Книга: Distributed operating systems

6.3. CONSISTENCY MODELS

Although modern multiprocessors have a great deal in common with distributed shared memory systems, it is time to leave the subject of multiprocessors and move on. In our brief introduction to DSM systems at the start of this chapter, we said that they have one or more copies of each read-only page and one copy of each writable page. In the simplest implementation, when a writable page is referenced by a remote machine, a trap occurs and the page is fetched. However, if some writable pages are heavily shared, having only a single copy of each one can be a serious performance bottleneck.

Allowing multiple copies eases the performance problem, since it is then sufficient to update any copy, but doing so introduces a new problem: how to keep all the copies consistent. Maintaining perfect consistency is especially painful when the various copies are on different machines that can only communicate by sending messages over a slow (compared to memory speeds) network. In some DSM (and multiprocessor) systems, the solution is to accept less than perfect consistency as the price for better performance. Precisely what consistency means and how it can be relaxed without making programming unbearable is a major issue among DSM researchers.

A consistency model is essentially a contract between the software and the memory (Adve and Hill, 1990). It says that if the software agrees to obey certain rules, the memory promises to work correctly. If the software violates these rules, all bets are off and correctness of memory operation is no longer guaranteed. A wide spectrum of contracts have been devised, ranging from contracts that place only minor restrictions on the software to those that make normal programming nearly impossible. As you probably already guessed, the ones with minor restrictions do not perform nearly as well as the ones with major restrictions. Such is life. In this section we will study a variety of consistency models used in DSM systems. For additional information, see the paper by Mosberger (1993).

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