Home
Help
Resources
Extensions
FAQ
NetLogo Publications
Donate

Models:
Library
Community
Modeling Commons

User Manuals:
Web
Printable
Chinese
Czech
Farsi / Persian
Japanese
Spanish

## NetLogo User Community Models

WHAT IS IT?

The problem of Readers and Writers describes a computing problem with concurrency. This problem was described for the first time by P. J. Courtois et al. in their work from 1971.

Suppose we have a shared document. There are two types of agents in this model - readers, who access the document only for reading and writers, who access the document for modification. The problem is with the agents accessing this document at the same time.

HOW IT WORKS

FIRST TYPE OF R/W PROBLEM - readers-preference:
There are several rules for this model:
- only one type of agent can access the document at the same time
- only one writer can modify the document
- if the document is locked for reading, an unlimited number of readers can access the document

The main rule of this model is that no reader shall be kept waiting if the document is currently opened for reading.

HOW TO USE IT

Initial settings:
- probability-reader-comes: Interval (0-100) - what is the probability that the next agent, who wants to access the document, will be a reader?
- probability-reader-leaves: Interval (0-100) - probability, that the reader will finish his action ("how long he reads")
- probability-writer-comes: Interval (0-100) - what is the probability that the next agent who wants to access the document will be a writer?
- probability-writer-leaves: Interval (0-100) - probability that the writer will finish his action ("how long he writes")

Plots:
- Writers - how many readers are actually in the system?
- Total readers - total number of processed readers plus number of readers currently in the system.
- Total writers - total number of processed writers plus number of writers currently in the system.
- Actual number of readers-writers: chart showing number of agents in the system..
- Number of readers-writers: chart showing number of agents in the system and total number of processed agents

THINGS TO NOTICE

From the number of writers in the system (unprocessed writers) you can see that in the first readers-writers problem the writers are starving.

THINGS TO TRY

Try moving the sliders. For example, if you set the "probability-reader-comes" to a very low value and "probability-reader-leaves" to a very high value, the writers won't starve.