计算机的Cache和Memory访问时Write-back，Write-through及write allocate的区别

计算机的存储系统采用Register，Cache，Memory和I/O的方式来构成存储系统，无疑是一个性能和经济性的妥协的产物。Cache和Memory机制是计算机硬件的基础内容，这里就不再啰嗦。下面重点说明Write-back，Write-through及write allocate这三种操作的区别。

一、CPU读Cache

1. Read through，即直接从内存中读取数据；

2. Read allocate，先把数据读取到Cache中，再从Cache中读数据。

二、CPU写Cache

1. 若hit命中，有两种处理方式：

Write-through: write is done synchronously both to the cache and to the backing store。Write-through（直写模式）在数据更新时，把数据同时写入Cache和后端存储。此模式的优点是操作简单；缺点是因为数据修改需要同时写入存储，数据写入速度较慢。

         Write-back (also called write-behind): initially, writing is done only to the cache. The write to the backing store is postponed until the cache blocks containing the data are about to be modified/replaced by new content。Write-back（回写模式）在数据更新时只写入缓存Cache。只在数据被替换出缓存时，被修改的缓存数据才会被写到后端存储（即先把数据写到Cache中，再通过flush方式写入到内存中）。此模式的优点是数据写入速度快，因为不需要写存储；缺点是一旦更新后的数据未被写入存储时出现系统掉电的情况，数据将无法找回。

2. 若miss，有两种处理方式：

Write allocate (also called fetch on write): data at the missed-write location is loaded to cache, followed by a write-hit operation. In this approach, write misses are similar to read misses.。Write allocate：先把要写的数据载入到Cache中，写Cache，然后再通过flush方式写入到内存中；  写缺失操作与读缺失操作类似。

No-write allocate (also called write-no-allocate or write around): data at the missed-write location is not loaded to cache, and is written directly to the backing store. In this approach, only the reads are being cached。No write allocate：并不将写入位置读入缓存，直接把要写的数据写入到内存中。这种方式下，只有读操作会被缓存。

注、本文两幅图来自wiki。