Cause of Thrashing

• Thrashing results in severe performance problems. Consider the following scenario (see Fig. 15);
• The OS monitors CPU utilization. If CPU utilization is too low, we increase the degree of multiprogramming by introducing a new process to the system.

  Figure 15: Thrashing.

  
  • Now suppose that a process enters a new phase in its execution and needs more frames.
  • It starts faulting and taking frames away from other processes (global page-replacement algorithm).
  • These processes need those pages, however, and so they also fault, taking frames from other processes.
  • These faulting processes must use the paging device to swap pages in and out.
  • As processes wait for the paging device, CPU utilization decreases.
• The CPU scheduler sees the decreasing CPU utilization and increases the degree of multiprogramming as a result.
  • The new process tries to get started by taking frames from running processes, causing more page faults and a longer queue for the paging device.
  • As a result, CPU utilization drops even further, and the CPU scheduler tries to increase the degree of multiprogramming even more.
• Thrashing has occurred. The page-fault rate increases tremendously. As a result, the effective memory-access time increases.
• No work is getting done, because the processes are spending all their time paging.
• As the degree of multiprogramming increases, CPU utilization also increases, although more slowly, until a maximum is reached.
• If the degree of multiprogramming is increased even further, thrashing sets in, and CPU utilization drops sharply.
• At this point, to increase CPU utilization and stop thrashing, we must decrease the degree of multiprogramming.