Content #
User-accessible memory on a Linux system is divided into two broad classes: file-backed and anonymous.
File-backed pages (or page-cache pages) correspond to a segment of a file on disk; if they do not contain newly written data that has not yet made it back to persistent storage, these pages can be easily reclaimed for other uses.
Anonymous pages do not correspond to a file on disk; they hold the run-time data generated and used by a process. Reclaiming an anonymous page requireswriting its contents to the swap device.
As a general rule, reclaiming anonymous pages (swapping) is seen as being considerably more expensive than reclaiming file-backed pages. One of the key reasons for this difference is that file-backed pages can be read from (and written to) persistent storage in large, contiguous chunks, while anonymous pages tend to be scattered randomly on the swap device. On a rotating storage device, scattered I/O operations are expensive, so a system that is doing a lot of swapping will slow down considerably. It is far faster to read a bunch of sequentially stored file-backed pages — and, since the file is usually current on disk, those pages may not need to be written at reclaim time at all.
使用SSD硬盘会改变这种状况。