Three Approaches to Read-Write Concurrency Problems
Kyle Robinson
Summary
In concurrent systems, the rules for organizing access of separate processes to shared
memory can be complicated because there are many different scenarios. In an earlier project,
fellow classmates and I simulated a solution to the Dining Philosopher’s problem which is
made up of processes in a circle which access two separate items, each of which is shared with
only their neighbor, one to their left, and one to their right. The solution was drastically simple
compared to real-world concurrency issues because of the predefined low number of shared
resources and neighbors. Because of this, our solution does not translate well to other types of
concurrency problems.
For my next project, I propose to simulate a larger concurrency problem, the Readers-
Writers Problem (RWP). In a RWP, processes attempt to access shared pieces of memory for
one of two reasons, to read or to write. It would be easy enough to implement a simple queue
on each piece of memory and allow access to only one process at a time using a mutex or
semaphore. However, for example, if two processes both wish to access memory for reading
purposes, then there is no reason to wait for one to finish before the other can begin because
reading processes do not interfere with each other. I will implement and test a Concurrent
Read, Exclusive Write (CREW) model in which a processes being read can be read by multiple
processes at the same time1 (See figure 1). This is different from the Exclusive Read, Exclusive
Write (EREW) model in which at most one process can be accessing a piece of memory at any
given time2.
Figure 1: CREW model for memory access. Reading processes can be concurrent with other reading
processes, but writing processes must be exclusive.
1 NIST. Concurrent Read, Exclusive Write. Retrieved from
http://www.itl.nist.gov/div897/sqg/dads/HTML/concurrentReadExcluWrt.html
2 NIST. Exclusive Read, Exclusive Write. Retrieved from
http://www.itl.nist.gov/div897/sqg/dads/HTML/exclusiveReadExcluWrt.html
