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Month: 十月 2016 (Page 1 of 2)

2016年10月简讯

这个月基本就是一边找工作,一边学习。感觉在家工作效率还是不行,也就相当于平时工作的一半或者三分之二,看来自由职业者不是那么好当的。最大的收获就是把《Working with Unix Processes》的英文版读了,再有就是把Unix微信工作号重新经营起来了。希望下个月自己生活上各个方面的状态能更好吧。

Go语言中连接多个字符串

本文参考自How to efficiently concatenate strings in Go?Efficient String Concatenation in Go

(1)直接使用“+=”:

package main

import (
    "fmt"
    "strconv"
)

func main() {
    s := ""
    for i := 0; i <= 9; i++ {
        s += strconv.Itoa(i)
    }
    fmt.Println(s)
}

因为字符串类型在Go中是不可改变的,因此每次操作实际都要新分配字符串,所以在字符串比较多的时候效率不高。

(2)使用strings.Join()函数:

package main

import (
    "fmt"
    "strconv"
    "strings"
)

func main() {
    var s []string
    for i := 0; i <= 9; i++ {
        s = append(s, strconv.Itoa(i))
    }
    fmt.Println(strings.Join(s, ""))
}

这种方式需要花费构建slice的时间。

(3)使用bytes.Buffer

package main

import (
    "bytes"
    "fmt"
    "strconv"
)

func main() {
    var buffer bytes.Buffer
    for i := 0; i <= 9; i++ {
        buffer.WriteString(strconv.Itoa(i))
    }
    fmt.Println(buffer.String())
}

这种方式在字符串比较多时效率最高。

Linux kernel 笔记 (63)——改变启动的kernel

原文在这里

得到当前系统运行的kernel(系统为CentOS):

# egrep ^menuentry /etc/grub2.cfg | cut -f 2 -d \'
CentOS Linux (4.8.3) 7 (Core)
CentOS Linux (3.10.0-327.el7.x86_64) 7 (Core)
CentOS Linux (0-rescue-d07a2009dd34415fa45624985dccbdf6) 7 (Core)

使用grub2-set-default改变启动的kernel

# grub2-set-default 0

如果仅仅想生效一次,可以使用grub2-reboot命令:

# grub2-reboot 0

inode,“hard link”和“symbol link”

*nix文件系统上,每个文件的存储实际可以看成包含两部分:inode和实际存储文件内容的数据块。其中inode存储文件的metadata,包含创建时间,访问权限,等等,当然还有指向文件具体数据块的指针。正是通过这个指针,将indoe和数据块关联起来。

要注意,inode中并不保存文件的名字。关于文件名字和inode的映射存储在目录文件中。因此,当访问一个文件时,其实是通过这个文件所在的目录文件访问到这个文件的inode信息,继而进行文件操作的。

接下来,看一下hard linksymbol linkinode之间的关系。首先创建一个文件和指向这个文件的hard linksymbol link

# echo 'Hello, World!' > myfile.txt
# ln myfile.txt my-hard-link
# ln -s myfile.txt my-soft-link

查看这3个文件的inode信息:

# ls -ailt my*
325332 lrwxr-xr-x  1 root  wheel  10 Oct 24 05:26 my-soft-link -> myfile.txt
325331 -rw-r--r--  2 root  wheel  14 Oct 24 05:25 my-hard-link
325331 -rw-r--r--  2 root  wheel  14 Oct 24 05:25 myfile.txt

可以看到myfile.txtmy-hard-link其实对应的的是同一个inode节点:325331,而my-soft-link对应的是另一个inode节点:325332。接下来删除myfile.txt,然后分别读取my-hard-linkmy-soft-link文件内容:

# rm myfile.txt
# ls -ailt my*
325332 lrwxr-xr-x  1 root  wheel  10 Oct 24 05:26 my-soft-link -> myfile.txt
325331 -rw-r--r--  1 root  wheel  14 Oct 24 05:25 my-hard-link
# cat my-hard-link
Hello, World!
# cat my-soft-link
cat: my-soft-link: No such file or directory

可以看到,因为my-hard-linkmyfile.txt对应相同的inode节点:325331,因此删除myfile.txt后,仍然可以通过my-hard-link读取325331这个inode节点所对应的文件内容。而my-soft-link仅仅是指向myfile.txt这个文件名字,因此一旦myfile.txt被删除,也就无法读取文件内容了。

参考资料:
Inodes – an Introduction
What is the difference between a symbolic link and a hard link?

得到一个有符号数的符号位

Compute the sign of an integer描述了如何得到一个有符号数的符号位:

int v;      // we want to find the sign of v
int sign;   // the result goes here 

// CHAR_BIT is the number of bits per byte (normally 8).
sign = -(v < 0);  // if v < 0 then -1, else 0. 
// or, to avoid branching on CPUs with flag registers (IA32):
sign = -(int)((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1));
// or, for one less instruction (but not portable):
sign = v >> (sizeof(int) * CHAR_BIT - 1); 

关于方法1

sign = -(v < 0);  // if v < 0 then -1, else 0. 

参考C规范6.5.8:6

Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false.92) The result has type int.

因此(v < 0)会得到10

关于方法23

// or, to avoid branching on CPUs with flag registers (IA32):
sign = -(int)((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1));
// or, for one less instruction (but not portable):
sign = v >> (sizeof(int) * CHAR_BIT - 1); 

参考stackoverflow这篇帖子。因为有符号数类型中的负数右移是C规范中未定义行为,取决与具体体系结构的实现,因此需要先把它转化成无符号数进行右移操作:

((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1))

Linux下使用vmstat命令获得系统CPU的使用状态

本文是使用vmstat命令监控CPU使用的续文。

Linux下使用vmstat命令可以得到系统CPU的使用状态:

# vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 2  0      0 1860352    948 131040    0    0  2433   137  252  897  2  7 90  1  0

其中描述CPU状态的是最后5列:

------cpu-----
us sy id wa st
2  7 90  1  0

要注意,上面数字的含义是百分比。即CPU运行user space程序的时间占2%,。。。

各列含义如下:

ususer time):CPU运行user space代码的时间;
sysystem time):CPU运行kernel代码的时间,比如执行系统调用;
ididle time):CPU处于idle状态的时间;
waIO-wait time):CPU处于idle状态,因为所有正在运行的进程都在等待I/O操作完成,因此当前无可以调度的进程;
ststolen time):CPU花费在执行系统上运行的虚拟机的时间。

参考资料:
The precise meaning of I/O wait time in Linux
Linux Performance Analysis in 60,000 Milliseconds

Memoization和dynamic programming

这篇笔记摘自Tutorial for Dynamic Programming

Dynamic programming (usually referred to as DP ) is a very powerful technique to solve a particular class of problems. It demands very elegant formulation of the approach and simple thinking and the coding part is very easy. The idea is very simple, If you have solved a problem with the given input, then save the result for future reference, so as to avoid solving the same problem again.. shortly ‘Remember your Past’ 🙂 . If the given problem can be broken up in to smaller sub-problems and these smaller subproblems are in turn divided in to still-smaller ones, and in this process, if you observe some over-lappping subproblems, then its a big hint for DP. Also, the optimal solutions to the subproblems contribute to the optimal solution of the given problem ( referred to as the Optimal Substructure Property ).

There are two ways of doing this.

1.) Top-Down : Start solving the given problem by breaking it down. If you see that the problem has been solved already, then just return the saved answer. If it has not been solved, solve it and save the answer. This is usually easy to think of and very intuitive. This is referred to as Memoization.

2.) Bottom-Up : Analyze the problem and see the order in which the sub-problems are solved and start solving from the trivial subproblem, up towards the given problem. In this process, it is guaranteed that the subproblems are solved before solving the problem. This is referred to as Dynamic Programming.

Memoization是一种“自顶向下”解决问题的方式,并且顾名思义,它具有保存结果的含义:它把一个问题细化成子问题,如果子问题已经解决,就直接获得结果,反正则解决子问题,并把结果保存起来。
Dynamic Programming则是“自下而上”的解决问题方式,它从处理子问题入手,在解决最终问题之前,保证它的所有子问题都已解决。

CentOS配置静态IP

VirtualBox里安装CentOS,配置静态IP

(1)CentOS 6,修改/etc/sysconfig/network-scripts/ifcfg-eth0文件:

......
ONBOOT=yes
BOOTPROTO=static
IPADDR=192.168.1.9
NETMASK=255.255.255.0
GATEWAY=192.168.1.1

(2)CentOS 7,修改/etc/sysconfig/network-scripts/ifcfg-enp0s3文件:

......
BOOTPROTO="static"
ONBOOT="yes"
IPADDR="192.168.1.5"
NETMASK="255.255.255.0"
GATEWAY="192.168.1.1"
DNS1="192.168.1.1"
DNS2="8.8.8.8"

 

进程的priority和nice

本文选自Difference between nice value and priority in the top output,以Linux系统为例讲解进程的prioritynice
(1)

The difference is that PR is a real priority of a process at the moment inside of the kernel and NI is just a hint for the kernel what the priority the process should have.

Priority反映当时进程真正的优先级,而nice则是告诉kernel进程应该获得什么样的优先级。

(2)Nice的值从-2019-20表示优先级最高。通常情况下,priority = nice + 20,也就是priority的值为0~39。但是上述理论仅仅适用于调度策略是SHED_OTHER的进程,此外,kernel也有可能只改变priority的值,而nice的值保持不变,因此上述等式同样不适用。

“Page out”和“swap out”

下文摘自Linux performance and tuning guidelines

The pages are used mainly for two purposes: page cache and process address space. The page cache is pages mapped to a file on disk. The pages that belong to a process address space (called anonymous memory because it is not mapped to any files, and it has no name) are used for heap and stack. When kswapd reclaims pages, it would rather shrink the page cache than page out (or swap out) the pages owned by processes. A large proportion of page cache that is reclaimed and process address space that is reclaimed might depend on the usage scenario and will affect performance. You can take some control of this behavior by using /proc/sys/vm/swappiness.

Page out and swap out: The phrases “page out” and “swap out” are sometimes confusing. The phrase “page out” means take some pages (a part of entire address space) into swap space while “swap out” means taking entire address space into swap space. They are sometimes used interchangeably.

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