Abstract
Providing efficient emulations of atomic read/write objects in asynchronous, crash-prone, message-passing systems is an important problem in distributed computing. Communication latency is a factor that typically dominates the performance of message-passing systems, consequently the efficiency of algorithms implementing atomic objects is measured in terms of the number of communication exchanges involved in each read and write operation. The seminal result of Attiya, Bar-Noy, and Dolev established that two pairs of communication exchanges, or equivalently two round-trip communications, are sufficient. Subsequent research examined the possibility of implementations that involve less than four exchanges. The work of Dutta et al. showed that for single-writer/multiple-reader (SWMR) settings two exchanges are sufficient, provided that the number of readers is severely constrained with respect to the number of object replicas in the system and the number of replica failures, and also showed that no two-exchange implementations of multiple-writer/multiple-reader (MWMR) objects are possible. Later research focused on providing implementations that remove the constraint on the number of readers, while having read and write operations that use variable number of communication exchanges, specifically two, three, or four exchanges.
Highlights
Each read operation takes two rounds involving in four communication exchanges
They presented a SWMR algorithm, called OhSam, where reads take three exchanges: two of these are between clients and servers, and one is among servers; their MWMR algorithm, called OhMam, uses a similar approach
Read operation latency: A reader sends a readRequest message to all the servers in the first communication exchange e1
Summary
Emulating atomic [10] (or linearizable [9]) read/write objects in asynchronous, message-passing environments with crash-prone processors is a fundamental problem in distributed computing. Lynch et al [12] showed how to implement MWMR (Multi-Writer, Multi-Reader) atomic memory where both read and write operations take two communication round trips, for a total of four exchanges. Hadjistasi et al [8] showed that atomic operations do not necessarily require complete communication round trips, by introducing server-to-server communication They presented a SWMR algorithm, called OhSam, where reads take three exchanges: two of these are between clients and servers, and one is among servers; their MWMR algorithm, called OhMam, uses a similar approach. These algorithms do not impose constrains on reader participation and perform a modest amount of local computation, resulting in negligible computation overhead. Simulation results suggest that in practical settings, such as data centers with well-connected servers, the communication overhead is not prohibitive
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