L-5.16: What is Thrashing | Operating System

Summary: Thrashing in Operating Systems

The degree of multiprogramming is the number of processes the system keeps in memory or ready to run. Increasing it can improve CPU utilization: when one process blocks for I/O, the CPU can run another.

Because RAM is limited, operating systems use paging. Processes are divided into pages, and only some pages are kept in RAM while others remain on disk.

If a process requests a page that is not in RAM, a page fault occurs. The operating system must fetch the page from disk, which takes much longer than accessing RAM.

Keeping more processes in memory can help up to a point. But if there are too many processes for the available memory, the pages they need are often absent. The system then spends much of its time handling page faults instead of doing useful work.

This condition is called thrashing. Page faults become frequent, CPU utilization drops, and overall system performance and throughput degrade. In the illustrated relationship, CPU utilization rises as multiprogramming increases, reaches a peak, and then falls sharply when thrashing begins.

Ways to Reduce or Prevent Thrashing

• Increase main memory to give processes more room for the pages they need. The lecture notes that this may not be practical to do frequently.
• Reduce the degree of multiprogramming. The long-term scheduler controls how many processes are admitted to memory or the ready state. Limiting admissions rather than pushing multiprogramming to its maximum can help avoid thrashing.

Speakers and Sources Featured

• Unnamed lecturer/presenter — introduces the lesson as being from Gate Smashers; the subtitles do not identify the presenter by name.
• Gate Smashers — the channel and source named in the subtitles.