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.