Question

6) What are the benefits and the disadvantages of each of the following? Consider both the system level and he rograminer or nd asynchronous communication a. Synchronous and asynchronous communication b. Automatic and explicit buffering c. Send by copy and send by reference d. Fixed-sized and variable-sized messages(20 points) 7) Explain the role of the init process on UNIX and Linux systems in regard to process termination. (10 points)

Answer 1

6)--------------------------------

a)--------------------A benefit of synchronous communication is that it allows a rendezvous between the sender and receiver. A disadvantage of a blocking send is that a rendezvous may not be required and the message could be delivered asynchronously. As a result, message-passing systems often provide both forms of synchronization.

b)-------------------Automatic buffering provides a queue with indefinite length, thus ensuring the sender will never have to block while waiting to copy a message. There are no specifications on how automatic buffering will be provided; one scheme may reserve sufficiently large memory where much of the memory is wasted. Explicit buffering specifies how large the buffer is. In this situation, the sender may be blocked while waiting for available space in the queue. However, it is less likely that memory will be wasted with explicit buffering.

c)-------------------Send by copy does not allow the receiver to alter the state of the parameter; send by reference does allow it. A benefit of send by reference is that it allows the programmer to write a distributed version of a centralized application. Java’s RMI provides both; however, passing a parameter by reference requires declaring the parameter as a remote object as well

d)----------------------The implications of this are mostly related to buffering issues; with fixed-size messages, a buffer with a specific size can hold a known number of messages. The number of variable-sized messages that can be held by such a buffer is unknown. Consider how Windows 2000 handles this situation: with fixed-sized messages (anything < 256 bytes), the messages are copied from the address space of the sender to the address space of the receiving process. Larger messages (i.e. variable-sized messages) use shared memory to pass the message.

7)-----------------------When a process is terminated, it briefly moves to the zombie state and remains in that state until the parent invokes a call to wait(). When this occurs, the process id as well as entry in the process table are both released. However, if a parent does not invoke wait(), the child process remains a zombie as long as the parent remains alive. Once the parent process terminates, the init process becomes the new parent of the zombie. Periodically, the init process calls wait() which ultimately releases the pid and entry in the process table of the zombie process.