copy of a file. • See page 151 of the core book (Tanebaum 2002).

6. For better understanding the code migration, a process is used to be modelled as a three segments model. What are these three segments? • Code segment is the part that contains the set of instructions that make up the program that is being executed. • Resource segment contains references to external resources needed by the process, such as file, printers, devices, other processes, and so on. • Execution segment is used to store the current execution state of a process, consisting of private data, the stack, and the program counter.

7. For code migration in distributed system, explain what are the weak mobility model and the strong mobility model? What are the sender-initiated mobility and the receiver-initiated mobility? • Weak mobility model: In this model, it is possible to transfer only the code segment, along with perhaps some initialization data. Feature: a transferred program is always started from its initial state, e.g. Java applets. • Strong mobility model: Besides the code segment being transferred, the execution segment can be transferred as well. Feature: A running process can be stopped, subsequently moved to another machine, and then resume execution where it left off. • In sender-initiated migration, migration is initiated at the machine where the code currently resides or is being executed. • In receiver-initiated migration, the initiative for code migration is taken by the target machine

8. List all the alternatives for code migration.