Final answer:
Page replacement in computer operating systems involves writing out modified process pages when memory is full, either at the time of replacement or in clusters using a precleaning policy.
Step-by-step explanation:
Page replacement happens when a requested page is not in memory (page fault) and a free page cannot be used to satisfy the allocation, either because there are none, or because the number of free pages is lower than some threshold. Page replacement is where modified process pages can be written out at the time of replacement, or a precleaning policy can be used, which clusters the output activity by writing out a number of pages at once.
This is a concept in computer operating systems where the system needs to manage the limited amount of physical memory available. When the physical memory, or RAM, is full and a new page needs to be loaded, the operating system may use a page replacement algorithm to decide which memory page to write to the disk. This action can be done lazily, at replacement time, or it can employ a precleaning policy where multiple pages are written out to disk in batches, which can improve the efficiency of disk operations by reducing the frequency and randomness of write operations.