swapin

Swapin is the process by which a memory page that has been moved to secondary storage is loaded back into main memory. It is the counterpart to swapout. In systems that use virtual memory, pages may be written to swap space to free RAM for active processes, and later swapped back in when they are accessed again. Swapin occurs when a page fault references a swapped-out page, prompting the operating system to recover it from storage.

When swapin happens, the operating system locates the page within the swap area, allocates a free physical frame, and reads the page from disk into memory. The page table entry for that page is updated to mark it present, and the process’s TLB and caches are updated accordingly. If a swap cache is used, the system may reuse a cached copy to accelerate subsequent accesses. The exact timing and performance of swapin depend on memory pressure, disk speed, and I/O scheduling.

Swapin is slower than regular RAM access because it requires disk I/O. High levels of swap activity can lead to thrashing, where the system spends most of its time swapping pages in and out rather than doing productive work. Tuning parameters such as swappiness, I/O scheduler choices, and the use of alternative swap devices (for example, compressed RAM-based swap) influence swapin performance.

In modern operating systems, swapin is an integral part of memory management, interacting with swapout, page faults, and the page cache. Its efficiency affects overall responsiveness under memory pressure, particularly on systems with limited physical memory or slow storage.

See also: swapout, page fault, swap cache, swappiness, zram.