Memo Shared Records: Readers preferred.

Memo Prog Tech

• Shared records
• Unordered queues
• Locks and semaphores
• Readers/Writers
  • readers preferred
  • writers preferred
  • taking turns
• Send/Receive
• Ordered Queues
• Barriers
• Job jars / self scheduling
• Implementation
• References

The rules of the readers-preferred shared record are these:

• multiple readers may read the record at the same time
• at most one writer may be writing a record at a time
• no writer may be writing the record while any readers are reading it
• no writer may be given the record while readers are waiting.

A solution to this problem in Memo uses three folders and an extra status record in addition to the data record.

• The three folders are:

1. status folder, holding the status record
2. read folder, where the data record is placed while being read and between accesses
3. write folder, where the data record is placed to give it to a writer.

• The status record (nr,nw) has these two integer fields:

1. nr, number of readers waiting for the record or reading.
2. nw, number of writers waiting or writing.

• There are four operations:
  1. start reading,
  2. end reading,
  3. start writing, and
  4. end writing.

The algorithms are described in terms of their implementations of the four operations.

start reading.

get the status memo

add one to nr

replace the status memo

get a copy of the data memo from the read folder

end reading

get the status memo

subtract one from nr

if nr=0 and nw>0,

get the data memo from the read folder

put it into the write folder

replace the status memo

start writing

get the status memo

add one to the number of writers

if nr=0 and nw=1

get the data memo from the read folder

replace the status memo

otherwise

replace the status memo

get the data memo from the write folder

end writing

get the status memo

subtract one from nw

if nr>0 or nw=0

put the data memo into the read folder

otherwise

put the data memo into the write folder

replace the status memo

This page last updated 04/06/97