Книга: Distributed operating systems

3.2.4. A Comparison of the Three Algorithms

3.2.4. A Comparison of the Three Algorithms

A brief comparison of the three mutual exclusion algorithms we have looked at is instructive. In Fig. 3-11 we have listed the algorithms and three key properties: the number of messages required for a process to enter and exit a critical region, the delay before entry can occur (assuming messages are passed sequentially over a LAN), and some problems associated with each algorithm.

Algorithm Messages per entry/exit Delay before entry (in message times) Problems
Centralized 3 2 Coordinator crash
Distributed 2(n–1) 2(n–1) Crash of any process
Token ring 1 to ? 0 to n–1 Lost token, process crash

Fig. 3-11. A comparison of three mutual exclusion algorithms.

The centralized algorithm is simplest and also most efficient. It requires only three messages to enter and leave a critical region: a request and a grant to enter, and a release to exit. The distributed algorithm requires n–1 request messages, one to each of the other processes, and an additional n–1 grant messages, for a total of 2(n–1). With the token ring algorithm, the number is variable. If every process constantly wants to enter a critical region, then each token pass will result in one entry and exit, for an average of one message per critical region entered. At the other extreme, the token may sometimes circulate for hours without anyone being interested in it. In this case, the number of messages per entry into a critical region is unbounded.

The delay from the moment a process needs to enter a critical region until its actual entry also varies for the three algorithms. When critical regions are short and rarely used, the dominant factor in the delay is the actual mechanism for entering a critical region. When they are long and frequently used, the dominant factor is waiting for everyone else to take their turn. In Fig. 3-11 we show the former case. It takes only two message times to enter a critical region in the centralized case, but 2(n–1) message times in the distributed case, assuming that the network can handle only one message at a time. For the token ring, the time varies from 0 (token just arrived) to n–1 (token just departed).

Finally, all three algorithms suffer badly in the event of crashes. Special measures and additional complexity must be introduced to avoid having a crash bring down the entire system. It is slightly ironic that the distributed algorithms are even more sensitive to crashes than the centralized one. In a fault-tolerant system, none of these would be suitable, but if crashes are very infrequent, they are all acceptable.

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