Introduction:
Thrashing is a phenomenon that occurs in computer systems when there is excessive paging activity, resulting in a decrease in system performance. It can severely impact the overall system efficiency and throughput. The primary cause of thrashing is the excessive swapping of pages between the main memory and disk.

Explanation:
Thrashing occurs when processes on a system frequently access pages, but these pages are not available in the memory. As a result, the system needs to constantly swap pages between the disk and memory, leading to a high disk I/O activity and low CPU utilization. This swapping activity hampers the overall system performance, causing the system to spend more time swapping pages rather than executing useful work.

Process Accessing Pages Not in Memory:
When a process is running and needs to access a page that is not currently in the memory, a page fault occurs. The operating system then retrieves the required page from the disk and brings it into the memory. However, in the case of thrashing, the process frequently accesses pages that are not present in the memory. This continuous swapping of pages between the disk and memory leads to a situation where the system spends more time swapping pages than executing the actual processes.

Impact on System Performance:
Thrashing severely impacts the performance of the system in several ways:

1. High Disk I/O Activity: The continuous swapping of pages between the disk and memory leads to a high disk I/O activity, which slows down the overall system performance.

2. Low CPU Utilization: As the system spends more time swapping pages, the CPU utilization decreases, resulting in a significant decrease in the execution of useful work.

3. Increased Response Time: Due to the excessive paging activity, the response time of processes increases. Processes have to wait longer to access the required pages, leading to delays in execution.

4. Decreased Throughput: Thrashing reduces the system's throughput, as the majority of the system's resources are consumed by swapping pages instead of executing processes.

Preventing Thrashing:
To prevent thrashing, the following strategies can be employed:

1. Increasing Memory: Adding more physical memory to the system can reduce the frequency of page faults and decrease the likelihood of thrashing.

2. Optimizing Page Replacement Algorithms: Using efficient page replacement algorithms, such as the Least Recently Used (LRU) algorithm, can help in minimizing the number of page faults and reducing thrashing.

3. Adjusting Process Priorities: Adjusting the priorities of processes can ensure that critical processes are given higher priority and are less likely to be affected by thrashing.

4. Monitoring and Tuning: Regularly monitoring system performance and tuning the system parameters can help in identifying and resolving thrashing-related issues.

Conclusion:
Thrashing is a situation where processes frequently access pages that are not present in the memory, leading to excessive swapping of pages between the disk and memory. This phenomenon severely impacts system performance, resulting in high disk I/O activity, low CPU utilization, increased response time, and decreased throughput. Preventive measures such as increasing memory, optimizing page replacement algorithms, adjusting process priorities, and regular monitoring can help mitigate thrashing and improve system efficiency.