Memo Shared Records: Writers 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 writers-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 reader may be given the record while writers are waiting.

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

• The four folders are

1. F[0], read folder 0
2. F[1], read folder 1
3. F[0,1], status folder
4. F[0,2], write folder

(A folder name s[i] means its symbol is s and its X array is {i,0,0}; similarly s[i,j] means its symbol is s and its X array is {i,j,0}.)

• The fields of the status record, (nr,wtr,nw,i), are

1. nr, number of processes reading
2. wtr, number of processes waiting to read
3. nw, number of processes writing and waiting to write
4. i, index of current read folder, either 0 or 1.

The four operations, start reading, end reading, start writing, and end writing, are as follows:

start reading.

get the status memo

if nw=0

add one to nr

get a copy of the data memo from the read folder F[i]

replace the status memo

otherwise

add one to wtr

replace the status memo

get a copy of the data memo from the read folder F[1-i]

end reading

get the status memo

subtract one from nr

if nr=0 and nw>0,

get the data memo from the read folder F[i]

put it into the write folder

replace the status memo

start writing

get the status memo

add one to the nw

if nr=0 and nw=1

get the data memo from the read folder F[i]

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 nw=0

assign wtr to nr

assign 0 to wtr

i := 1 - i

replace the status memo

put the data memo into the read folder F[i]

otherwise nw>0

put the data memo into the write folder

replace the status memo

This page last updated 04/06/97