L-5.16: What is Thrashing | Operating System

Summary

This video explains the concept of thrashing in operating systems, which occurs when excessive page faults slow down system performance. Thrashing is closely linked to the degree of multi-programming—the number of processes loaded into RAM. While increasing multi-programming can boost CPU utilization, it also risks overwhelming the system with too many processes, leading to page faults. The video outlines how paging can help manage memory limitations, the consequences of thrashing, and solutions like increasing RAM or adjusting the long-term scheduler to optimize CPU utilization and prevent performance degradation.

Takeaways

• 😀 Thrashing is a critical issue in operating systems, occurring when excessive paging leads to inefficient CPU usage.
• 😀 The degree of multi-programming refers to the number of processes loaded into RAM, and increasing this can improve CPU utilization.
• 😀 RAM has limited capacity, so only portions (pages) of each process are loaded into RAM, which helps maximize CPU utilization.
• 😀 In paging, only a small portion (such as one page) of each process is loaded into RAM to increase the number of processes that can be managed simultaneously.
• 😀 A page fault occurs when the CPU requests a page that is not currently in RAM, leading to the need to fetch it from the hard disk, which is time-consuming.
• 😀 If too many processes are loaded into RAM, and required pages are not available, frequent page faults occur, causing thrashing.
• 😀 As the degree of multi-programming increases, CPU utilization increases until it reaches an optimal point, after which thrashing can occur.
• 😀 Thrashing causes the system to waste CPU time on fetching pages from the disk instead of performing useful operations, reducing overall performance.
• 😀 One solution to prevent thrashing is to increase the size of the main memory, although this is not always feasible or scalable.
• 😀 Another solution to mitigate thrashing is to adjust the long-term scheduler to limit the number of processes brought into RAM at any given time.
• 😀 By managing the degree of multi-programming through the long-term scheduler, systems can avoid thrashing and maintain good performance.

Q & A

What is thrashing in the context of operating systems?

Thrashing occurs when the system spends a large amount of time swapping data between RAM and secondary storage (like a hard disk) due to excessive page faults, which leads to the CPU being underutilized and system performance degrading.

How is the degree of multi-programming related to CPU utilization?

The degree of multi-programming refers to the number of processes loaded into RAM. Increasing the number of processes typically increases CPU utilization because the CPU can continue working even if some processes are blocked waiting for I/O. However, too many processes can lead to thrashing.

What role does RAM size play in the performance of an operating system?

RAM size is crucial because it determines how many processes can be loaded into memory. If the RAM is too small for the number of processes, it leads to frequent page faults as processes are swapped in and out of memory, reducing system performance and potentially causing thrashing.

What is paging and how does it help manage memory?

Paging is a memory management technique that divides processes into small fixed-size units called 'pages'. These pages are loaded into RAM as needed. This allows a higher degree of multi-programming by enabling the system to load only parts of a process, rather than the entire process, into memory.

What happens when a page fault occurs?

A page fault occurs when the CPU requests a page that is not currently in RAM. The operating system must then load the missing page from secondary storage (e.g., a hard disk) into RAM, which can take a significant amount of time and cause delays in execution.

How does thrashing affect CPU utilization?

During thrashing, the system spends most of its time servicing page faults rather than executing processes. As a result, CPU utilization decreases because the CPU is often idle, waiting for pages to be loaded into memory.

What is the relationship between degree of multi-programming and thrashing?

Increasing the degree of multi-programming by loading more processes into RAM can improve CPU utilization up to a point. However, beyond a certain threshold, the system becomes overloaded, leading to excessive page faults and thrashing, which ultimately decreases CPU utilization and performance.

What is the role of the long-term scheduler in preventing thrashing?

The long-term scheduler controls how many processes are admitted into the system. By reducing the number of processes allowed in memory at any given time, the long-term scheduler can help prevent thrashing and ensure that the system does not become overloaded.

What solutions are available to prevent or mitigate thrashing?

To prevent thrashing, one solution is to increase the physical size of RAM, allowing more processes to be loaded without causing excessive page faults. Another solution is to adjust the long-term scheduler to control the degree of multi-programming and ensure that the system doesn't become overloaded.

What is the trade-off between increasing the degree of multi-programming and system performance?

Increasing the degree of multi-programming improves CPU utilization by keeping the CPU busy with more processes. However, if too many processes are loaded, it can lead to thrashing, which reduces CPU utilization and degrades system performance. There is an optimal point where the balance between multi-programming and system performance is maintained.