局部性：

局部性分為時間局部性和空間局部性：Locality is typically described as having two distinct forms: temporal locality and spatial locality. In a program with good temporal locality, a memory location that is referenced once is likely to be referenced again multiple times in the near future. In a program with good spatial locality, if a memory location is referenced once, then the program is likely to reference a nearby memory location in the near future.

一個使用局部性的例子：

At the operating system level, the principle of locality allows the system to use the main memory as a cache of the most recently referenced chunks of the virtual address space. Similarly, the operating system uses main memory to cache the most recently used disk blocks in the disk file system.

CSAPP分兩個方面說明了局部性的問題：

Locality of References to Program Data 和?Locality of Instruction Fetches

關(guān)于前者，CSAPP是舉例說明了局部性的問題：

1 int sumvec(int v[N])
2 {
3 int i, sum = 0;
4
5 for (i = 0; i < N; i++)
6     sum += v[i];
7 return sum;
8 }

在這個例子里：

sum每次循環(huán)都會被訪問一次，所以有好的時間局部性

v是一個接著一個地讀取，空間局部性好，時間局部性差

Stride-1 reference patterns are a common and important source of spatial locality in programs. In general, as the stride increases, the spatial locality decreases.

關(guān)于取指令的局部性分析，舉例：

 int sumarraycols(int a[M][N])
2 {
3 int i, j, sum = 0;
4
5 for (j = 0; j < N; j++)
6     for (i = 0; i < M; i++)
7         sum += a[i][j];
8 return sum;
9 }    

從取指的角度，這個函數(shù)時間局部性和空間局部性都很好，解釋如下：

The instructions in the body of the?for loop are executed in sequential memory order, and thus the loop enjoys good spatial locality. Since the loop body is executed multiple times, it also enjoys good temporal locality.

還順帶解釋了指令和數(shù)據(jù)的區(qū)別：

An important property of code that distinguishes it from program data is that it is rarely modified at run time. While a program is executing, the CPU reads its instructions from memory. The CPU rarely overwrites or modifies these instructions.

關(guān)于locality的總結(jié)：

Programs that repeatedly reference the same variables enjoy good temporal locality（不斷引用同一個變量的程序具有好的時間局部性）

For programs with stride-k reference patterns, the smaller the stride the better the spatial locality. Programs with stride-1 reference patterns have good spatial locality. Programs that hop around memory with large strides have poor spatial locality（步長越短，空間局部性越好）

Loops have good temporal and spatial locality with respect to instruction fetches. The smaller the loop body?and the greater the number of loop iterations, the better the locality（取指的時候，循環(huán)的時間局部性和空間局部性很好，循環(huán)體越短，循環(huán)次數(shù)越多，局部性越好）

?存儲器的層次圖：

值得一提的固態(tài)硬盤的位置：

As another example, solid state disks are playing an increasingly important role in the memory hierarchy, bridging the gulf between DRAM and rotating disk.?

關(guān)于cold misses:?An empty cache is sometimes referred to as a cold cache, and misses of this kind are called compulsory misses or cold misses. Cold misses are important because they are often transient events that might not occur in steady state, after the cache has been warmed up by repeated memory accesses

一種設(shè)計緩存的方法是利用哈希，使得k+1層的數(shù)據(jù)按照地址映射到k層的某個位置

working set是程序運行過程中訪問的一個大小相對固定的緩存塊的一部分

capacity misses:?When the size of the working set exceeds the size of the cache, the cache will experience what are known as capacity misses. In other words, the cache is just too small to handle this particular working set.

那么，不同層次的緩存是由誰管理的呢？

The compiler manages the register file, the highest level of the cache hierarchy. It decides when to issue loads when there are misses, and determines which register to store the data in. The caches at levels L1, L2, and L3 are managed entirely by hardware logic built into the caches. In a system with virtual memory, the DRAM main memory serves as a cache for data blocks stored on disk, and is managed by a combination of operating system software and address translation hardware on the CPU. For a machine with a distributed file system such as AFS, the local disk serves as a cache that is managed by the AFS client process running on the local machine. In most cases, caches operate automatically and do not require any specific or explicit actions from the program.

?

轉(zhuǎn)載于:https://www.cnblogs.com/geeklove01/p/9069296.html

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