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Monday, December 21, 2009

Page/Disk Cache and Web Cache

Page Cache or disk cache is transparent buffer of disk-backed pages kept in main memory (RAM) by the operating system for quicker access. All memory that is not directly allocated to applications is usually utilized for page cache. Since non-dirty pages in the page cache have identical copies in secondary storage(hard disk), discarding and re-using their space is much quicker than paging out application memory, and is often preferred.The page cache also aids in writing to a disk. Pages that have been modified in memory for writing to disk, are marked "dirty" and have to be flushed to disk before they can be freed. When a file write occurs, the page backing the particular block is looked up. If it is already found in cache, the write is done to that page in memory. Otherwise, when the write perfectly falls on page size boundaries, the page is not even read from disk, but allocated and immediately marked dirty. Otherwise,the page(s)are fetched from disk and requested modifications are done.

Web caching is the caching of web documents(e.g.,HTML pages, images) to reduce bandwidth usage, server load, and perceived lag. A web cache stores copies of documents passing through it; subsequent requests may be satisfied from the cache if certain conditions are met.
With a local cache in operation, user web object requests go via the local cache which then retains a copy of the said web object. This results in all subsequent requests for the same object being fulfilled from the local cache instead of from the site of origin. This process of web caching minimizes the amount of times identical web objects are transferred from remote websites by retaining copies of requested URLs in a cache. A web cache can be installed utilizing both software and hardware, and can run on various different platforms.
With a local cache in operation, subsequent requests for previously cached URLs result in the cached copy of the object being returned to the user; creating little or no extra network traffic, improving efficiency and reducing waiting time.

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