用一只麻雀来观察Linux内存分配惰性策略

本文主要是介绍用一只麻雀来观察Linux内存分配惰性策略，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

结论先行

我们知道现代的计算机系统，通常分为虚存和物理内存。其中虚存和物理内存之间存在地址映射。

原则上机器的物理内存是宝贵的，所以，Linux使用懒惰的策略来进行分配物理内存，直到虚存被真实使用到！

实验设计

用malloc来申请内存

仅体现为虚存的增长

对malloc申请的内存进行逐字节写入访问

造成物理内存被依次分配出来，体现出阶梯状

观察手段

# notice the VIRT RES %MEM cols
top -p $(pidof a.out)

代码

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>
#include <stdint.h>int main(int argc , char* argv[])
{printf("Hello world!\n""To see the `top` command memory statistics!\n\n");int sleeptime = 3;if(argc > 1){sleeptime = atoi(argv[1]);}printf("Using sleeptime: %d argument to watch the incresing RES(physical) statistic\n""But the VIRT statistic keeps unchanged after malloc\n\n", sleeptime);printf("You can see the program's VIRT&RES&%%MEM statistics using `top` command\n\n""The initial VIRT&RES statistics must be small\n""Enter any key to continue to malloc a big memory area\n");int c = getchar();const size_t nBigSize = 1*1024*1024*1024;char* const pbBigMem = malloc(nBigSize);if(pbBigMem == NULL){perror("Malloc a big memory");return -1;}printf("\nThe VIRT statistic is changed after malloc\n");char* const pbBigMemEnd = pbBigMem + nBigSize;char* cursor = pbBigMem;do{// Access the memory to cause the physical memory to be assignedfor(size_t i = 0; i < (100*1024*1024) && cursor < pbBigMemEnd; ++i, ++cursor){// one by onecursor[0] = (char)i;}  printf("The RES statistic is changed after the memory is really scaned one by one, cursor: %p\n", cursor);sleep(sleeptime);}while(cursor < pbBigMemEnd);printf("\n\nExit from program !\n");return 0;
}