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In computing, a page cache, sometimes also called disk cache, is a transparent cache for the pages originating from a secondary storage device such as a hard disk drive (HDD). The operating system keeps a page cache in otherwise unused portions of the main memory (RAM), resulting in quicker access to the contents of cached pages and overall performance improvements. A page cache is implemented in kernels with the paging memory management, and is mostly transparent to applications. Usually, all physical memory not directly allocated to applications is used by the operating system for the page cache. Since the memory would otherwise be idle and is easily reclaimed when applications request it, there is generally no associated performance penalty and the operating system might even report such memory as "free" or "available". When compared to main memory, hard disk drive read/write speeds are low and random accesses require expensive disk seeks; as a result, larger amounts of main memory bring performance improvements as more data can be cached in memory. Separate disk caching is provided on the hardware side, by dedicated RAM or NVRAM chips located either in the disk controller (in which case the cache is integrated into a hard disk drive and usually called disk buffer), or in a disk array controller. Such memory should not be confused with the page cache. == Memory conservation == Pages in the page cache modified after being brought in are called dirty pages.〔(【引用サイトリンク】 Glossary - TechNet Library )〕 Since non-dirty pages in the page cache have identical copies in secondary storage (e.g. hard disk drive or solid-state drive), discarding and reusing their space is much quicker than paging out application memory, and is often preferred over flushing the dirty pages into secondary storage and reusing their space. Executable binaries, such as applications and libraries, are also typically accessed through page cache and mapped to individual process spaces using virtual memory (this is done through the mmap system call on Unix-like operating systems). This not only means that the binary files are shared between separate processes, but also that unused parts of binaries will be flushed out of main memory eventually, leading to memory conservation. Since cached pages can be easily evicted and re-used, some operating systems, notably Windows NT, even report the page cache usage as "available" memory, while the memory is actually allocated to disk pages. This has led to some confusion about the utilization of page cache in Windows. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「page cache」の詳細全文を読む スポンサード リンク
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