Cache Algorithms

1. 平均内存引用时间

T = average memory reference time

m = miss ratio = 1 - (hit ratio)

T_m = time to make a main memory access when there is a miss (or, with multi-level cache, average memory reference time for the next-lower cache)

T_{h =}  the latency: the time to reference the cache when there is a hit

E = various secondary effects, such as queuing effects in multiprocessor systems

平均内存引用时间 = 丢失率 * 丢失时加入时间 + 命中时引用时间（延迟） + 多种次要影响（比如多级处理器排队等待时间）

参考 https://en.wikipedia.org/wiki/Cache_algorithms

2. 替换算法

关键词：research 、 traditional 、 大内存 、 遍历周期

OPT (The theoretically optimal page replacement algorithm) 

This algorithm cannot be implemented in a general purpose operating system because it is impossible to compute reliably how long it will be before a page is going to be used, except when all software that will run on a system is either known beforehand and is amenable to static analysis of its memory reference patterns, or only a class of applications allowing run-time analysis.

无法实现

NRU (Not recently used)

It is an algorithm that favours keeping pages in memory that have been recently used.

3. referenced, modified
2. referenced, not modified
1. not referenced, modified
0. not referenced, not modified

从最低级（0级）选择随机页删除

FIFO (First-in, first-out)

最简单的页替换算法，可用List和LinkedHashMap 实现

LRU (The least recently used)

LRU works on the idea that pages that have been most heavily used in the past few instructions are most likely to be used heavily in the next few instructions too.

在前面几条指令中使用频繁的页面很可能在后面的几条指令中频繁使用。

反过来说，已经很久没有使用的页面很可能在未来较长的一段时间内不会被用到。

Random

Random replacement algorithm replaces a random page in memory.

NFU (Not frequently used)

The not frequently used (NFU) page replacement algorithm requires a counter, and every page has one counter of its own which is initially set to 0.