Книга: Distributed operating systems



Let us now briefly compare the various systems we have examined. IVY just tries to mimic a multiprocessor by doing paging over the network instead of to a disk. It offers a familiar memory model — sequential consistency, and can run existing multiprocessor programs without modification. The only problem is the performance.

Munin and Midway try to improve the performance by requiring the programmer to mark those variables that are shared and by using weaker consistency models. Munin is based on release consistency, and on every release transmits all modified pages (as deltas) to other processes sharing those pages. Midway, in contrast, does communication only when a lock changes ownership.

Midway supports only one kind of shared data variable, whereas Munin has four kinds (read only, migratory, write-shared, and conventional). On the other hand, Midway supports three different consistency protocols (entry, release, and processor), whereas Munin only supports release consistency. Whether it is better to have multiple types of shared data or multiple protocols is open to discussion. More research will be needed before we understand this subject fully. 

Finally, the way writes to shared variables are detected is different. Munin uses the MMU hardware to trap writes, whereas Midway offers a choice between a special compiler that records writes and doing it the way Munin does, with the MMU. Not having to take a stream of page faults, especially inside critical regions, is definitely an advantage for Midway.

Now let us compare Munin and Midway to object-based shared memory of the Linda-Orca variety. Synchronization and data access in Munin and Midway are up to the programmer, whereas they are tightly integrated in Linda and Orca. In Linda there is less danger that a programmer will make a synchronization error, since in and out handle their own synchronization internally. Similarly, when an operation on a shared object is invoked in Orca, the locking is handled completely by the runtime system, with the programmer not even being aware of it. Condition synchronization (as opposed to mutual exclusion synchronization) is not part of the Munin or Midway model, so it is up to the programmer to do all the work explicitly. In contrast, it is an integral part of the Linda model (blocking when a tuple is not present) and the Orca model (blocking on a guard).

In short, the Munin and Midway programmers must do more work in the area of synchronization and consistency, with little support, and must get it all right. There is no encapsulation and there are no methods to protect shared data, as Linda and Orca have. On the other hand, Munin and Midway allow programming in only slightly modified C or C++, whereas Linda's communication is unusual and Orca is a whole new language. In terms of efficiency, Midway is best in terms of the number and size of messages transmitted, although the use of fundamentally different programming models (open C code, objects, and tuples) may lead to substantially different algorithms in the three cases, which also can affect efficiency.

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