Allocation of Frames

1
Allocation of Frames

• Each process needs a minimum number of frames.
• Must have enough frames to hold all the pages
  that any single instruction can reference
• Example IBM 370 needs 6 pages to handle MVC
  instruction (move up to 256 characters from one
  location to another)
• The instruction is 6 bytes, can straddle 2 pages.
• The block of characters to move can straddle 2
  pages.
• The area to be moved to can straddle 2 pages.

2
Allocation Algorithms

• How to allocate m frames to n processes?
• Equal allocation each process gets m/n frames
• May waste frames
• Proportional allocation allocate according to
  the size of each process

3
Global vs. Local Allocation

• Local replacement each process selects from
  only its own set of allocated frames
• Number of frames allocated to a process is
  constant over time
• Global replacement a process can select a
  replacement frame from the set of all frames
• Process may grow and shrink over time
• Greater system throughput, more commonly used

4
Thrashing

• Thrashing ? high page-fault rate
• Cause of thrashing degree of multiprogramming is
  too high so that processes do not have enough
  frames ( actively used pages gt allocated
  frames)
• Consequence of thrashing system throughput
  plunges
• CPU utilization drops when thrashing sets in
• OS increases the degree of multiprogramming to
  increase CPU utilization.
• The new process causes more page faults, CPU
  utilization drops even further
• Solution to thrashing decrease degree of
  multiprogramming by swapping out some processes

5
Thrashing
6
Prevention of Thrashing

• Working-set strategy provide a process as many
  frames as it needs
• Page-fault frequency strategy control the page
  fault rate

7
Working-Set Model

• Locality model as a process executes, it moves
  from locality to locality
• Locality a set of pages actively used together.
• E.g., a C/C function defines a locality
• Thrashing occurs when allocated frames lt size of
  the current locality
• Working set the set of pages in the most recent
  ? page references
• Pages in the working set are in active use
• The working set is an approximation of the
  programs locality
• ? too small not encompass entire locality.
• ? too large encompass several localities.

8
Example of Working Set
?10
9
Working-Set Model

• Number of frames needed by process Pi WSSi
  working-set size of Pi
• D ? WSSi is total demand for frames
• m total number of available frames
• D gt m ? Thrashing
• The policy
• if D gt m, swap out a process
• if D lt m, start a new process
• The working-set strategy prevents thrashing while
  keeping the degree of multiprogramming as high as
  possible

10
Page-Fault Frequency Strategy

• Establish acceptable page-fault rate.
• If actual rate too low, remove a frame from
  process
• If actual rate too high, allocate a frame to
  process if no free frame, swap out the process