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第9课

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Memory Hierarchy

Cache

Q3: Block replacemet

当要访问的数据不在Cache当中,对于直接映射不需要考虑替换,因为只有一块对应的,没有选择问题。

  • Random replacement;
  • Least-Recently Used(LRU):从局部性来考虑,最近最少使用的块;
  • First in, First Out(FIFO);

Example1:

  • FIFOimage-20230327101750387
  • LRU(3位,因为size只有3)image-20230327102006164
  • OPT(最优算法,考虑的是未来的事情,必须程序全部跑一遍才知道,所以实际中很难做到,可以用来衡量其他算法的优劣)image-20230327102342414

操作步骤

  • image-20230327102715393
  • image-20230327102721476

Example2:

  • 除了替换方案之外,对于一个访问序列,序列本身也会对命中率也会有影响;image-20230327103308660

Example3:

  • 当cache块大小改变,对命中率还是有影响的;image-20230327103430684

Stack replacement algorithm

  • 保证算法随着cache块的增大,命中率只增不减image-20230327103645769

  • LRU算法是Stack replacement algorithmimage-20230327103854756

  • FIFO就不是Stack replacement algorithmimage-20230327104020331
  • 使得栈顶的元素永远都是刚刚被访问的元素image-20230327105252872当n=1,没有两个连续块是一样的,所以命中率为0image-20230327105303244当n=2,红色的块是命中的,所以命中了一次image-20230327105429253n=3,命中了三次image-20230327105408769n=4,命中了四次image-20230327105417721n=5以及更大,命中了五次image-20230327105445021统计图:image-20230327105456590
  • 怎么比较各个cache的块的次序(访问远近的时间次序)image-20230327105916440image-20230327105930149image-20230327110227849image-20230327110305374image-20230327110400122image-20230327110431974

Q4: Write strategy

  • 如果hit命中
    • 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: 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。write back是只写cache,并使用dirty标志位记录cache的修改,直到被修改的cache 块被替换时,才把修改的内容写回main memory。 image-20230327110903625

Write stall

  • image-20230327111111085image-20230327111121677

  • 如果write miss:

    • 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。
  • image-20230327111226871

Example:

  • image-20230327111819465

Split vs. unified caches

  • image-20230327112217723

总结

image-20230327112254933

image-20230327112548426

内存的管理也有相应的几个问题image-20230327112621443

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