How do you manage limited physical memory using virtual memory and paging?

If performance is an issue in your application, avoid using disk-paged memory. RAM is at least 8x faster than the best current SSD technologies, and that discounts per-transaction latencies. Consider paged memory as an emergency mechanism only; specify your systems with more RAM than they are likely to use in their lifetimes, or plan to upgrade before swapping becomes a problem.

With advancements in technology and lower costs per gigabyte, real memory ("central storage" in mainframe terms) is no longer a constraint in most installations. Yet enough disk storage ("auxilliary storage" in mainframe terms) must be available to hold a dump of the largest possible address space.

Virtual memory was conceived in the days when addressability was physically limited to 16 MB (or less). In mainframe terminology, the unit of virtual storage is a "page;" the physical memory backing a page is a "frame;" the auxilliary storage that holds the paged-out frame is a "slot." From an application point of view, virtual storage makes the impossible appear to be possible. From the performance analyst point of view, the question is how much paging delay a process can tolerate and still meet its service level objectives.

With advancements in technology and lower costs per gigabyte, real memory ("central storage" in mainframe terms) is no longer a constraint in most installations. Yet enough disk storage ("auxilliary storage" in mainframe terms) must be available to hold a dump of the largest possible address space.

Alongside paging, there is another method known as swapping. Swapping is a technique that works with paging to manage memory resources when there isn't enough physical memory available to hold all active processes. If a process isn't using its allocated memory, the operating system may swap out some or all of its pages from main memory to secondary storage, like a hard disk.

- Disk uses block instead of sector to keep page frames - First page address is searched in TLB, if it is not found there then page table walk is performed - If page table has swapped page table entry then page is brought from disk to memory

-The procedure of evicting an existing page is known as the page replacement. -Here are some choices of page replacement algorithms: * First in first out * Least recently used * Least frequently used * Most recently used

It is probably unlikely that the paged memory had to retrieved from disk as it is probably sitting in the cache of the disk subsystem (it would be unusual for disks on main grains not to have extensive cache memory). Also, disks on mainframes would be in a RAID configuration and therefore not susceptible to data loss outside of catastrophic failures.

Managing memory is a delicate act, requiring deep understanding of system architecture and workload requirements. High volatility pages can lead to frequent page faults, thus impacting system performance. However, less frequently accessed pages on disk are less affected by swap latency. Techniques such as memory compression, used in Android devices, or memory deduplication, seen in virtualized environments, help optimize memory usage. Moreover, setting per-process memory quotas ensures fair resource allocation and prevents monopolization. Remember, memory management isn't one-size-fits-all; it demands careful evaluation and balance. #MemoryManagement

Never forget about speed. Physical memory is super fast due to dedicated paths between CPU and memory itself. When using virtual memory based on disk, remember that you are introducing slower processes based in your disk speed (nvme > ssd > raid hdd > mech hdd). Not only slower speed is introduced, as you are adding complexity, you system can be less reliable as disks tend to get issues more frequently than memory sticks!