PrePaging

Prepaging is a technique in computer systems that aims to reduce the latency of memory and I/O operations by loading data into faster memory before it is explicitly requested. It is used primarily in virtual memory management, file systems, and database engines to hide the cost of slower storage devices such as disks or slower tiers of memory.

In practice, prepaging works by predicting future accesses and issuing prefetch I/O for pages or blocks that are likely to be needed soon. When a page fault occurs, the system may load not only the faulting page but also adjacent or related pages, or data units that are on the expected path of execution. The predictions can rely on sequential access patterns (read-ahead along a stream), evident localities of reference, stride-based patterns, or workload-specific heuristics. Hardware prefetchers, operating system read-ahead, and database prefetchers are common implementations.

Benefits include reduced page faults and lower latency for predictable workloads, as well as improved throughput for sequential scans. Limitations involve wasted bandwidth and memory when predictions prove incorrect, potential contention with memory pressure, and diminishing returns on irregular access patterns. Effective prepaging requires balancing aggressiveness with reclamation by the page-replacement policy and may be tuned by system administrators or database developers.

Relation to other concepts: prepaging is a form of prefetching and is closely related to read-ahead; it differs from reactive paging in that it acts before a fault is observed. It has historical roots in virtual memory research and remains relevant in modern operating systems, file systems, and data-intensive applications.