COMP 252 Operating Systems

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COMP 252Operating Systems

• Lab 06

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Outline

• Review questions week 10
• Synchronization readers-writers problem
• Project_2 guidelines

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Review questions week 10

• Please refer to our course web
• http//course.cse.ust.hk/comp252/web07fall/Lab6/la
  b06_review.pdf

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Synchronization readers-writers problem

• The problem consists of readers and writers that
  share a data resource
• The readers only want to read from the resource
• The writers want to update (write) the resource

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Synchronization readers-writers problem

• There is no problem if two or more readers access
  the resource simultaneously
• However
• If a writer and some other thread (either a
  reader or a writer) access the resource
  simultaneously, the result becomes
  indeterminable.

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Synchronization readers-writers problem

• To ensure that these difficulties do not arise,
  we require
• writers have exclusive access to the shared data
  resource
• This synchronization problem is referred to as
  the readers-writers problem
• can be used to test synchronization primitive
• more details on textbook p202

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Project_2 guidelines

• In this project, you will learn
• how to use semaphores
• get a better understanding about synchronization
• Your task is to solve the readers-writers problem
  on Nachos
• semaphores on Nachos have already been introduced
  in our previous lab

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Project_2 guidelines

• You should solve this problem in two strategies
• reader-first
• writer-first
• In both strategies, a writer has exclusive access
  to the share data while readers can share with
  other readers.

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Project_2 guidelines

• In reader-first strategy
• a reader can access the shared data (i.e., starts
  reading) as long as there is no writer inside the
  critical section.
• A writer can only access the shared data when
  there is no reader or writer inside the critical
  section.
• This is the solution that the textbook provides.

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Project_2 guidelines

• In writer-first strategy, when a new writer
  comes, here are some circumstances
• No other persons reading or writing, he/she could
  write.
• There are some readers waiting (i.e. they did not
  start reading), the writer could start to write.
  That is to say, the priorities of writers are
  higher than readers.

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Project_2 guidelines

• Cont.
• There is an old writer, the new writer waits
  until the old one finishes.
• There are some readers reading. Then the new
  writer waits until all the readers finishes.
  However, readers come after the writer must wait
  for the writer finishes writing.

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Project_2 guidelines

• Project_2 instructions and Nachos source code can
  be found on our course web.
• For Nachos semaphores, please refer to the
  Semaphore class defined in Nachos.
• threads/synch.h
• threads/synch.cc