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Copy-on-write (sometimes referred to as "COW"), sometimes referred to as ''implicit sharing'', is an optimization strategy used in computer programming. Copy-on-write stems from the understanding that when multiple separate tasks use initially identical copies of some information (i.e., data stored in computer memory or disk storage), treating it as local data that they may occasionally need to modify, then it is not necessary to immediately create separate copies of that information for each task. Instead they can all be given pointers to the same resource, with the provision that on the first occasion where they need to modify the data, they must first create a local copy on which to perform the modification (the original resource remains unchanged). When there are many separate processes all using the same resource, each with a small likelihood of having to modify it at all, then it is possible to make significant resource savings by sharing resources this way. Copy-on-write is the name given to the policy that whenever a task attempts to make a change to the shared information, it should first create a separate (private) copy of that information to prevent its changes from becoming visible to all the other tasks. If this policy is enforced by the operating system kernel, then the fact of being given a reference to shared information rather than a private copy can be transparent to all tasks, whether they need to modify the information or not. ==Copy-on-write in virtual memory management== Copy-on-write finds its main use in virtual memory operating systems; when a process creates a copy of itself, the pages in memory that might be modified by either the process or its copy are marked copy-on-write. When one process modifies the memory, the operating system's kernel intercepts the operation and copies the memory; thus a change in the memory of one process is not visible in another's. Another use involves the calloc function. This can be implemented by means of having a page of physical memory filled with zeros. When the memory is allocated, all the pages returned refer to the page of zeros and are all marked copy-on-write. This way, the amount of physical memory allocated for the process does not increase until data is written. This is typically done only for larger allocations. Copy-on-write can be implemented by notifying the MMU that certain pages in the process's address space are read-only. When data is written to these pages, the MMU raises an exception which is handled by the kernel, which allocates new space in physical memory and makes the page being written correspond to that new location in physical memory. One major advantage of COW is the ability to use memory sparsely. Because the usage of physical memory only increases as data is stored in it, very efficient hash tables can be implemented which only use little more physical memory than is necessary to store the objects they contain. However, such programs run the risk of running out of virtual address space — virtual pages unused by the hash table cannot be used by other parts of the program. The main problem with COW at the kernel level is the complexity it adds, but the concerns are similar to those raised by more basic virtual-memory concerns such as swapping pages to disk; when the kernel writes to pages, it must copy any such pages marked copy-on-write. 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「Copy-on-write」の詳細全文を読む スポンサード リンク
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